DOI: 10.1002/cphc.201200762 Photophysics of Push–Pull Distyrylfurans, Thiophenes and Pyridines by Fast and Ultrafast Techniques Benedetta Carlotti,* [a] Ilijana Kikas ˇ, [b] Irena S ˇ koric ´, [b] Anna Spalletti, [a] and Fausto Elisei [a] 1. Introduction Organic molecules with delocalised p-electron bridges and electron-donor (D) and -acceptor (A) groups at opposite ends (push–pull systems) have been widely investigated owing to their ability to absorb light and generate charge separation. These properties are particularly interesting for performing useful photoinduced functions, such as fluorescent sensing of ions and molecules, solar energy conversion, optical informa- tion storage and effects of non-linear optics (NLO). Owing to their greater application potential, easier synthesis and lower cost relative to inorganic materials, increasing interest has been devoted to organic compounds, particularly for NLO. [1, 2] After the first observation of photoinduced intramolecular charge transfer (ICT) in 4-(N,N-dimethylamino)benzonitrile, [3] a wide series of push–pull systems have been studied in differ- ent solvents. [4–22] Recent studies carried out in our laboratory on distyryl and ethynyl aromatics with A/D side groups with the aim of charac- terising the photophysics of the substrates in fluid media. [23–30] Experimental (steady-state and time-resolved techniques) and quantum mechanical [time-dependent density functional cal- culations (TD-DFT)] approaches were used to investigate the effect of solvent on the main deactivation processes of the lowest excited singlet state (S 1 ), such as ICT, fluorescence (F), singlet–triplet intersystem crossing (ISC) and internal conver- sion (IC). The effect of solvent on the ISC rate constant, which changes drastically with the locally excited (LE)/charge-transfer (CT) nature of the S 1 state, is particularly interesting. [27, 28, 30] Herein, we deal with the effect of solvent on the photobeha- viour of six trans,trans-distyrylbenzene analogues (Scheme 1), in which the benzene rings are replaced with heteroaromatic rings (furan, thiophene and pyridine) and functionalised with nitro (A) and dimethylamino/methoxy (D) groups by steady- state and time-resolved absorption and emission techniques, with nano- and femtosecond resolution. The synthesis, spectral properties, fluorescence quantum yield and lifetime, and photoisomerisation of the compounds shown in Scheme 1 in deaerated cyclohexane have previously been reported. [31] Another paper on two series of four non-re- active and six isomerisable push–pull compounds, including three of those examined herein, aimed to use positive solvato- chromism to obtain information on changes in the excited- state dipole moment induced by the ICT process to derive the hyperpolarisability coefficient. [32] This paper describes the effect of the nature of the central heteroaromatic group on Time-resolved transient absorption and fluorescence spectros- copy with nano- and femtosecond time resolution were used to investigate the deactivation pathways of the excited states of distyrylfuran, thiophene and pyridine derivatives in several organic solvents of different polarity in detail. The rate con- stant of the main decay processes (fluorescence, singlet–triplet intersystem crossing, isomerisation and internal conversion) are strongly affected by the nature [locally excited (LE) or charge transfer (CT)] and selective position of the lowest excit- ed singlet states. In particular, the heteroaromatic central ring significantly enhances the intramolecular charge-transfer pro- cess, which is operative even in a non-polar solvent. Both the thiophene and pyridine moieties enhance the S 1 !T 1 rate with respect to the furan one. This is due to the heavy-atom effect (thiophene compounds) and to the 1 (p,p)*! 3 (n,p)* transition (pyridine compounds), which enhance the spin-orbit coupling. Moreover, the solvent polarity also plays a significant role in the photophysical properties of these push–pull compounds: in fact, a particularly fast 1 LE*! 1 CT* process was found for di- methylamino derivatives in the most polar solvents (time con- stant, t  400 fs), while it takes place in tens of picoseconds in non-polar solvents. It was also shown that the CT character of the lowest excited singlet state decreased by replacing the di- methylamino side group with a methoxy one. The latter causes a decrease in the emissive decay and an enhancement of triplet-state formation. The photoisomerisation mechanism (singlet/triplet) is also discussed. [a] Dr. B. Carlotti, Prof. A. Spalletti, Prof. F. Elisei Department of Chemistry and Centro di Eccellenza sui Materiali Innovativi Nanostrutturati (CEMIN) University of Perugia, via Elce di Sotto 8 06123 Perugia (Italy) Fax: (+ 39) 755855598 E-mail : carlotti@unipg.it [b] Dr. I. Kikasˇ, Prof. I. S ˇ koric ´ Department of Organic Chemistry Faculty of Chemical Engineering and Technology University of Zagreb, Marulicev trg 19 10000 Zagreb (Croatia) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/cphc.201200762. # 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim ChemPhysChem 2013, 14, 970 – 981 970 CHEMPHYSCHEM ARTICLES