1
The effect of hydrogen-bonding on the ultrafast electronic deactivation dynamics of indigo carmine

Type: Object | Advantage: None | Novelty: New | ConceptID: Obj1

2
Excited-state dynamics and mechanisms of the rapid deactivation process of indigo carmine (InC) were investigated by means of femtosecond transient absorption spectroscopy and steady-state Raman spectroscopy.

Type: Object | Advantage: None | Novelty: New | ConceptID: Obj2

3
Solvent dependence of the excited-state lifetime revealed that intermolecular hydrogen-bonding with the solvent molecule is more effective than the intramolecular ones to accelerate the deactivation process.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con1

4
Steady-state Raman spectra in the low-frequency region indicated a loss of molecular planarity in protic solvents.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res1

5
It was concluded that the hydrogen-bonding in the excited state, which leads to the twisting around the central CC bond and/or to the out-of-plane deformation, was of crucial importance in the rapid deactivation.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con2

Introduction

6
Indigo is one of the natural dyestuffs utilized by human beings from the dawn of civilization.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac1

7
The description “indigo” can refer either to the blue dyestuff obtained from several species of plants, or to the chemical compound accountable for the color.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac2

8
In this report, it represents the latter.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac2

9
The Nobel Prize in 1905 was awarded to a German research chemist, A. von Baeyer, for the synthesis and the structural formulation of indigo.1,2

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac3

10
Influenced by this success, search for artificial indigo derivative with superior chemical properties took place and the first artificial derivative, thioindigo, was synthesized in .19063

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac4

11
It is well known that thioindigo and other indigo-derivatives shown in Scheme 1 (a) undergo photochromism, i.e., reversible trans–cis isomerization around the central CC bond on photo-excitation.4–9

Type: Model | Advantage: None | Novelty: None | ConceptID: Mod1

12
However, for indigo (X = NH) itself, photo-isomerization and/or the cis-form have never been observed.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac5

13
On the basis of the peculiar molecular structure enabling intramolecular hydrogen(H)-bonds between the two adjacent pairs of carbonyl and NH groups,10–13 two possibilities were considered as the origin of the absence of photo-isomerization; (1) double intramolecular H-bonds “lock” the molecular structure in planer trans-form, or (2) intramolecular proton-transfer from the NH group to the carbonyl group occurs on the excited state prior to the isomerization, which leads back to the trans-configuration in the ground state.14,15

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac6

14
For highly fluorescent thioindigo, it is generally accepted that trans → cis isomerization takes place through triplet intermediate.15–21

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac7

15
It was concluded that the planar trans configuration is in thermal equilibrium with the twisted configuration in the T1 state.20

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac8

16
T1 state is also considered to be the intermediate in the case of N,N′-dimethylindigo.22

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac9

17
Intersystem crossing is usually a slow process which can be inhibited by a faster reaction channel.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac10

18
Elsaesser and coworkers have carried out time-resolved picosecond fluorescence and IR spectroscopy of 4,4′,7,7′-tetramethylindigo, a indigo derivative that can form intramolecular H-bonds.23

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac11

19
They concluded that proton transfer does not occur in the first excited electronic state from the following experimental results, (a) their time-resolved IR spectra revealed that the NH stretching in the S1 state was shifted to lower frequency only by 40 cm−1 compared to the ground state, (b) no evidence of a transient OH band was found around 3200 cm−1 in the excited molecule, (c) the CO stretching mode at 1640 cm−1 remained virtually unchanged upon photo-excitation and (d) the steady state fluorescence spectrum was Stokes shifted only by 900 cm−1 which was rather too small for a fluorescence form a proton transferred tautamor which can be as large as ∼10 000 cm−1.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac12

20
Although, there are many reports on excited state intramolecular proton transfer (ESIPT) taking place from phenolic –OH group to the H-bonded adjacent –N group, i.e., 2-(2′-hydroxy-5′-methylphenyl)benzotriazole,24,25 2-(2′-hydroxyphenyl)benzothiazole,26,27 and 10-hydroxybenzo[h]quinoline,28 we could not find any report on ESIPT from NH group to CO group.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac13

21
Intramolecular H-bond forming 5- or 6-membered ring is considered to be strong, because of the good overlap of the molecular orbitals.29

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac14

22
If the isomerization of indigo is blocked by the intramolecular H-bond bridge, formation of intermolecular H-bonds with the solvent may break the bridge and lead to photo-isomerization.

Type: Hypothesis | Advantage: None | Novelty: None | ConceptID: Hyp1

23
However, addition of alcohols to the solution of photochromic indigo dyes is known to quench the fluorescence and diminish the isomerization.30–32

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac15

24
It was concluded that intermolecular H-bond opens a rapid nonradiative deactivation channel to the trans-formed ground state by transferring the energy to the solvent bath through the H-bond.30

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac16

25
In the case of indigo, which is only weakly fluorescent, intramolecular H-bonds are suggested to provide a channel for the rapid radiationless deactivation.32,33

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac17

26
For 4,4′,7,7′-tetramethylindigo in chloroform, the fluorescence lifetime was only 30 ps and the overtones of the NH stretching mode and the out-of-plane deformation mode were considered to be the decay channels for the rapid internal conversion.23

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac18

27
Another H-bonded system that exhibits rapid nonradiative decay without formation of any detectable intermediate is the intermolecular H-bonded 1-pyrenol-pyridine system.34

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac19

28
It was concluded that, immediately after the electron-transfer form 1-pyrenol to pyridine, a large scale and ultrafast proton shift takes place which induces an ultrafast nonradiative crossing to the ground state.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac19

29
Hydrogen-bonding is one of the most important topics in chemistry and in molecular biology.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac20

30
H-bonds in the excited state are known to mediate not only proton transfer but also electron transfer.35,36

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac21

31
However, the role of H-bonds in energy deactivation and dissipation is not well understood.

Type: Motivation | Advantage: None | Novelty: None | ConceptID: Mot1

32
The ultrafast deactivation channel of indigo can be utilized as a model of energy dissipation and related structural reorganization in much complicated H-bonding systems.

Type: Hypothesis | Advantage: None | Novelty: None | ConceptID: Hyp2

33
It is needless to say that H-bonds determine the structure and dynamics of protic liquids like water as well as of biopolymers like protein and DNA.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac22

34
Here we report a study on ultrafast deactivation process of indigo carmine (InC) to elucidate the relation between deactivation dynamics and intra-/intermolecular H-bonding.

Type: Object | Advantage: None | Novelty: None | ConceptID: Obj2

35
InC is a trans-indigo derivative which can form intramolecular double H-bonds as shown in Scheme 1(b).

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac23

36
We have chosen this molecule because of its high solubility in protic solvents compared to indigo.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac23

37
It is also reported that InC forms solute/solvent complexes with methanol and water because of its solvent dependence of the absorption and emission spectrum.37

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac24

38
Solvent dependence of the excited state lifetime was carefully examined by ultrafast time-resolved spectroscopy.

Type: Goal | Advantage: None | Novelty: None | ConceptID: Goa1

39
Steady-state resonance Raman measurement was also carried out to elucidate the solvent dependence of the molecular structure of InC.

Type: Goal | Advantage: None | Novelty: None | ConceptID: Goa2

Experimental section

Cr : forsterite laser system with 25 fs time-resolution

40
The details of the home-made cavity-dumped Kerr lens mode-locked Cr : forsterite laser and the femtosecond pump–probe measurement setup were reported elsewhere.38

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met1

41
The repetition rate of the cavity-dumping was 100 kHz and the output was focused into a LBO crystal to generate the second harmonic centered at 630–640 nm.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp1

42
The second harmonic pulse energy was 4 nJ and the FWHM was calculated to be 27 fs.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp2

43
The beam was divided into pump and probe pulses by a 50% beam splitter.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp3

44
The energy of the pump pulse was 0.6–0.8 nJ and that of the probe pulse was attenuated to 60–80 pJ by a ND filter.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp4

45
The pulses were focused into the sample by a 10 cm focusing lens.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp5

46
The pump–probe signals were detected at a magic angle polarization with a photodiode coupled to a lock-in amplifier.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp6

47
The sample path length was 0.5 mm and the absorbance of the sample was set to 1.0 at ∼610 nm, i.e., the peak of the absorption.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp7

48
In this experiment, signal intensity was measured in terms of transmittance difference, ΔT.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp8

49
The measured ΔT was regarded to be equivalent to absorbance difference, ΔAbs, because the signals were very weak, i.e., the ΔAbs was less than 10−4 for our experimental condition.

Type: Method | Advantage: None | Novelty: New | ConceptID: Met2

Ti : sapphire laser DOPA system with 120 fs time-resolution

50
Transient absorption measurement with 120 fs time-resolution was carried out utilizing a dual OPA femtosecond laser system based on a Ti : sapphire laser.

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met3

51
The details of the system were described elsewhere.39

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac25

52
Briefly, the output of a femtosecond Ti : sapphire laser (Tsunami, Spectra-Physics) pumped by SHG of cw Nd3+ : YVO4 laser (Millennia V, Spectra-Physics) was regeneratively amplified with 1 KHz repetition rate (Spitfire, Spectra-Physics).

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp9

53
The amplified pulse (1 mJ/pulse energy and 85 fs fwhm) was divided into two pulses with same energy.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp10

54
These pulses were guided into two OPA systems (OPA-800, Spectra-Physics) or into a 1 cm fused silica cell filled with H2O/D2O (3 : 7) mixture for super-continuum generation.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp10

55
The wavelengths of the OPA output pulses were converted by SHG, THG, FHG, or by sum-frequency generation with the fundamental 800 nm pulse.

Type: Method | Advantage: None | Novelty: New | ConceptID: Met4

56
The output pulses were obtained with energy of 1–10 mW, fwhm of ∼120 fs, and a wavelength range that covered 300–1200 nm.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp11

57
When two OPA systems were utilized, one of the energy of the output pulse was attenuated to <1/5000 and utilized as a probe pulse.

Type: Method | Advantage: None | Novelty: New | ConceptID: Met4

58
The pulse duration at the sample position was estimated to be 120 fs from the cross correlation trace at the same position.

Type: Method | Advantage: None | Novelty: New | ConceptID: Met4

59
The intensity of the probe, reference, and the pump pulses were monitored simultaneously by photodiodes and sent to a microcomputer for further analysis.

Type: Method | Advantage: None | Novelty: New | ConceptID: Met4

60
When the super continuum was utilized as the probe pulse, both the pump and the probe pulses were passed through an optical chopper rotating with a frequency of 100 Hz to reduce the optical damage of the sample.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp12

61
The signal and the reference pulse were detected with two pairs of a monochrometer and a MCPD.

Type: Method | Advantage: None | Novelty: New | ConceptID: Met4

62
The obtained transient spectra were calibrated for group velocity mismatch.

Type: Method | Advantage: None | Novelty: New | ConceptID: Met4

63
Sample cell with an optical length of 2 mm was utilized and the optical density of the sample was set to ∼1.0.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp13

Picosecond YAG laser system with 15 ps time-resolution

64
Picosecond laser photolysis system with a mode-locked Nd3+:YAG laser was utilized for transient absorption measurement.40

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp14

65
The second harmonic at 532 nm with 15 ps fwhm and 0.5–1.0 mJ of power was utilized for excitation.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp15

66
The excitation pulse was focused into a spot with a diameter of ca. 1.5 mm.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp16

67
A picosecond white continuum generated by focusing the fundamental pulse into a 10 cm quartz cell containing H2O/D2O mixture (1 : 3) was employed as a probe pulse.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp17

68
The white continuum was divided into two beams by a half-mirror and the reflected beam was monitored by a monochrometer and a MCPD.

Type: Method | Advantage: None | Novelty: New | ConceptID: Met5

69
The penetrated beam was utilized as a probe beam and also detected by a monochromator and a MCPD.

Type: Method | Advantage: None | Novelty: New | ConceptID: Met5

70
To cancel the effect of molecular rotational diffusion, the polarization of the white continuum was changed from linear to circular by a λ/4 plate.

Type: Method | Advantage: None | Novelty: New | ConceptID: Met5

Steady state absorption spectrum, fluorescence spectrum, and Raman scattering measurements

71
Steady state absorption and emission spectra were measured by Hitachi U-3500 spectrophotometer and Hitachi 850E fluorescence spectrophotometer, respectively.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp18

72
The optical density at the peak absorption wavelength of the InC solution was 0.25–0.15 for the emission measurement.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp19

73
Resonance Raman scattering was measured by the 514.5 nm line of an Ar ion laser (STABLELITE 2017, Spectra-Physics) and a CCD (1152F, Princeton Instruments).

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp20

74
The sample concentration was ∼1.0 × 10−3 M and a rotating cell with diameter of 11.7 mm was utilized.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp21

75
The laser beam was injected into the sample cell from the bottom and the perpendicular scattering was measured.

Type: Method | Advantage: None | Novelty: New | ConceptID: Met6

76
Indigo carmine (InC, >97.5%) and dimethylsulfoxide (DMSO, >99.7%) were purchased from Kanto chemicals, N,N′-dimethylformamide (DMF, infinity pure grade), methanol (MeOH, infinity pure grade), D2O (>99.5%), and polyvinylalcohol (PVA, MW ≅ 40 000, completely hydrolyzed) were from Wako Pure Chemical Industries, N-methylformamide (MFA, >99%) and formamide (FA, >99%) were from Tokyo Chemical Industries.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp22

77
H2O was filtered by ion exchange resin.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp23

78
The purity of InC was checked by measuring the IR absorption spectrum of InC before and after recrystallization, which bared no difference.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac26

79
The InC/PVA film was produced by mixing 2.0 mg of InC and 1.5 g of PVA in 20 ml of hot water and dropping 2 ml of the solution onto a glass plate.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp24

80
The sample was dried over night under atmosphere and then kept inside a vacuum desiccator for several days.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp25

81
The peak optical density of the film was about 0.7–1.0.

Type: Method | Advantage: None | Novelty: New | ConceptID: Met6

Results and discussions

Steady-state absorption and fluorescence spectra

82
The steady-state absorption and fluorescence spectra of InC in various solvents are shown in Fig. 1.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs1

83
The electronic spectrum of InC shows a modest solvatochromic shift with virtually similar shape.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs1

84
The absorption, λabs, and fluorescence peak wavelengths, λflu, and Stokes shift, νStokes, are listed in Table 1, respectively.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs2

85
The value of νStokes increased in the order of DMF < DMSO < MFA < FA ≅ MeOH < H2O which is in good agreement with the previous report.37

Type: Result | Advantage: None | Novelty: None | ConceptID: Res2

86
It was reported that plot of νStokesvs. the Mataga–Lippert polarity function, f(D,n), exhibits some degree of linearity while that of νStokesvs. ET(30) exhibits more linearity.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac27

87
ET(30) is an empirical solvent polarity parameter that considers both solvent polarity and H-bonding influences on solvatochromic shifts.41,42

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac27

88
These solvent parameters are also listed in Table 1.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs3

89
The chromophore of InC is composed from two pairs of adjacent CO and NH group separated by the central CC bond that form a H-shaped structure (Scheme 1).

Type: Model | Advantage: None | Novelty: None | ConceptID: Mod1

90
The molecular orbital calculations suggest that charge-transfer takes place upon photo-excitation from the NH group to the CO group.32,43,44

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac28

91
However, the dipole contribution to the Stokes shift should be minor because InC possesses a quadropole moment, if the C2h symmetry of the molecule is perfect.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac28

92
The finite Stokes shift suggests the breakdown of the symmetry upon photo-excitation, which may be caused by the twisting around the central CC bond or the out-of-plane deformation.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res3

93
For N,N′-diacetylindigo (X = N–COCH3 in Scheme 1 (a)), X-ray crystallography shows that the molecule is not completely flat even in the ground state.43

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac29

94
Moreover, poor linearity of νStokesvs. f(D,n) suggests that the Stokes shift and the solvatochromism are not the results of a simple dipole solvation but also involves structural reorientation arising from the H-bonding to the solvent molecules as the ET(30) correlation suggests.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res4

95
The electrostatic repulsion between the CO groups is also utilized to explain the blue shift of the absorption spectrum upon trans → cis isomerization, i.e., instability of the cis-configuration compared to the trans-configuration is much larger in the S1 state than in the S0 state which leads to a larger vertical S1 ← S0 transition energy.44

Type: Result | Advantage: None | Novelty: None | ConceptID: Res5

96
Note that indigo derivatives that exhibit efficient trans → cis isomerization have atomic groups (see Scheme 1: X = S, O, Se, N–COCH3,) that posses weaker electron donating ability than that of indigo (X = NH).

Type: Result | Advantage: None | Novelty: None | ConceptID: Res6

97
N,N′-dimethylindigo (X = N–CH3) also exhibits trans → cis isomerization although its quantum yield is low, 0.08, and the lifetime of the cis-form is rather short, a few seconds.22

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs4

98
Moreover, isomerization of thioindigo is known to take place through T1 state,15–21 and avoids the electrostatic repulsion in the S1 state.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac30

99
In the case of oxoindigo with less charge-transfer character, the quantum yield of the trans → cis isomerization is reported to be 0.63 and trans-oxoindigo is non-fluorescent, indicating a direct isomerization from the S1 state.8

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac31

100
Usually cis-form is more polar than the trans-form because of the lower symmetry.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac32

101
This property is utilized to crystallize cis-form of N,N′-diacethylindigo by inducing trans → cis photoisomerization in non-polar solvent.45

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac32

102
For indigo, polar solvation may not be enough to reduce the electrostatic repulsion of the carbonyl groups to stabilize the cis-form.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con3

Transient-state absorption spectra

103
On the contrary to the ground state absorption spectra, the band shapes of the transient absorption shown in Figs. 2–4 depended drastically on solvents.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res7

104
Immediately after the photo-excitation, transient absorption with maximum at ∼465 nm and 635–645 nm appeared in DMF and MeOH solution, which were assigned to the Sn ← S1 absorption.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs5

105
The absorption peak at ∼645 nm in DMF was slightly blue shifted to ∼635 nm in MeOH.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs5

106
These transient absorptions are similar to the Sn ← S1 absorption of thioindigo which peaks at ∼470 nm and ∼600 nm.21,46,47

Type: Result | Advantage: None | Novelty: None | ConceptID: Res7

107
Because no bleaching can be seen around 640 nm where the S1 ← S0 and Sn ← S0 absorption overlap in DMF, extinction coefficient of the Sn ← S1 transition was concluded to be larger than that of the S1 ← S0 transition.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res8

108
The shoulder appearing around 605 nm in the DMF spectrum can be caused by the bleaching of the S1 ← S0 absorption with its maximum at 618 nm.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res8

109
The negative signal appearing in the region of >700 nm is assigned to the stimulated emission.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res8

110
In aqueous solution, the lifetime of the transient absorption was extremely short and the measurement was carried out utilizing the femtosecond Ti : sapphire DOPA system.

Type: Method | Advantage: None | Novelty: New | ConceptID: Met3

111
In Fig. 4, the transient absorption revealed that the absorption at ∼640 nm is weak compared to those in organic solvents.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs6

112
Negative signal can be seen at 610–640 nm which is safely ascribed to the bleaching of the S1 ← S0 absorption.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res9

113
These results indicate that the Sn ← S1 absorption in H2O is broader and blue shifted compared to those in the organic solvents.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res10

114
Note that the transient absorption around 500–550 nm is relatively stronger than that in the organic solvents.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res11

The excited state deactivation dynamics

115
Fig. 5 shows the kinetics of the ultrafast part (<1.5 ps) of the transient absorption signal pumped and probed at 635 nm with time-resolution of 30 fs.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs7

116
In addition to the solution phase, the temporal profile in PVA polymer film was also measured.

Type: Method | Advantage: None | Novelty: New | ConceptID: Met6

117
In the early stage after the excitation, the signals are strongly modulated by the coherent intramolecular vibrations caused by the resonant impulsive stimulated Raman scattering process.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac33

118
The vibrational dephasing times were 0.5–0.8 ps, which did not strongly depend on solvent, and the vibrational frequencies and their assignments will be discussed later.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res12

119
Note that the phase of the oscillation is similar in all solvents, including H2O where the signal possesses negative intensity.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs8

120
The negative signal observed in H2O corresponds to the bleaching of the ground state absorption as can be seen in Fig. 4.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs8

121
Fig. 6 shows the picosecond decay of the transient absorption pumped and probed at 635 nm in various solvents except H2O. The time constants listed in Table 2 were obtained by multi-exponential fitting of the decays shown in Figs. 5 and 6.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs9

122
The intense coherent oscillation and overlap of the ground state bleach and transient absorption prevent the rational analysis of the dynamics around the time origin.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac34

123
The fitting range was set to ≥53 fs, therefore, any ultrafast dynamics that occurred within the pulse duration were neglected.

Type: Method | Advantage: None | Novelty: New | ConceptID: Met6

124
In the organic solvents, three components including two with negative amplitudes were necessary to fit the signals satisfactorily.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs10

125
It can be seen in Fig. 5 that the excited state absorption rapidly increased in the organic solvents.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs11

126
The time constants for the rise, τ1 and τ2, were not very sensitive to the solvent property, although in PVA, the rise was significantly slow compared to other fluid solvents and can be observed in Fig. 6.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs12

127
Because the probe wavelength of 635 nm is near the peak of the Sn ← S1 absorption spectrum, the rise component corresponds to the narrowing of the transient absorption caused by a conformational relaxation in the excited state.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac35

128
It can be noticed in Fig. 6 that the decay profile in DMSO is identical to that in DMF and those in MeOH and FA are parallel.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs6

129
The longest time constant, τ3, in organic solvents and PVA was assigned to the lifetime of the S1 state.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac35

130
The fluorescence lifetime of InC in DMF was reported to be 110 ps48 which is in good agreement with our present value of 92 ± 0.4 ps.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res13

131
The ultrafast time profile in aqueous solution was rather complicated and needed four components to fit the data satisfactorily.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs13

132
To elucidate the accurate S1 lifetime in aqueous solution, the decay of the transient absorption and the recovery of the ground state bleach was probed at 470 nm and 610 nm, respectively, with excitation at 585 nm (Fig. 7).

Type: Goal | Advantage: None | Novelty: None | ConceptID: Goa1

133
The time constants of the decay and the rise measured at 470 nm and 610 nm were 3.9 ps and 4.0 ps, respectively; thus, the average of these two were regarded as the S1 lifetime of InC in aqueous solution.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs14

134
The dynamics taking place faster than 4.0 ps in aqueous solution is also related to the relaxation in the excited state.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac35

135
Interestingly, from Tables 1 and 2, it can be seen that the lifetime of InC decreases in solvents with larger Stokes shift.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res14

136
These observations indicate that ultrafast structural reorganization and solvent reorientation have a strong influence on the deactivation process.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res14

137
Thus, we have carried out experiments on deuterium isotope effect and viscosity effect.

Type: Goal | Advantage: None | Novelty: None | ConceptID: Goa1

138
Provided that the H-bonding is involved in the deactivation process, deuteration could cause a dynamical influence.

Type: Hypothesis | Advantage: None | Novelty: None | ConceptID: Hyp3

139
The time dependence of the ground state bleach signal pumped and probed at 635 nm in H2O was compared with that in D2O in Fig. 8.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac36

140
The PP signals showed ultrafast decay into the negative amplitude regime and it reached the minimum value at ∼250 fs.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs15

141
After the decay, the signals recovered multi-exponentially, and the PP signal overshot to positive value at ∼4 ps.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs15

142
To fit the time-dependence of the signal, four components, including two with negative amplitude, were necessary (Table 2).

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs16

143
The signal exhibited an ultrafast decay of ∼200 fs which was not observed in other organic solvents.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs15

144
The solvation dynamics of aqueous solution is known to be extremely fast, i.e., inertial response with time constant of ∼30 fs.49

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac37

145
It is clear from Fig. 8 that the negative bleach signal in H2O recovers to zero faster than that in D2O. This can be the evidence of the H-bond influencing the ultrafast structural reorganization and the deactivation process.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res15

146
The amino-hydrogen of InC can be easily exchanged to deuterium by simply dissolving in D2O. Therefore, this could be either the effect of intra- or intermolecular H-bond.

Type: Hypothesis | Advantage: None | Novelty: None | ConceptID: Hyp4

147
It should be noted that the viscosities of H2O and D2O are slightly different, 1.00 cP and 1.25 cP at 20 °C and this difference also affects the deactivation process in some cases.50

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac38

148
The frequencies and dephasing times of the coherent oscillation observed in the signal were similar in both solvents.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs17

149
The longest lifetime in highly viscous PVA (260 ± 10 ps) and slower dynamics in D2O compared to that in H2O indicate that the S1 lifetime may depend on the fluidity of the solvent.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res16

150
To examine this presumption, the S1 state lifetime was plotted against viscosity in Fig. 9(a), which exhibited no correlation.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs18

151
The lifetime in FA with viscosity of 3.3 cP (22 ± 0.2 ps) was the same as that in MeOH with viscosity of only 0.54 cP.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs19

152
On the other hand, the lifetime in DMF with viscosity of 0.8 cP was as long as 92 ± 0.4 ps.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs20

153
The lifetime is shorter in protic solvents than those in aprotic solvents.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res17

154
Thus, the lifetime was plotted against ET(30) in Fig. 9(b) which showed a good correlation.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs21

155
See Table 1 for the values of the solvent parameters.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs21

156
ET(30) is an empirical measure of solvent polarity which also depends on H-bonding ability of the solvent.41,42

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac39

157
It was reported that the value of the Stokes shift also had a good linear relation with ET(30).37

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac40

158
When lifetime is compared with the Mataga–Lippert polarity function, f(D,n), the correlation was not as good as ET(30).

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs22

159
The f(D,n) of MeOH (0.324) is close to that of H2O (0.326), although the lifetimes were rather different, ∼22 ps and ∼4 ps, respectively.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs22

160
The lifetime in MeOH was the same as that in FA (∼22 ps) with rather different value of f(D,n) which is 0.287.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs22

161
Correlation can be also found for the solvent H-bond donor acidity parameter, α, which describes the ability of the solvent to donate a proton in a solvent-to-solute H-bond.51

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs23

162
The value of α is as large as 1.17 in water, whereas, those of DMF and DMSO are 0.00.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs24

163
On the contrary, there was an opposite correlation with the average solvation time, 〈τ〉, obtained from the dynamic Stokes shift measurement of coumarin .15352

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac41

164
The values of 〈τ〉 for MeOH and FA were both 5.0 ps, while those in DMSO and DMF were shorter, i.e. 2.0 ps.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs25

165
These results demonstrate that H-bond ability of the solvent plays a crucial role rather than the polarity.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con4

Intramolecular vibrations of InC

166
Because the ultrafast part of the transient absorption signal of InC was modulated strongly by intramolecular vibrations, the Fourier transform was performed on the signals shown in Fig. 5 and the real parts of the Fourier transformed spectra are shown in Fig. 10.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs26

167
The strongest mode was observed at ∼260 cm−1 in protic solvents and interestingly this mode became a doublet, ∼254 cm−1 and ∼265 cm−1, in aprotic solvents.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs26

168
In our femtosecond measurement, coherent oscillation can either result from the electronic ground or excited state, because both absorptions are overlapped at the prove wavelength.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res18

169
To confirm their origin, resonance Raman scattering measurement was carried out in various solvents with an excitation wavelength of 514.5 nm and the results are shown in Figs. 11 and 12.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs27

170
In the low-frequency region (Fig. 11), a mode appeared at ∼260 cm−1 in protic solvents which became a doublet, 255 cm−1 and 270 cm−1, in aprotic solvents, in good agreement with the Fourier transformed spectra.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs27

171
In the near-IR Fourier transform Raman spectrum of solid state indigo, three modes are observed in this frequency range, 275 cm−1 (δOC–CC), 266 cm−1 (δN–CC), and 252 cm−1 (γCC), which are assigned to in-plane and out-of-plane bending modes.53

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac42

172
If the intramolecular H-bonds are replaced by the intermolecular ones, slight twisting or out-of-plane deformation of the molecule can occur.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac42

173
Therefore, the molecular structure will be thermally distributed along the twisting/deformation coordinate and result as a broadening of the Raman bands.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac42

174
Other mode that showed strong solvent dependence was the one at 613–617 cm−1.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac43

175
This mode was sharp and comparatively strong in aprotic solvents whereas it was broad and weak in protic solvents.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac43

176
For indigo, out-of-plane N–H motion was located at 635 cm−1 (Raman) and 633 cm−1 (IR).53

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac44

177
If the mode at 613–617 cm−1 of InC can be assigned to the same mode, this also suggests that molecular planarity has declined in protic solvents.

Type: Hypothesis | Advantage: None | Novelty: None | ConceptID: Hyp5

178
However, in high-frequency region, 1200–1800 cm−1 (Fig. 12), we were not able to observe any notable solvent dependence.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs28

179
The most important modes for indigo in this region are the CC and CO stretching modes, i.e., 1582 cm−1 and 1701 cm−1, respectively.53

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac44

180
For InC, strong mode can be observed at ∼1582 cm−1 and ∼1705 cm−1.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac44

181
The frequency of these modes may be slightly higher in protic solvents although it is hardly noticeable.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs29

182
It seems that exchange of intra- to intermolecular H-bond does not significantly change the conjugated double bonding structure of InC. Usually it is known that intramolecular H-bond that forms a five- or six-membered ring is rather strong, because of the good overlap of the molecular orbitals, and the frequency of the hydrogen accepting CO stretching mode is reduced by 10–100 cm−1.29

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac45

183
However, symmetric ν6 CO stretching mode of indigo observed in Raman spectrum is not significantly lower than other non-H-bonding indigo derivatives.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs30

184
In solid state near-IR Fourier transformed Raman spectra, the frequency was 1701, 1674, and 1660 cm−1 for indigo, thioindigo, and selenoindigo, respectively (the frequency of indigo was the highest).

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac46

185
In solution, the frequency was reported to be 1705, 1688, and 1714 cm−1 for indigo, N,N′-dimethylindigo and N,N′-diacetylindigo, respectively.54

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac47

186
For the IR active out-of-phase ν61 CO stretching mode, the frequency of indigo was notably lower than that of other non-H-bonding indigo derivatives, i.e., 1627, 1638, 1690, 1642, 1656, 1692, and 1724 cm−1 for indigo, N,N′-dimethylindigo, N,N′-diacetylindigo, selenoindigo, thioindigo, oxoindigo, and dehydroindigo, respectively.11,29,54

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac48

187
Usually this is treated as the evidence of double intramolecular H-bonds, however, it should be noted that, in crystal, indigo is considered to form intermolecular H-bonds with each other.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac48

188
The shifting and the broadening of the absorption spectrum in solid phase are regarded as the evidence of the self-association of indigo.13

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac49

189
The NH stretching mode appears as a rather broad band at 3268 and 3270 cm−1 in IR and in Raman spectrum, respectively, and the presence or absence of the H-bond is difficult to judge.53

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac50

190
To investigate the nature of the H-bonding, temperature dependent measurement of the Raman spectrum of InC in DMF was carried out (Fig. 13).

Type: Goal | Advantage: None | Novelty: None | ConceptID: Goa3

191
Usually the bonding energy of H-bond is in the order of 3–5 kcal mol−1 which can be partially broken by increasing temperature.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac51

192
However, noticeable temperature dependence was not observed for the CC and CO stretching modes of InC in the range of 278–363 K (Fig. 13(a)).

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs31

193
The relative intensity of the CO stretching mode of InC at 1703 cm−1 was insensitive to temperature and no new bands appeared.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs31

194
There are solvent bands at 1630 cm−1 and 1730 cm−1 which can disturb the observation of the temperature dependence.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs31

195
Note that the solvent band at 1630 cm−1 is shifting to higher frequency with increasing temperature.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs32

196
We also carried out similar temperature dependent experiment in aqueous solution and no intensity change or appearance of a new band was observed.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs33

197
If H-bonding and non-H-bonding molecules are in chemical equilibrium, CO stretching is likely to appear as a doublet.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res19

198
However, the observed CO stretching modes were all singlet indicating that chemical equilibrium is strongly one-sided in the studied temperature range.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res19

199
There may be slight temperature dependence for the solvent sensitive low-frequency bands, i.e., the band at 255 cm−1 slightly down shifted to 252 cm−1 and the intensity of the band at 270 cm−1 seemed to be decreasing at high temperatures, although the changes were close to the limit of our experimental accuracy.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs34

200
From the solvent dependent low-frequency Raman modes, it can be concluded that molecular planarity depends on solvent property, although from the constancy of the CC and CO stretching modes, the twisting or the out-of-plane deformation is not as significant as to disturb the conjugated CC structure of InC. The observation of only a single CO stretching mode in all the studied solvents and temperatures rises a question to the existence of intra- or intermolecular H-bonding in the ground state.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con5

201
Even if there were an interaction between CO and NH groups or with the solvent, it should be a feeble one in the ground state.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con6

Temperature dependence of the fluorescence intensity

202
Temperature dependence of the fluorescence intensity was measured and shown in Fig. 14.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs35

203
The Arrhenius plot reveals the activation energy for the radiationless deactivation to be 8.8 kJ mol−1 and 3.5 kJ mol−1 in DMSO and H2O, respectively.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs36

204
The deactivation is a barrier-crossing process and the activation energy in protic solvents was reduced to less than half the value of that in aprotic solvents.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res20

205
These activation energies are similar to that expected for H-bond breaking.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res20

206
We assume that slight twisting or out-of-plane deformation lead to the deactivation of the excited InC. In the case of aprotic solvents, intramolecular H-bonds exist in the excited state, which interrupt the twisting/deformation process, whereas such interruption is weakened in the protic solvents and the activation barrier is reduced.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con7

Conclusions and the deactivation scheme proposed

207
From the steady state absorption and fluorescence spectroscopy, poor linearity of νStokesvs. f(D,n) suggests that the Stokes shift and the solvatochromism are not the results of a simple dipole solvation but also involves structural reorientation arising from the H-bonding to the solvent molecules as the ET(30) correlation suggests.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con8

208
From the steady state resonance Raman spectrum, it was observed that the low-frequency bands which can be assigned to the in-plane and out-of-plane bending modes, ∼255 cm−1 and ∼270 cm−1, respectively, became broad and indistinguishable at ∼260 cm−1 in protic solvents.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con9

209
This indicates the reduction of molecular planarity in protic solvents.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con9

210
However, the CO stretching at ∼1705 cm−1 did not show any solvent or temperature dependence.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs29

211
On the contrary to the ground state absorption spectrum, the transient Sn ← S1 absorption spectrum in the aqueous solution was quite different from those in the organic solvents.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res7

212
The ultrafast dynamics in D2O was noticeably slower than that in H2O which suggests the involvement of H-bonding in the deactivation process.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res15

213
Elsaesser et al. concluded from their time-resolved IR absorption experiment that the NH stretching in the S1 state was shifted to lower frequency by 40 cm−1 compared to the ground state.23

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac12

214
This also supports the formation of the H-bonds in the excited state.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res15

215
The deactivation process was strongly solvent dependent, and it was faster in protic solvents, especially in H2O, and the lifetime had a linear correlation with the empirical solvent polarity parameter, ET(30).

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con10

216
The deactivation was a barrier crossing process and the activation energy was lower in protic solvents.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con7

217
From these observations, we propose the following deactivation scheme.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac52

218
The H-bond interaction is weak in the ground state whereas it is strong in the excited state.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con11

219
The central CC bond will weaken upon photo-excitation by the increased anti-bonding nature of the molecular orbital in the excited state which leads to the loss of molecular planarity.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con2

220
Loss of the planarity opens a channel to the ultrafast radiationless deactivation.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con2

221
However, in aprotic solvents, intramolecular H-bonds retain the molecular planarity and disturb the deactivation process.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con7

222
In protic solvents, intermolecular H-bonds take place of the intramolecular ones and the activation barrier for the radiationless deactivation is reduced.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con12

223
The rise components observed in ultrafast kinetics of the transient absorption (Fig. 5) could be the formation of the intra-/intermolecular H-bonds and the related conformational reorganization.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con13

224
Dissipation of vibrational energy through the solute-solvent H-bond may also play a role in the deactivation process.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con14

225
It is reported for betaine 30, the probe molecule utilized to determine the ET(30) values of solvents, that the vibrational energy is transferred to solvent through the H-bond.55

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac53

226
The cis-form of indigo is energetically unstable than the trans-form and energy barrier does not exist between these two isomers.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res5

227
It is known that thermal trans → cis isomerization of N,N′-dimethylindigo can be catalyzed by acid22 indicating a low energy barrier in protic solvents.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac54

228
Therefore, the deactivation process only leads back to the trans-form.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con15