Intramolecular energy transfer in cis-trans isomerization: A study of fluorescence from single vibronic levels of styrene, trans-β-styrene-d1, styrene-d8, and ethynlbenzene

Abstract

The fluorescence lifetimes of, and quantum yields of emission from, individual vibronic levels of isolated molecules of styrene, trans-β-styrene-d1, styrene-d8, and ethynlbenzene have been measured. These data are subjected to a decomposition, under a plausible hypothesis, into parameters descriptive of an isomerization process and an intersystem crossing process. It is argued that the two processes proceed independently in parallel. The rate of isomerization is found to be independent of the vibrational energy content of the isolated molecule and to have a dependence on isotopic mass consistent with the prediction that the torsional mode is the best ground state acceptor in the reaction channel. A plausible schematic potential energy surface is constructed from a combination of spectroscopic observations and analogies with theoretically derived surfaces for similar molecules. Analysis of the experimental data, and deductions from those data, in terms of the properties of the potential surface, leads to the inference that intramolecular vibrational relaxation is slower than chemical reaction for excess vibrational energy up to ∼2500 cm−1.

Benchmarking the Molecular Mechanics−Valence Bond Method: Photophysics of Styrene and Indene

“…MMVB and CASSCF 8i both find that the S 2 and S 1 states are almost degenerate at the S 2 planar minimum 1c (Table ), suggesting that barrierless IC to S 1 can take place after initial excitation to S 2 . Subsequent pathways on S 1 lead either to the planar minimum 1b or a twisted minimum 1f (Figure ) with a quinoid ring structure. , The energy difference between 1b and 1f is effectively the barrier height for adiabatic isomerization on S 1 and is 14 kcal mol -1 with MMVB and 17 kcal mol -1 with CASSCF/4-31G. Both methods show that the S 1 → S 0 gap at 1f is too large (see Computational Details section and ref h) for IC to be likely at this point (although S 1 and T 2 are degenerate, suggesting that ISC may take place 8i,13c ).…”

“…In the case of styrene ,− we have recently demonstrated 8i that changes in bonding within the benzene ring following excitation cannot be neglected, as the two mechanisms for S 1 decay set out in Scheme (where 1a , 1b , 1g , 1h , and 1m refer to optimized geometries of Figure that we shall discuss subsequently) show. The lowest energy pathway to efficient internal conversion (IC) is via an S 1 /S 0 conical intersection that results from a distortion of the benzene ring alone 8h (Scheme , left).…”

Phenylacetylene: A Hydrogen Bonding Chameleon

“…The M 1 and M 2 complexes correspond to the PMA4 and PMA1 structures, respectively. [37] Phenylacetylene has a long fluorescence lifetime of 54 ns, [73] which is quenched by the formation of the CÀH···N hydrogenbonded complex M 2 (PMA1). In the case of the binary complex between phenylacetylene with 1-azabicyclo[2.2.2]octane the formation of CÀH···N hydrogen bonding involving the aromatic CÀH bonds did not lead to fluorescence quenching.…”

A Fortunate Life in Physical Chemistry

“…At the same time, and in collaboration with Karl Freed, Bill Gelbart (49) extended the theoretical analysis of the dependence of the decay rate on the character and energy of the prepared state, providing support for the possibility of mode-selective dynamics within a dense manifold of states. Follow-up studies of the rates of radiationless decay from different initial vibronic levels of other aromatic molecules generated extensive evidence for mode-sensitive excited-state evolution (50)(51)(52)(53)(54)(55)(56)(57). These results led to new research programs, specifically, the study of collision-induced selective vibrational relaxation of electronically excited molecules (58)(59)(60)(61)(62)(63)(64)(65)(66) and the study of the state-selective predissociation of van der Waals molecules (67-71).…”

“…Gelbart developed a formal theory of photoinduced molecular rearrangements, paying particular attention to the case of internal rotation in cis-trans isomerization (73, 74). Man Him Hui (51) successfully used the latter theory to analyze the results of an experimental study of the initial state dependence of the competition between energy transfer and cis-trans isomerization in styrene-d 1 . Other studies of initial state-dependent rates of single-collision-induced energy transfer (58-66) and of the predissociation of van der Waals molecules (67-71) further supported the view that mode-selective evolution can prevail even when the density of states is fairly large.…”