1
Specific deuterium isotope effects on the intramolecular charge-transfer dynamics of 4-(dimethylamino)benzonitrile

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

2
The photoinduced charge-transfer (CT) dynamics of 4-(dimethylamino)benzonitrile (DMABN) are studied for methyl-deuterated isotopomers DMABN-d6 (with two CD3 groups) and DMABN-h3d3 (with one CH3 and one CD3 group) by steady-state and time-resolved fluorescence spectroscopy.

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

3
The asymmetrically deuterated species DMABN-h3d3 is found to exhibit less CT fluorescence in sufficiently polar solvents than the symmetric isotopomers of DMABN-h6 and DMABN-d6.

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

4
The results of fluorescence decay measurements are also reported, which suggest that the rate of backward electron transfer is increased upon asymmetric substitution of the dimethylamino group.

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

Introduction

5
The photoinduced charge-transfer (CT) reaction of DMABN in polar solvents has been the subject of extensive experimental and theoretical investigations.1,2

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

6
This process is usually considered as a twisting motion of the dimethylamino (DMA) group with respect to the benzonitrile group to produce an emissive transient species called twisted intramolecular charge-transfer (TICT) state.3

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

7
After finding similar behavior in many other molecules containing electron donor and acceptor groups, the rotamerism accompanying charge separation is most widely accepted.

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

8
A wide variety of model compounds have been studied to explain the motion leading to the CT state.

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

9
The so-called pretwisted compounds, such as sterically hindered ortho-methyl-derivatives of DMABN, are known to emit preferentially or only CT fluorescence in a medium polarity solvent.3,4

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

10
The CT fluorescence is also observed in the static vapor5 and under supersonic jet conditions.6

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

11
Since the DMA group in these compounds is already twisted in the ground-state structure, direct excitation of the CT state becomes possible.

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

12
Other derivatives of DMABN, in which the amino group is rigid with respect the twist around the Namino–Cring bond, reveal only locally excited (LE) fluorescence.3,4

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

13
These results are considered as evidence of the TICT scenario.

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

14
On the other hand, neither the meta nor the ortho isomer of DMABN shows dual fluorescence even in strongly polar solvents.7

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

15
The specific ring-substitution behavior has been ascribed to the large energy gap between the S1 and S2 states as compared to that of para-DMABN, leading to suggest a planar CT state.7

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

16
Nonetheless, the nature of the structural change is not well characterized yet and under controversial discussion.

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

17
Recently, we have measured S1–S0 spectra of methyl-deuterated isotopomers of DMABN (DMABN-d6 and DMABN-h3d3) in supersonic jets using resonant two-photon ionization (R2PI) spectroscopy.8

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

18
The spectra of the deuterated isotopomers reveal low-frequency progression due to the torsional motion of the DMA group in the S1 state, essentially analogous to the protonated species DMABN-h6.

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

19
Analysis shows that in all isotopomers the DMA group is twisted by ≈26° with respect to the ring plane under the isolated molecular condition.

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

20
However, the DMABN-h3d3 spectrum exhibits additional bands with a less regular pattern, suggesting that the asymmetric deuteration of the DMA group induces extensive mixing of its torsional motion and other low-frequency motions in S1.

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

21
Possible candidates for these motions are predicted to be torsion of the methyl groups and pyramidarization at the amino nitrogen.

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

22
It is therefore interesting to compare the CT dynamics of the symmetric and asymmetric rotors in polar solvents.

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

23
Several examples of deuterium isotope effects on electron-transfer reactions have been reported, especially for those in the Marcus inverted region.9–11

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

24
It has been shown that in intermolecular complexes between methyl-substituted benzene donors and cyanoanthracene acceptors, the backward electron transfer rate decreases substantially when the methyl hydrogens of the donor are deuterated.9

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

25
However, no or insignificant isotope effect is observed upon ring deuteration.

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

26
An asymmetric substitution effect has been suggested for intramolecular electron transfer reactions in bianthryls.12,13

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

27
In this case, the so-called symmetry breaking is believed to be an important step for CT.

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

28
Likewise, such symmetry reduction can be achieved upon complexation of bianthryl with other solvent molecules.

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

29
It has been shown that the 1:1 complex with acetone formed in a supersonic jet reveals a red-shifted emission assignable to the CT fluorescence.14

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

30
The complex is likely to have the solvent molecule sticking to one of the anthracene moieties, undergoing symmetry breaking and charge separation.

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

31
On the other hand, no CT formation occurs for the 1:2 complex, which is explained by invoking a symmetric structure.

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

32
In this paper we report the results of the systematic study of the effect of deuterium substitution on the dual fluorescence behavior of DMABN in solvents of different polarity.

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

33
The asymmetric isotopomer DMABN-h3d3 is found to reveal less CT fluorescence in moderately polar solvents than the symmetric ones (DMABN-h6 and DMABN-d6).

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

34
In less polar solvents (e.g., 1,4-dioxane), on the other hand, the fully protonated compound gives rise to red-shifted fluorescence with higher efficiency than the deuterated isotopomers.

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

35
We have also employed a time-resolved spectroscopic technique to investigate the origin of the specific deuteration effects.

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

36
The results suggest that the backward electron transfer is facilitated upon asymmetric deuteration of the DMA group.

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

37
These observations are interpreted within the framework of the TICT model.

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

Experimental

38
Commercial DMABN (DMABN-h6) was obtained from Aldrich and purified by vacuum sublimation before use.

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

39
The methods of preparing the deuterium-substituted DMABN samples, (CD3)2N-C6H4-CN (DMABN-d6), and (CH3)(CD3)N-C6H4-CN (DMABN-h3d3) have been described elsewhere.15

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

40
The isotope content (>99%) was confirmed by NMR and mass spectroscopy analysis.

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

41
The DMABN-h3d3 sample showed traces (≈0.17%) of monomethylated impurities [e.g., (CD3)HN-C6H4-CN].

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

42
A series of solvents of different polarities [cyclohexane, tetrahydrofuran (THF), 1,4-dioxane, n-butyronitrile, acetonitrile, 1-butanol, 1-propanol, 2-propanol, ethanol, methanol, and water] were purchased from Kanto Chemical Co. These solvents were of spectroscopic grade and used as received.

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

43
The sample concentration was 1 × 10−4 M.

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

44
A Hitachi F-2000 spectrofluorometer was used for the steady-state fluorescence measurements.

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

45
The time-resolved fluorescence measurements were carried out by exciting the samples with the frequency-tripled output of a femtosecond laser (Spectra Physics Tsunami/Spitfire) at 266 nm and by recording the fluorescence with a streak camera (Hamamatsu Photonics).16

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

46
The system response function was found to be ≈30 ps.

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

47
Since the samples exhibit dual fluorescence, each fluorescence emission was collected separately through a monochromator.

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

48
The fluorescence decays were fitted with exponential functions by taking account of the response function of the streak camera.

Type: Model | Advantage: None | Novelty: None | ConceptID: Mod2

Results

49
The steady-state fluorescence spectra of three isotopomers of DMABN in 1,4-dioxane, ethanol, and acetonitrile at 25 °C are shown in Fig. 1.

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

50
In each solvent, the absorption and fluorescence excitation spectra are essentially identical for the three isotopomers.

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

51
The fluorescence spectra of the fully protonated species (Fig. 1) are in good agreement with previous measurements.4,17

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

52
In the case of weakly polar dioxane (dielectric constant ε = 2.2 at 25 °C), the relative LE fluorescence intensity is strongest for DMABN-d6.

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

53
With increasing the solvent polarity (ethanol and acetonitrile), the ratio for the asymmetric isotopomer DMABN-h3d3 becomes larger than those for -h6 and -d6.

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

54
The symmetric species give nearly the same intensity ratios.

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

55
This behavior cannot be ascribed to the monomethylated impurity described in the Experimental section, which is known to exhibit only LE fluorescence.17

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

56
The fluorescence spectra of the three isotopomers in cyclohexane (not shown) appear to be identical.

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

57
In all polar solvents studied, the relative intensity of the LE and CT fluorescence is found to be different for the three isotopomers.

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

58
Ratios of the CT and LE fluorescence yields (ΦCT/ΦLE) can be obtained by separating the respective emission bands.

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

59
The ΦCT/ΦLE ratios plotted against the dielectric constant of the solvent are shown in Fig. 2.

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

60
It can be seen that in 1-butanol (ε = 17.1) or more polar solvents the ratio for the -h3d3 isotopomer is considerably lower than for the symmetric isotopomers.

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

61
Furthermore, the ratios in protic solvents (propanols, 1-butanol, methanol, and ethanol) appear to increase monotonically with the polarity, but differ noticeably from those obtained with aprotic solvents of similar polarity [e.g., 1-propanol (ε = 20.1) and n-butyronitrile (ε = 20.4)].

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

62
Fluorescence yield ratios have also been obtained for the three isotopomers with an excitation wavelength of 306 nm (not shown).

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

63
The symmetry dependent behavior is essentially the same for the two different excitation wavelengths.

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

64
Time profiles of the LE and CT fluorescence emissions have been measured separately for three DMABN isotopomers in each solvent.

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

65
The selected spectral range chosen to follow the LE fluorescence kinetics is centered at 350 nm in dioxane and 355 nm in more polar solvents, whereas the CT fluorescence was measured with the observation window centered at 450 nm for dioxane and 490 nm for the other solvents.

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

66
Each time-resolved fluorescence intensity appears to consist of two components, with a picosecond decay followed by a nanosecond decay for the LE emission and a picosecond rise followed by a nanosecond decay for the CT emission.

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

67
Thus, the time-resolve fluorescence intensity can be described by the following equationswhere I(t)LE is the LE fluorescence intensity with the fast and slow decay lifetimes τfast and τslow, and the amplitudes Afast and Aslow.

Type: Model | Advantage: None | Novelty: None | ConceptID: Mod3

68
I(t)CT is the CT fluorescence intensity with the risetime τrise and decay lifetime τdecay, and the amplitudes Arise and Adecay.

Type: Model | Advantage: None | Novelty: None | ConceptID: Mod3

69
The decay profiles of the LE fluorescence obtained for dioxane, ethanol and acetonitrile are compared in Fig. 3.

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

70
In each solvent, the profiles of the CT fluorescence are nearly identical for the three isotopomers.

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

71
The fluorescence decay lifetimes and preexponential factors calculated by fitting the data to eqns. (1) and (2) are summarized in Table 1.

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

72
As shown in Fig. 3, a small isotope effect is observed for the decay curves in dioxane.

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

73
The fast decay lifetime of the LE fluorescence τfast increases slightly upon deuterium substitution.

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

74
The value of 25 ps for the -h6 species is in good agreement with that reported by Schuddeboom et al.17

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

75
Pronounced isotope effects occur in the case of ethanol and acetonitrile.

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

76
The asymmetric deuteration leads to an increase in the magnitude of the slow decay component of the LE fluorescence.

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

77
The ratio of the amplitudes of the fast and slow decay components (Afast/Aslow) for -h3d3 is calculated to be 6.3 for ethanol and 22.2 for acetonitrile.

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

78
The corresponding values for the symmetric isotopomers are much larger (28.6 and 34.5 for -h6 and 76.9 and 83.3 for -d6).

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

79
As listed in Table 1, the lifetimes of the fast and slow decay components of the LE fluorescence (τfast and τslow) are not significantly influenced by the isotope substitution.

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

80
The result obtained for -h6 is in qualitative agreement with that reported by Changenet et al.18

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

81
In all cases, the amplitude ratio of the rise and decay components of the CT fluorescence Arise/Adecay is nearly equal to –1, which implies that the CT state is formed through the LE state decay.

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

82
Isotope dependent behavior obtained with methanol is very similar to the case of ethanol, except that the fast components τfast and τrise are much shorter than in ethanol.

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

83
For all isotopomers in the protic solvents (i.e., ethanol and methanol), the CT fluorescence risetime τrise is found to vary with the emission wavelength: it increases as the observation window is shifted to a longer wavelength region.

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

84
This implies that the CT fluorescence intensity distribution redshifts with time, which has been recognized previously18,19 and explained as a result of the longer solvent reorganization time in these solvents.

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

85
In this case, the risetime of the CT fluorescence τrise does not match the fast decay lifetime of the LE fluorescence τfast.

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

86
On the other hand, rapid equilibrium between the LE and CT states can be reached upon excitation in aprotic solvents, and thus τrise and τdecay of the CT fluorescence are nearly equal to the decay lifetimes τfast and τslow of the LE fluorescence, respectively.

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

Discussion

87
We have observed that in solvents of moderate and high polarity, the intensity ratio of the CT to LE fluorescence (ΦCT/ΦLE) of DMABN decreases upon deuteration of one of the methyl groups.

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

88
This symmetry effect is contrasted with the case of the weakly polar solvent dioxane where the fully methyl-deuterated isotopomer gives the lowest ΦCT/ΦLE fluorescence ratio.

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

89
Moreover, the time-resolved fluorescence measurements reveal that the asymmetric substitution leads to a substantial decrease in the ratio of the fast to slow decay component of the LE fluorescence (Afast/Aslow) in polar solvents.

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

90
Here, we employ the kinetic scheme (Scheme 1) proposed by Grabowski et al3. to rationalize these observations.

Type: Model | Advantage: None | Novelty: None | ConceptID: Mod4

91
In addition, the TICT model is invoked in which the CT stabilization occurs as a consequence of the torsional motion of the DMA group.

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

92
For DMABN in polar aprotic solvents, the excited-state equilibrium can be fully established within the lifetime, i.e., k1, k2 ≫ kLE, kCT, where k1 and k2 are the forward and backward reaction rate constants, respectively, and kLE and kCT are the decay rate constants of the respective excited states.

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

93
In this high-temperature regime, the ratio of the LE fluorescence components Afast/Aslow can be associated with the ratio of the reaction rate constants k1/k2.

Type: Model | Advantage: None | Novelty: None | ConceptID: Mod3

94
The reciprocal of the short decay lifetime τfast corresponds to the sum of the forward and backward reaction rate constants k1 + k2.

Type: Model | Advantage: None | Novelty: None | ConceptID: Mod5

95
For DMABN in protic solvents, the risetime of the CT fluorescence is determined by the solvent reorganization time as described above.

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

96
In this case, the reaction kinetics can be deduced from the LE state decays.18

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

97
The rate constants k1 and k2 for the two aprotic solvents (dioxane and acetonitrile) derived based on the high-temperature approximation are listed in Table 2.

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

98
Comparison of the kinetic data in Table 2 suggests that the backward reaction rate constant k2 increases upon asymmetric deuteration, whereas the change in k1 is minor.

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

99
It is also important to note that the emission maxima of the LE and CT fluorescence are identical for all isotopomers (Fig. 1), supporting that the energies of the LE and CT states are not shifted by the symmetry reduction.

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

100
Therefore, we conclude that the activation barrier for CT changes upon deuteration of one of the methyl groups.

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

101
For DMABN-h6 in polar solvents, the activation energy of the backward reaction E2 is much larger than that of the forward reaction E1.

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

102
Thus, the ratios of ΦCT/ΦLE and Afast/Aslow increase by lowering the temperature in the so-called high-temperature region.20

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

103
The activation energies E1 and E2 have been obtained separately from Arrhenius-type plots of the decay rate constants.21

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

104
In both protic and aprotic solvents, the observed Arrhenius activation energy E1 appears to be similar to the activation energy for the solvent mobility Eη.

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

105
This implies the absence of intrinsic potential barrier for the forward reaction.21

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

106
For example, the activation energy E1 in butyronitrile is obtained to be 10.5 kJ mol−1, which is comparable to Eη (9.2 kJ mol−1).

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

107
On the other hand, the activation energy of the backward reaction E2 (15.9 kJ mol−1 in butyronitrile) is substantially larger than Eη, and a significant intrinsic barrier exists for this process.

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

108
On the basis of the barrierless reaction model, we propose that the pronounced asymmetric substitution effect on the CT dynamics in sufficiently polar solvents is due to vibrational coupling between the primary reactive mode, namely the twisting motion around the Namino–Cring bond, and other low-frequency motions (e.g., pyramidalization at the DMA group and torsion of the methyl groups).

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

109
Among these nonreactive motions, pyramidalization is believed to be important in determining the CT dynamics of DMABN.4,22

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

110
With very low pyramidalization, the activation energy for the CT reaction is lowered, leading to a barrierless reaction coordinate, as discussed above.

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

111
Therefore, asymmetric substitution of the DMA group exerts a minor influence on the forward reaction process.

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

112
In contrast, vibrational coupling with nonreactive modes will increase the density of states for the back CT reaction, thus lowering its barrier height.

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

113
The above interpretation is consistent with the absence of noticeable symmetry effect in dioxane.

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

114
As shown in Fig. 1, the protonated species DMABN-h6 reveals the largest ΦCT/ΦLE value among all isotopomers.

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

115
In the weakly polar solvent, the forward reaction may also have an intrinsic barrier.

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

116
In this case, deuterium isotope effects prevail.

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

117
The slower CT reaction rate for the deuterated species can be explained in terms of the torsional frequency of the DMA group, which decreases by ≈10% in going from -h6 to -d6.8

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

118
While DMABN is known to be pyramidal in the ground state,23 increased conjugation upon excitation would be expected to lead to a more planar (with respect to inversion) conformation in S1.

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

119
It has been suggested that its S1 inversion potential is very flat-bottomed.8

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

120
On the other hand, the torsional geometry is planar in S0,23 while twisted in S1 by about 26° with a small barrier to planarity.8

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

121
In this case, vibrational coupling between the torsion and inversion of the DMA group is expected to occur extensively upon excitation.

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

122
Evidence in support of such coupling can be obtained from vibrational analysis of the S1 ← S0 spectra of three isotopomers.8

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

123
Each spectrum, obtained in a supersonic jet, consists of a prominent low-frequency progression due to the DMA torsion.

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

124
In addition to this, bands assignable to the inversion and its combination bands with the DMA torsion are apparent in the spectrum of the asymmetric isotopomer.

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

125
The -h3d3 spectrum also reveals bands involving torsion of each methyl group (CH3 and CD3).

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

126
The increased coupling of the reactive DMA torsion with these nonreactive modes upon asymmetric substitution is expected to reduce the activation energy.

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

127
It is also possible to ascribe the observed symmetry effect to an increase of the preexponential (frequency) factor due to such mode coupling.

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

128
In conclusion, it is demonstrated that the fluorescence intensity ratio of the CT and LE emissions of DMABN decreases substantially upon deuteration of one of the methyl groups.

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

129
This asymmetric substitution effect can be observed only in polar solvents of medium and high polarity.

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

130
The time-resolved fluorescence measurement reveals that the reverse electron transfer is facilitated in the asymmetric isotopomer, whereas no significant change occurs for the forward reaction.

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

131
This behavior is explained by mode coupling of the torsional motion of the DMA group with other low-frequency motions, which occurs strongly upon asymmetry reduction.

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

132
However, more accurate decay data would be essential to determine unequivocally the origin of the asymmetric substitution effect.

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