Teorijsko istraživanje strukture i relaksacionih mehanizama fotopobuđenog avobenzona pomoću ab initio metoda

Abstract

Avobenzon (4-terc-butil-4’metoksidibenzoilmetan) je jedan od najpoznatijih najˇsire koriˇs´cenih filtera za zaˇstitu od UVA zraˇcenja. Avobenzon spada u grupu β-dikarbonilnih jedinjenja i ˇcija fotohemija ukljuˇcuje keto-enol tautomerizaciju, cis-trans izomerizaciju, rotaciju oko jednostruke veze kao i raskidanje karbonilnih grupa. Kako bi istraˇzili fotohemiju avobenzona izuˇcavani su tautomeri avobenzona: helatni i ne-helatni enol, rotamer Z, rotamer E kao i keto oblik u osnovnom i pobudenim stanjima u gasnoj fazi i acetonitrilu pomo´cu DFT i metode spregnutih klastera. Istraˇzivanje je pokazalo da torzija oko dvostruke C2-C3 veze fotopobudenog enolnog oblika vodi do unutraˇsnje konverzije u osnovno elektronsko stanje i formiranje rotamera E. Takode, otvaranje helatnog prstena i raskidanje vodoniˇcne veze rotacijom hidroksilne grupe uzrokuje stvaranje ne-helatnog enolnog oblika. Izuˇcavani su i mogu´ci mehanizmi za formiranje rotamera Z. Studija je pokazala da je zavisnost fotostabilnosti avobenzona od rastvaraˇca u direktnoj vezi sa relativnim rasporedom najniˇzeg tripletnog ππ∗ i nπ∗ stanja. Opis apsorpcionih osobina samo na osnovu vertikalnog ekscitacionog spektra raˇcunatog u minimumu povrˇsi potencijalne energije osnovnog elektronskog stanja, dovodi do zakljuˇcka da ne dolazi do inicijalnog pobudivanja nπ∗ stanja. S obzirom da pod uticajem termalnih i kvantnih efekata dolazi do znaˇcajnih promena u strukturi molekula avobenzona, za potpunije opisivanje sistema neophodno je uraˇcunati ne samo jednu konfiguraciju ve´c ansambl konfiguracija molekula. Tako je u ovom radu izvrˇsena simulacija spektara primenom DFT/MRCI metode zajedno sa ansamblom konfiguracija dobijenih pomo´cu metoda molekulske dinamike. Takode, izraˇcunati su i Frank-Kondonovi faktori za prelazak na ππ∗ stanje. Istraˇzivanje je pokazalo da termalne i kvantne fluktuacije simetriˇcno ˇsire apsorpcioni spektar u okviru poluklasiˇcne aproksimacije zbog ˇcega je neophodno uraˇcunati i vibronske efekte kako bi eksperimentalni spektar u potpunosti mogao biti reprodukovan. Proces apsorpcije uglavnom ukljuˇcuje pobudivanje ππ∗ stanja, mada ima je i nπ∗ stanje takode pobudeno u manjoj meri. Prelaz izmedu ova dva pobudena stanja se javlja prilikom intramolekulskog transfera protona. Takode postulirana su dva termalna tautomerizaciona mehanizma i prodiskutovane su njihove intermedijerne strukture.

Avobenzone (4-tert-butyl-4’-methoxydibenzoylmethane, AB) is one of the most popular and widely used organic filter for skin photoprotection in the UVA band. AB belongs to the class of a β-dicarbonyl compounds which undergoes keto-enol tautomerization, rotation around single bond, bond breaking of carbonyl groups and cis-trans isomerization. All of these structural changes of AB lead to formation of different forms of AB which we labeled as: rotamer E, rotamer Z, chelated enol, non-chelated enol and keto tautomer. In order to study the photochemistry of AB we examined AB tautomers in the ground and excited states in the gas phase and acetonitrile using the coupled cluster and DFT computational methods. Based on the results of this study, it can be concluded that the internal conversion to the ground state of photoexcited chelated enol and creation of rotamer E is a consequence of the rotation around C2-C3 double bond of the chelated enol form. In addition, non-chelated enol tautomer is formed as a result of torsion of the hydroxyl group and breaking of the hydrogen bond of the chelated ring. The possible mechanisms of rotamer Z creation are discussed. Examination of the keto tautomer triplet states revealed that the lowest triplet ππ∗ and nπ∗ states exchange their relative order depending on the solvent. Results acquired from the calculation of the vertical excitation spectrum at the minimum of the ground state potential energy surface indicate that nπ∗ state does not exhibit initial photoexcitation. Because thermal and quantum fluctuations cause major structural changes of AB molecule and in order to create overall picture of photoexcitation, it is necessary to consider an ensemble of configurations instead of one molecular configuration. In order to do so, we sampled different configurations using DFT-based classical and path integral molecular dynamics, which are later used to calculate UVA response at DFT/MRCI level of theory. Also, Franck-Condon factors are calculated for the ππ∗ state. Results obtained using the semi-classical method show that there is symmetrical broadening of the excited states absorption due to the thermal and nuclear fluctuation, which lead to the conclusion that vibronic effects need to be included in order to achieve better matching of simulated and experimental spectrum. During the intramolecular proton transfer, crossing between the ππ∗ and nπ∗ state occurs, which results in minor contribution of the nπ∗ state in absorption, although the ππ∗ state contributes considerably more. Also, two thermal tautomerization mechanisms are postulated and their transient structures are discussed.