Multiple Charge‐Transfer Transitions in Alkylbenzene‐TCNE Complexes

The charge recombination dynamics of excited donor-acceptor complexes consisting of hexamethylbenzene (HMB), pentamethylbenzene (PMB), and isodurene (IDU) as electron donors and tetracyanoethylene (TCNE) as electron acceptor in various polar solvents has been investigated within the framework of the stochastic approach. The model accounts for the reorganization of intramolecular high-frequency vibrational modes as well as for the solvent reorganization. All electron-transfer energetic parameters have been determined from the resonance Raman data and from the analysis of the stationary charge transfer absorption band, while the electronic coupling has been obtained from the fit to the charge recombination dynamics in one solvent. It appears that nearly 100% of the initially excited donor-acceptor complexes recombine in a nonthermal (hot) stage when the nonequilibrium wave packet passes through a number of term crossings corresponding to transitions toward vibrational excited states of the electronic ground state. Once all parameters of the model have been obtained, the influence of the dynamic solvent properties (solvent effect) and of the carrier frequency of the excitation pulse (spectral effect) on the charge recombination dynamics have been explored. The main conclusions are (i) the model provides a globally satisfactory description for the IDU/TCNE complex although it noticeably overestimates the spectral effect, (ii) the solvent effect is quantitatively well described for the PMB/TCNE and HMB/TCNE complexes but the model fails to reproduce their spectral effects, and (iii) the positive spectral effect observed with the HMB/TCNE complex cannot be described within the framework of two-level models and the charge redistribution in the excited complexes should most probably be taken into account.

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“…51,52 This splitting is very small and was set to 0.15 eV in the fit. The intensities of the two bands were supposed to be equal.…”

Section: Simulation Results and Discussionmentioning

confidence: 99%

Solvent and Spectral Effects in the Ultrafast Charge Recombination Dynamics of Excited Donor−Acceptor Complexes

Feskov¹,

Ionkin²,

Ivanov³

et al. 2008

J. Phys. Chem. A

132

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“…This absorption band can be assigned to a CT transition of the MPe/TCNE DAC as documented for many other complexes composed of various aromatic hydrocarbons and TCNE. [29][30][31] The rise of the CT band is accompanied by a slight decrease of the 420-nm band associated with the local S 0 -S 1 MPe transition. This effect can be interpreted as a perturbation of the electronic structure of MPe by the presence of a nearby TCNE molecule.…”

Section: Resultsmentioning

confidence: 99%

Simultaneous Generation of Different Types of Ion Pairs upon Charge-Transfer Excitation of a Donor−Acceptor Complex Revealed by Ultrafast Transient Absorption Spectroscopy

Mohammed

Vauthey

2008

J. Phys. Chem. A

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The excited-state dynamics of the methylperylene/tetracyanoethylene (MPe/TCNE) donor-acceptor complex has been investigated in various solvents using femtosecond transient absorption spectroscopy. The transient spectra reveal the formation of two types of ion pairs: The first (IP1), constituting the major fraction of the total ion-pair population, is characterized by a broad and red-shifted absorption spectrum compared to that of the free MPe cation and by a subpicosecond lifetime, whereas the second (IP2) has a spectrum closer to that of MPe cation and a lifetime of a few picoseconds. A substantial polarization anisotropy was observed with IP1 but not with IP2, indicating a relatively well-defined structure for the former. The reaction scheme that best accounts for the observed dynamics and its solvent dependence involves the simultaneous excitation of complexes that differ by their electronic coupling. The more coupled complexes have a high absorption coefficient and thus yield IP1, which undergoes ultrafast charge recombination, whereas the less coupled complexes have a lower probability to be excited and lead to the longer-lived IP2.

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“…Figure 2A shows the steady-state absorption spectrum of a mixture of DEA and PMDA in ACN. This spectrum is characterized by two CT bands, designed as CT1 (lower energy) and CT2 (higher energy), which have been reproduced by asymmetric Gauss functions as described in ref 32. This simple line shape analysis was carried out to estimate the contribution of each CT band to the absorption spectrum, a more quantitative analysis 44,45 going beyond the scope of this study.…”

Section: Resultsmentioning

confidence: 99%

Effect of the Excitation Wavelength on the Ultrafast Charge Recombination Dynamics of Donor−Acceptor Complexes in Polar Solvents

et al. 2005

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The effect of the excitation wavelength on the charge recombination (CR) dynamics of several donoracceptor complexes (DACs) composed of benzene derivatives as donors and of tetracyanoethylene or pyromellitic dianhydride as acceptors has been investigated in polar solvents using ultrafast time-resolved spectroscopy. Three different wavelength effects have been observed. (1) With complexes exhibiting two well-separated charge-transfer bands, the CR dynamics was found to be slower by a factor of about 1.5 upon excitation in the high-energy band. This effect was measured in both fast and slow relaxing solvents and was discussed in terms of different DAC geometries. (2) When the CR is faster than diffusive solvation, a slowing down of the CR with increasing excitation wavelength accompanied by an increase of the nonexponential character of the dynamics was measured. This effect appears only when exciting on the red edge of the charge-transfer absorption band. (3) When the driving force for CR is small, both nonequilibrium (hot) and thermally activated CR pathways can be operative. The results obtained with such a complex indicate that the relative contribution of these two paths depends on the excitation wavelength.

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Multiple Charge‐Transfer Transitions in Alkylbenzene‐TCNE Complexes

Cited by 24 publications

References 29 publications

Solvent and Spectral Effects in the Ultrafast Charge Recombination Dynamics of Excited Donor−Acceptor Complexes

Solvent and Spectral Effects in the Ultrafast Charge Recombination Dynamics of Excited Donor−Acceptor Complexes

Simultaneous Generation of Different Types of Ion Pairs upon Charge-Transfer Excitation of a Donor−Acceptor Complex Revealed by Ultrafast Transient Absorption Spectroscopy

Effect of the Excitation Wavelength on the Ultrafast Charge Recombination Dynamics of Donor−Acceptor Complexes in Polar Solvents

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