Photobehavior of the Geometrical Isomers of Two 1,4-Distyrylbenzene Analogues with Side Groups of Different Electron Donor/Acceptor Character

Abstract

The photobehavior of two 1,4-distyrylbenzene analogues where the central benzene ring is asymmetrically substituted with a pyrid-4-ylethenyl group at one side and thien-2-ylethenyl or a p-nitrostyryl group at the other side, has been studied in two solvents at room temperature. The four geometrical isomers (EE, ZE, EZ, and ZZ) of each compound were separated by the combined use of HPLC and spectrophotometric techniques. The radiative/reactive competition in their excited state relaxation was particularly examined: the diabatic/adiabatic contributions were estimated and a reasonable interpretation of the photoisomerization mechanism was proposed. The role of the conformational isomers was also investigated by measured and computed spectral data. Since the different electron donor/acceptor character of the side groups of these molecules can induce charge transfer phenomena that can affect the relaxation pathways of their excited states, the photobehavior was compared in inert and polar solvents to clarify the role of the intramolecular charge transfer. The latter was found to affect markedly the relaxation properties and to induce interesting fluorosolvatochromic effects, particularly in the p-nitro derivative. The participation of the triplet state in the reaction mechanism of the latter was also investigated by flash photolysis and sensitized experiments.

Unusual high fluorescence of two nitro-distyrylbenzene-like compounds induced by CT processes affecting the fluorescence/intersystem-crossing competition

“…1 The reactivity in non-polar solvent was now measured also in Tol for comparison with 2 that is more soluble in this solvent. The photoisomerization quantum yield of 1 has shown a similar trend with an even strong decrease of reactivity in the polar solvent (from 0.20 to 0.007 on going from CH to MeCN).…”

“…S8 and S9 in ESI ‡). 1 The calculations gave also information on the energy level and nature of the states in the singlet and triplet manifolds at the ground state geometry (Tables S1-S4 and Chart 2 and Fig. The MOs of compound 1 show that the pyridyl ring behaves as a donor of charge towards the acceptor nitrophenyl moiety, while those of compound 2 indicate a movement of charge from the central benzene ring to the side nitrophenyl groups.…”

“…[1][2][3][4][5] The effect of substituents on the photobehaviour of stilbene and related molecules has been widely investigated in the last two decades, 6,7 particularly the effect of D/A substituents on the excited state deactivation (for some significant contributions see ref. [1][2][3][4][5] The effect of substituents on the photobehaviour of stilbene and related molecules has been widely investigated in the last two decades, 6,7 particularly the effect of D/A substituents on the excited state deactivation (for some significant contributions see ref.…”

“…An extensive study of the photobehaviour of mono-nitro and di-nitro derivatives has been performed by the group of D. Schulte-Frohlinde and H. Görner in Muhleim. 1 Compound 2 has been the object of large attention, both theoretically and experimentally, with the aim of comparing its emission properties with those of the distyrylfluorene analogue 16 or investigating its two photon absorption 17 and its applications in organic light-emitting diodes 18 but its emission/photoreaction competition and the involvement of the triplet manifold have not been investigated at our knowledge. However, if the opposite side of the molecule bears a unity of more or less efficient donor properties, thus favouring intramolecular charge transfer (ICT) processes, the rate of the competitive deactivation pathways may be strongly modified.…”

Synthesis, spectral properties and photobehaviour of push–pull distyrylbenzene nitro-derivatives

“…We reported recently the synthesis and photophysical study of conjugated systems bearing electron-donor/acceptor groups at the extreme ends, connected by a spacer [1][2][3][4][5][6][7]. The design and study of these push-pull (D--A) systems have attracted our attention because of their potential application as fluorescent probes, as dye laser systems and in optoelectronics [8,9], especially for their hyperpolarizability properties which make them candidates as components of non linear optical (NLO) materials [10][11][12][13].…”