J. Phys. Chem. A 2009, 113, 8557–8568
8557
Adiabatic Pathways in the Conformational and Geometrical Photoisomerizations of the 1,2-Distyrylbenzene Isomers G. Bartocci, S. Ciorba, U. Mazzucato, and A. Spalletti* Dipartimento di Chimica, UniVersita` di Perugia, I-06123 Perugia, Italy ReceiVed: April 27, 2009
The photophysical behavior of the geometrical isomers of 1,2-distyrylbenzene (1,2-DSB) has been investigated as a function of temperature by stationary and pulsed fluorimetric techniques in a nonpolar solvent. The temperature effect on the spectral properties, fluorescence quantum yields, and decay profiles allowed the role played by the conformational isomers of these molecules in the relaxation processes of the lowest excited states to be clarified. In the case of the EE geometrical isomer, a conformer (or rotamer) re-equilibration, implying an adiabatic interconversion during the lifetime in the S1 state, was found to be operative. The deactivation channels of the excited cis isomers (ZE and ZZ) markedly depend on the conformational geometry, which is responsible for the adiabatic (rotamer-specific) cis f trans photoisomerization. The radiative and radiationless relaxation rate parameters of the isomers and conformers of 1,2-DSB, derived from the experimental and calculation results, allowed a complete and reasonably quantitative description of their behavior in the S1 state. 1. Introduction The photobehavior of 1,n-distyrylbenzenes [1,n-DSB, with n ) 2, 3, or 4] has received wide attention because delocalization of the π electrons in their electronically excited states can induce new and specific properties, which can find potential applications in the field of material science, particularly in optoelectronics.1,2 The photophysical and photochemical behavior of all-trans (EE) geometrical isomers of 1,n-DSB bearing the two styryl substituents in the para (n ) 4, linear conjugation)3-6 and meta (n ) 3, crossed conjugation)7-10 positions has been particularly investigated in the last decades. Both series of compounds show a very small trans f cis photoisomerization yield, while the radiative relaxation is predominant. In these compounds, conformational equilibria are present in the ground state (S0), and their role on the deactivation channels of the excited states has been investigated.3-8,10-14 On the contrary, the nonconjugated ortho positional isomer (n ) 2, 1,2-DSB) has been relatively less investigated from the photophysical point of view.9,11,12,15,16 The results have shown that the lack of conjugation markedly affects the properties of this molecule in the excited states. In fact, in fluid solution at room temperature, the main relaxation pathway of all of the geometrical isomers (EE, ZE, and ZZ) of 1,2-DSB seemed to be internal conversion (IC). The ZE and ZZ isomers photoisomerize by a predominant adiabatic pathway (a “one photontwo bonds” process is also operative for ZZ),16 and an unexpected cyclization photoproduct was obtained exciting the EE isomer. This cyclized compound can only originate from the compressed conformation (Chart 1), which is the less stable, practically absent in the ground state, as suggested by calculations.15 This led us to suppose that the EE rotamers could play a relevant role in the excited state deactivation also in the case of 1,2-DSB, despite the fact that only the elongated conformer is practically present in S0. The aim of this Article is to prove the unusual presence of very fast adiabatic interconversion processes among the rotamers * Corresponding author. E-mail:
[email protected].
during their S1 lifetime as well as the involvement of rotamerspecific adiabatic pathways in the cis f trans photoisomerization. Therefore, a detailed study on the spectral trends and photophysical properties of the three geometrical isomers of 1,2DSB as a function of the excitation wavelength (λexc) and temperature was carried out in a nonpolar solvent. The derived rate constants for the competitive relaxation pathways of the lowest excited singlet state of all of the conformational and geometrical isomers allowed a reasonable mechanism for the photophysical and photochemical behavior of 1,2-DSB to be proposed. The occurrence of both rotamer re-equilibration in S1 and rotamer-specific adiabatic cis f trans isomerization confirms the relevant role of conformation in the relaxation processes of these molecules in the excited state. 2. Experimental Section The compounds investigated were synthesized for previous works.9 The solvent was a mixture of methylcyclohexane and 3-methylpentane (MCH/3MP, 9/1 (v/v)) from Fluka, spectrophotometric grade, further purified by standard procedure. A few measurements were also carried out in pentane/heptane (P/ Hept, 1:1 (v/v)) from Aldrich. A Perkin-Elmer Lambda 800 spectrophotometer was used for the absorption measurements. The fluorescence spectra were obtained by a Spex Fluorog-2 F 112 AT spectrofluorimeter, using dilute solutions (absorbance
