Filename: b316620a

Back | Show only these items: | Conformational control of electron delocalisation in geometrically-constrained, binuclear ruthenium(ii) bis(2,2′:6′,2″-terpyridine) complexes

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Conformational control of electron delocalisation in geometrically-constrained, binuclear ruthenium(ii) bis(2,2′:6′,2″-terpyridine) complexes

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

On the basis of temperature-dependent emission studies made with the target binuclear compounds, it is concluded that the extent of electron delocalisation at the triplet level depends on the torsion angle imposed by the bridge.

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

It is well established that the triplet lifetime of ruthenium(ii) bis(2,2′:6′,2″-terpyridine) at ambient temperatures can be prolonged by attaching an alkynylene substituent at the 4′-position.¹

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

This strategy works particularly well for the corresponding binuclear complexes, where the triplet lifetimes at 20 °C exceed that of the parent complex by a factor of at least .1000²

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

Prolongation of the triplet lifetime is due to two complementary effects; namely, decoupling of the lowest-energy triplet state from upper-lying metal-centred states and extended delocalisation of the promoted electron over the substituted ligand.

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

Electron delocalisation can occur because the lowest-energy triplet is of metal-to-ligand, charge-transfer (MLCT) character and is formed by selective charge injection into the alkyne-substituted ligand.

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

We now show that the extent of electron delocalisation is subject to conformational control, as imposed by constraining the geometry of the alkynylene-based bridge.

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

The target molecules comprise the following units: (i) The main chromophore is a ruthenium(ii) bis(2,2′:6′,2″-terpyridine) unit.

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

(ii) To ensure luminescence at ambient temperature, an ethynylene group is attached at the 4′-position.

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

(iii) A central biphenylene unit is used as a variable rotor.

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

(iv) A strap of predetermined length is attached at the 2,2′-positions of the rotor so as to restrict internal twisting around the connecting bond.

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

The generic molecular structures, and their abbreviations, are shown in Fig. 1.

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

These compounds were synthesized as outlined previously³ and fully characterised by ¹H and ¹³C NMR, mass spectrometry and elemental analysis.

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

Each compound shows a prominent MLCT transition centred at 490 nm, with a tail stretching towards 600 nm.

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

The tail is due to the spin-forbidden MLCT transitions.

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

At higher energy, the parent terpyridine ligand absorbs around 270 nm whilst the polytopic ligand displays absorption bands between 290 and 400 nm.

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

Very weak luminescence can be observed in deoxygenated solution at ambient temperature.

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

This emission is centred around 675 nm and can be assigned to phosphorescence from the lowest-energy MLCT triplet state of the metal complex.⁴

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

At room temperature, the emission quantum yields are very low (Φ_LUM ≈ 0.0004) and the lifetimes are relatively short (τ_LUM ≈ 20 ns).

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

Even so, emission is much more intense than that observed from the parent complex under these conditions.⁵

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

Luminescence from the binuclear complexes becomes more pronounced as the temperature is lowered.

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

In all cases, there is good agreement between the increase in quantum yield and lifetime.

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

The corrected excitation spectrum matches the absorption spectrum over the entire visible and near-UV regions and all spectroscopic measurements indicate that only a single species emits.

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

Even in a frozen glass, there is no suggestion of emission from either a ligand-centred triplet or an intramolecular charge-transfer state.⁶

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

As such, the temperature dependence is due to perturbation of the photophysical properties of the lowest-energy MLCT triplet state.

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

The photophysical properties of metal poly(pyridine) complexes are subject to the energy-gap law.⁷

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

But, in order to expose this relationship, it is necessary to allow for coupling between the emitting state and higher-lying triplets.

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

The energy of the lowest triplet excited state (E_T) can be derived by detailed analysis of the emission spectrum.

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

For the target compounds in fluid solution, E_T (≈15,800 cm^–1) is independent of temperature and insensitive to the length of the strap.

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

Likewise, the total reorganisation energy (λ_T ≈ 700 cm^–1) accompanying decay of the first triplet excited state remains essentially constant for all the complexes studied here.

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

The rate constant (k_NR) for nonradiative deactivation of the lowest-energy triplet state was found to follow eqn. (1) over a wide temperature range (Fig. 2).

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

This expression provides for the activationless rate of decay (k₀), seen at low temperature, and for coupling to two additional excited triplet states.

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

Of these two higher-energy triplets, one is accessed by passage over a small barrier (E₁).

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

This upper-lying state is probably a second MLCT triplet but with increased singlet character.

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

The overall rate constant (k₁) associated with reaching this state is rendered complex by the reversible nature of the barrier crossing.

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

At ambient temperature, a metal-centred (MC) state can be accessed by crossing a more substantial barrier (E₂).

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

In this case, barrier crossing is considered to be irreversible such that the derived rate constant (k₂) is set by the rate of decay of the MC state.

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

This latter process results in direct population of the ground state.

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

The various values extracted from the kinetic fits are collected in Table 1.

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

It is seen that only k₀ depends on the length of the strap.

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

It is known from related systems that k₀ decreases with increasing extent of electron delocalisation of the promoted electron.⁸

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

The fact that k₀ depends on the length of the strap is clear indication that the conformation of the bridging unit affects the degree of electronic communication along the molecular axis.

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

Two possibilities need be considered on the basis of molecular dynamics simulations made for the target compounds in a bath of acetonitrile molecules.

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

First, the strap controls the torsion angle between the phenylene rings.

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

As such, the length of the strap should influence the degree of orbital overlap, in the event that the promoted electron extends to the LUMO on the biphenylene group.

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

Second, the strapped biphenylene unit does not lie coplanar with the covalently-linked terpyridine groups.

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

The mean torsion angle between nearby phenylene and pyridine rings varies markedly with strap length, especially for optimised structures.

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

This torsion angle controls the degree of electronic coupling between the terpyridine ligand and the ethynylene group and will have a pronounced effect on the extent of electron delocalisation over the acetylenic unit.

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

As mentioned above, the variation in k₀ should be explainable within the framework of the energy-gap law, eqn. (2).⁷

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

The most important variable within this expression is the amount of energy to be dissipated during nonradiative decay.

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

However, since E_T remains invariant throughout this series it is clear that other factors combine to affect the magnitude of k₀.

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

The remaining parameters in eqn. (2) are the Huang–Rhys factor (S_M),⁹ the electron-vibrational coupling matrix element (C) and the averaged medium-frequency vibrational mode (hω_M) coupled to nonradiative decay.

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

The coefficient γ depends on both the energy gap and the extent of nuclear displacement.

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

Both S and hω_M can be determined by analysis of the emission spectra recorded in fluid solution.

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

Usually, the full emission spectrum, at any temperature, can be expressed¹⁰ in terms of a weighted average, medium-frequency mode (hω_M) of around 1380 cm^–1.

Type: Model | Advantage: None | Novelty: None | ConceptID: Mod6

This was not the case for the target compounds studied here since, even in fluid solution, it was necessary to include an additional low frequency mode (hω_L) to properly express the emission spectra.

Type: Model | Advantage: None | Novelty: None | ConceptID: Mod7

Thus, the normalised emission spectra could be reconstituted on the basis of eqn. (3)¹¹ and using the data collected in Table 2.

Type: Model | Advantage: None | Novelty: None | ConceptID: Mod8

It should be stressed that the inclusion of hω_L in fluid solution is unique to those metal complexes equipped with the constraining strap.

Type: Model | Advantage: None | Novelty: None | ConceptID: Mod9

In fitting the experimental data to eqn. (3) (Fig. 3), I(ν) is the normalised emission intensity at wavenumber ν, n_M and n_L are the vibrational quantum numbers for the averaged medium (hω_M) and low (hω_L) frequency acceptor modes, Δv_1/2 is the half-width of individual vibronic bands and E₀₀ is the energy gap for the 0–0 transition.

Type: Model | Advantage: None | Novelty: None | ConceptID: Mod10

The quantities S_M and S_L are the Huang–Rhys factors for medium and low frequency modes, respectively.

Type: Model | Advantage: None | Novelty: None | ConceptID: Mod11

Only the half-width depends on temperature and the other parameters were averaged over the entire temperature range (Table 2).

Type: Model | Advantage: None | Novelty: None | ConceptID: Mod12

The best fits to the experimental data gave hω_M and hω_L values, respectively, of ca. 1400 and ca. 800 cm^–1.

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

The latter value increases systematically with increasing temperature; e.g. for C4 hω_L varies from ca. 535 cm^–1 at 90 K to 1020 cm^–1 at 290 K.

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

The medium-frequency mode corresponds to CC and CN stetching vibrations associated with the terpyridine ligand.¹²

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

The low frequency vibration can be assigned to substituted phenylene bending modes.¹³

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

It is now clear that motion around the bridging phenylene unit is coupled to nonradiative decay of the lowest-energy triplet state.

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

Presumably, this effect relates to the system trying to achieve the maximum orbital overlap at the triplet level.

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

It should be noted that the strap exerts a pronounced effect on k₀ since there is a 90-fold increase in rate between C2 and the corresponding complex lacking the strap.

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

The exact details underlying this behaviour will be described elsewhere.

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