Medium Effects on the Direct Cis−Trans Photoisomerization of 1,4-Diphenyl-1,3-butadiene in Solution

Abstract

Quantum yields for the photoisomerization of trans,trans-1,4-diphenyl-1,3-butadiene (tt-DPB), determined in benzene, cyclohexane, methylcyclohexane, hexane, and perfluorohexane, confirm the low values reported earlier for benzene and cyclohexane and reveal even lower values in the last two solvents. In contrast to trans-stilbene (t-St), fluorescence and torsional relaxation leading to photoisomerization do not account exclusively for S(1)tt-DPB decay. Competing radiationless singlet excited-state decay pathways exist in tt-DPB, which do not lead to photoisomerization and may not involve large-amplitude torsional motions. Our results invalidate analyses of tt-DPB fluorescence quantum yields and lifetimes that assign all radiationless decay to the isomerization channel. Gas-phase chromatography analysis of tt-DPB photoisomerization in hexane shows the reaction to be concentration-independent and reveals, for the first time, a significant, two-bond photoisomerization pathway, φ(tt→tc) = 0.092 and φ(tt→cc) = 0.020. The dominant one-bond-twist (OBT) process is accompanied by a bicycle pedal (BP) process that accounts for almost 20% of tt-DPB photoisomerization. The OBT tt-DPB photoisomerization quantum yield is largest in benzene (Bz) and smallest in perfluorohexane (PFH). Contrary to expectations, reduction in medium friction in PFH is accompanied by a decrease in φ(tt→tc). The 1(1)B(u)/2(1)A(g) order and energy gap appear to control the contribution of torsional relaxation to radiationless decay. Lowering the 1(1)B(u) energy as in Bz favors photoisomerization. Reversal of the 1(1)B(u)/2(1)A(g) order in PFH is accompanied by short τ(f) and small φ(f) and φ(tt→tc) values that suggest the presence of competing 2(1)A(g) → 1(1)Ag relaxation paths that are unproductive with respect to photoisomerization. We conclude that the Birks extension to diphenylpolyenes of the Orlandi-Siebrand cis-trans photoisomerization mechanism is not valid. Photoisomerization appears to occur in the 1(I)B(u) state, and we argue that this applies to t-St as well.

Fluorescence phenomena in nerve-labeling styryl-type dyes

“…The same transformation applied to the 3-ring BMB system (compound d1 ) led to compound d4 , in which the absorbance displayed a slight hypsochromic (blue) shift and the fluorescence was essentially suppressed. In agreement with previous reports on diene chromophores 30,31 we are compelled to conclude that extended alkene systems such as d4 , despite the addition of only one double bond on each side of a rigidifying middle ring, possess too flexible a backbone. Thus the fluorescence quantum yields and lifetimes in these systems are dependent on mechanisms that compete with non-radiative decay which do not lead to photoisomerization and might not involve large amplitude torsional motions.…”

“…Similarly, in the trans -diphenylbutadiene system, Saltiel et. al 30 have observed radiationless decay pathways that do not lead to photoisomerization resulting in the reduction of fluorescence quantum yields and lifetimes in contrast to the corresponding trans -stilbene derivatives.…”

Comment on ‘On Saltiel's isopolarizability approach and its applicability to diphenylpolyenes’ by J. Catalán, Chem. Phys. Lett. 635 (2015) 56–59

“…It was also achieved in solution by alkyl substitution in the rigid analogue of DPH, all-trans-diindanylidenyl-2-butene (stiff-5-DPH), Scheme 2 [22]. Solvent induced 2 1 A g /1 1 B u state order reversal also occurs in tt-DPB although fluorescence in solution, in this case, is due exclusively to the allowed 1 1 B u → 1 1 A g transition [23,24]. Also relevant is trans,trans-1phenyl-4-(1-pyrenyl)-1,3-butadiene which exhibits dual fluorescence from states of B u and A g parentage [25].…”

Donor-Acceptor Conjugated Linear Polyenes: A Study of Excited State Intramolecular Charge Transfer, Photoisomerization and Fluorescence Probe Properties

“…The photoisomerization in linear polyenes is highly regioselective and is much influenced by substitutent, polarity of the medium [28][29][30][31][32][58][59][60][61][62][63][64][65][66][67][68][69][70][71][72][73] and dipolar nature of the excited state [74][75][76][77][78]. Various mechanistic models have been proposed to understand the photoisomerization process of retinyl and related polyenes.…”