J. Phys. Chem. A 2010, 114, 10761–10768
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Photobehavior of the Geometrical Isomers of Two 1,4-Distyrylbenzene Analogues with Side Groups of Different Electron Donor/Acceptor Character S. Ciorba, G. Galiazzo,† U. Mazzucato, and A. Spalletti* Dipartimento di Chimica, UniVersita` di Perugia, 06123 Perugia, Italy ReceiVed: June 11, 2010; ReVised Manuscript ReceiVed: July 29, 2010
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. 1. Introduction
SCHEME 1: Compounds Investigated
Donor-acceptors R,ω-diarylpolyenes, particularly the substituted stilbenes, have been widely investigated as interesting models for studies of intramolecular charge transfer (ICT) in the excited state.1–4 The interest in these dipolar push-pull molecules is especially due to their hyperpolarizability properties, which make them candidate components of NLO materials.5,6 In the framework of our long-term investigation of the photobehavior of stilbene-like molecules, particularly the heteroanalogues of stilbene and distyrylbenzene (DStB) [for some representative papers, see refs 7–16] two asymmetric 1,4-DStB analogues were prepared, bearing two different arylethenyl groups at the opposite side of the central benzene ring, namely 1-(pyrid-4-ylethenyl)-4-(thien-2-ylethenyl)benzene (1) and 1-(pyrid-2-ylethenyl)-4-(p-nitrostyryl)benzene (2). The presence of four geometrical isomers (EE, ZE, EZ, and ZZ) and various conformational isomers (conformers or rotamers) of these compounds makes their photobehavior rather unforeseeable. Apart from the role of conformers in the photoreactivity, new interesting aspects offered by asymmetric DStBs bearing two different isomerizable double bonds, with respect to the analogous compounds with one bond only, are the possible selectivity in the production of the ZE and EZ isomers under irradiation of both EE and ZZ and the possible occurrence of the “one photon-two bonds” mechanism in the ZE f EZ and ZZ f EE photoprocesses. Interesting examples of regioselectivity have been reported for alkyl-substituted 1,3-distyrylbenzene, attributed to intramolecular energy transfer in the excited state,17 and for other series of compounds with two isomerizable double bonds, for instance, the asymmetrically substituted 1,4diarylbutadienes, attributed to ICT in the excited state.18–20 * To whom correspondence should be addressed. E-mail:
[email protected]. † Retired from Padua University (address: via Crescini 39, 35126 Padova, Italy).
This paper aims to describe the different relaxation properties of the four geometrical isomers of the two compounds, in particular, the fluorescence/reactivity competition, the photoisomerization mechanism, and the role of the triplet state in the photobehavior. The effect of ICT on their behavior under irradiation in the polar solvent was also investigated. 2. Experimental Section 2.1. Materials. The two 1,4-DStB analogues bearing a 4-pyridyl (4P) group at one side and a 2-thienyl (2T) or a 4-nitrophenyl (p-NO2Ph) substituent at the other side (Scheme 1), were synthesized by application of a 2-fold Wittig reaction to difunctional reactants [p-xylylene-bis(triphenylphosphonium bromide) and 2-thienyl-aldehyde or p-nitrobenzaldehyde, respectively]. The four geometrical isomers contained in the photoproduct mixture of both compounds (Scheme 2) were separated, purified, and characterized by the combined use of preparative HPLC (column: Alltima C18, 22 × 250 mm; 5 µm) and spectrophotometric techniques. The formula was calculated on the basis of the isotopic abundance in the LC/MS analysis
10.1021/jp105383e 2010 American Chemical Society Published on Web 09/21/2010
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J. Phys. Chem. A, Vol. 114, No. 40, 2010
Ciorba et al.
SCHEME 2: Geometrical Isomers of the Compounds Investigated
(see Supporting Information). The geometrical structure was assigned on the basis of their experimental and computed absorption spectra. The ZZ isomers were characterized also by 1 H NMR spectrometry (see Supporting Information). The solvents were cyclohexane (CH) and acetonitrile (AcN) from Fluka (spectrophotometric grade) except when otherwise specified. 2.2. Techniques and Methods. A Perkin-Elmer Lambda 800 spectrophotometer was used for the absorption measurements. The fluorescence spectra were measured by a Spex Fluorolog-2 F112AI spectrofluorometer. Dilute solutions (absorbance
