I. Kikaš , a I. Škorić, a M. Kov á cs, b L. Fodor, b O. Horv á th, b M. Šindler a

1. Sinteza, fotokemija i fotofizika novih butadienskih derivataSynthesis, photochemistry and photophysics of novel butadienes derivatives Synthesis of butadiene derivatives 1 - 4 In order to prepare new polycyclic structures by photochemical methodology, new systems with prolonged conjugation, ,’-diaryl-1,3-butadiene derivatives 1-4,were synthesized by Wittig reaction. The influence of the introduced methyl group on the double bond was examined in order to see the possibility of the intramolecular [2+2] photocycloaddition and formation of the benzobicyclo[3.2.1]octadiene photoproducts. The methyl group has great influence on the planarity and steric hindrance of the starting geometrical isomers and as the consequence on the yield and reaction course also. Detailed photophysical characterization of the starting derivatives 1-4 was examined for better understanding different photochemical routes. The photochemical reactions were performed as well as the spectroscopic characterization of the photoproducts. Photophysics Transient absorption Time-resolved absorption measurements gave some hints regarding the photochemical decay routes of the excited states. In the case of the unsubstituted trans,trans-3 isomer a transient absorption of 450-ns lifetime was observed, which may be assigned to the triplet excited state of this compound (Fig 1). The corresponding methyl substituted derivative displayed several transient absorptions. The shortest-lived one of 150-ns lifetime, similarly to the previous case, may be attributed to the formation and decay of triplet excited state (Fig 2). Fig 3. Two-stage absorption transient signal (decay curve) of trans,trans-4at 300 nm in a longer time-scale. At longer time-scale, however, two other transients were observed with 50-s and 15-ms lifetime, respectively (Fig 3). On the basis of the lifetime and the change of the absorption, the decay of the latter intermediate probably leads to the formation of the final product in the photochemical reaction of the starting compound. I. Kikaš,a I. Škorić,a M. Kovács,b L. Fodor,b O. Horváth,b M. Šindlera aDepartment of Organic Chemistry, Faculty of Chemical Engineering and Technology, University of Zagreb, Marulićev trg 19, 10000 Zagreb, Croatia bDepartment of General and Inorganic Chemistry, Institute of Chemistry, Faculty of Engineering, University of Pannonia, P.O.B. 158, Veszprém H-8201, Hungary Fig 2. Transient absorption spectra of trans,trans-4 recorded at 80, 150, and 300 ns after the excitation, the estimated spectrum of the triplet state along with the spectrum of the starting material and that of the final product. The decay curves at 255 and 310 nm are also displayed. Emission These butadiene derivatives 1–4 display diverse emission features strongy depending on their structures. The cis,cis-3 isomer efficiently fluoresces ( = 0.246), its emission spectrum is characterized by well resolved vibronic fine structure (Fig 4).The fluorescence of the cis,trans-3 isomer is very similar, but its quantum yield is 45% lower ( = 0.135). The trans,trans-3 isomer, however, displays an order of magnitude weaker ( = 0.017) and rather structurless emission, probably due to the steric infuence promoting other (probably photochemical) decay routes. Methyl substituents on the double bonds in compounds 4 dramatically modify the emission properties too, breaking the extended conjugation. The fluorescence spectra are strongly red-shifted, becoming a very weak ( < 0.001), broad, structureless band (Fig 4). Also the lifetime of the singlet excited state is significantly lower for these methylated derivatives compared to the corresponding unsubstituted compounds. Fig 1. Transient absorption spectum of trans,trans-3 recorded at 150 ns after the excitation, the estimated spectrum of the triplet state along with the spectrum of the starting material and that of the final product. The decay curves at 305 and 400 nm are also displayed. Fig 4. Fluorescence spectra of the geometrical isomers of compounds 3and 4. Photochemistry and reaction mechanisms The photochemistry of butadiene structures 1-4 under the intramolecular conditions of cycloaddition reactions was studied and different reaction pathways (Schemes 1-4) were followed as the influence of the presence of the methyl group in the starting molecules 1,2 and 4 in comparison with the unsubstituted derivative 3. Scheme 1. Intramolecular [2+2] photocycloaddition of butadiene derivative 1 Scheme 2. Two possible electrocyclic photoproducts from different configurations of 2. Scheme 3. Intramolecular [2+2] photocycloaddition of butadiene derivative 3. This work was supported by the Ministry of Science, Education and Sports of the Republic of Croatia and the National Office of Research and Technology (OMFB-01247/2009). Literature: [1] Škorić, I.; Šmehil, M.; Martinić, Ž.; Molčanov, K.; Kojić-Prodić, B.; Šindler-Kulyk, M. J. Photochem. Photobiolog. A - Chemistry 207 (2009) 190–196. [2] Filippini, M.-H.; Rodriguez, J. Chem. Rew. 99 (1999) 27–76. Scheme 4. Photoisomerisation of butadiene derivative 4 XXII. hrvatski skup kemičara i kemijskih inženjera Zagreb, Hrvatska, 13.-16. veljače 2011.