The Photochemistry of trans-ortho-, -meta-, and -para-Aminostilbenes

Abstract

The photochemical behavior of the three positional isomers of trans-aminostilbene is reported and compared to that for unsubstituted trans-stilbene. The absorption spectrum of the para isomer displays a single intense long-wavelength band; however the meta and ortho isomers display two less intense bands as a consequence of configuration interaction. All three isomers are fluorescent and display similar solvent-induced shifts of their fluorescence maxima. The fluorescence rate constant of the para isomer is larger than that of the ortho or meta isomer. The para isomer has a short singlet lifetime and high photoisomerization quantum yield, similar to those for trans-stilbene. In contrast, the ortho and meta isomers have long singlet lifetimes and low photoisomerization quantum yields. Isomerization of the para isomer is a singlet-state process, whereas isomerization of both the ortho and meta isomers is a triplet-state process. The lower bound for the barrier for singlet-state torsion of the ortho and meta isomers is 7 kcal/mol. The barrier height is found to be determined by the effects of the amino substituent upon the relative energies of the fluorescent singlet and twisted singlet states.

Fate of photoexcited trans-aminostilbenes

“…It is known that the C¼C torsion barrier is $3.5 kcal/mol for trans-stilbene [24]. A similar value of 3.6 kcal/mol was also determined for the case of trans-4-aminostilbene (4AS) and its N,N-dimethyl derivative 4DS [54,55]. However, the C¼C torsion barriers are 4.9 and 7.4 kcal/mol for p1H and p1CN, respectively, in hexane [52,56].…”

“…The weak D-A electronic coupling in meta-amino systems is sufficient to induce ICT from D to A upon photoexcitation, but once charge separation has occurred the charge delocalization and recombination in the relaxed PICT state becomes inefficient, leading to large dipole moments and low fluorescence rate for the PICT state. This explains not only the longer wavelength of fluorescence maximum (446 vs. 423 nm) but also the longer fluorescence lifetime (11.7 vs. $0.1 ns) for 3AS vs. 4AS in acetonitrile [54]. Note that the excited state deactivation is a game of kinetics: the relatively higher is the rate for a specific decay channel than all the other pathways, the larger is the quantum efficiency for that channel, regardless the absolute value of the rate constants.…”

“…This is illustrated by the comparison of trans-3-aminostilbene (3AS) [54], trans-4-aminostilbene (4AS) [54], and trans-4,4 0 -bis(dimethylamino) stilbene (4DDS) [61,62] in cyclohexane or hexane (Chart 5), which possess a K f of 0.78, 0.05, and 0.81, respectively.…”

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.…”

“…Substituent can raise or lower the torsional energy barrier and thus the Φ f as well as photoisomerization quantum yield (Φ t-c ) are altered. It is also found that stilbene containing heavy atom/groups (e.g., halogen, nitro) on one of its phenyl rings undergoes photoisomerization from its triplet excited state rather than singlet excited state [65][66][67].…”