Dynamics simulations of photoinduced proton transfer reactions of 2-(2 0 -hydroxyphenyl)benzoxazole in the gas phase and its hydrated clusters Rathawat Daengngern, Nawee Kungwan ⇑ Department of Chemistry, Faculty of Science, Chiang Mai University, Chiang Mai 50200, Thailand article info Article history: Received 29 May 2014 In final form 20 June 2014 Available online 28 June 2014 abstract The excited-state dynamics simulations of 2-(2 0 -hydroxyphenyl)benzoxazole (HBO) and hydrated HBO were performed on their lowest energy structures using RI-ADC(2)/SVP-SV(P). For free HBO, only syn- HBO can undergo tautomerization in the excited state. For HBO(H 2 O), two different pathways (A) the excited-state intramolecular proton transfer (PT) and (B) the excited-state intermolecular PT through hydrogen-bonded network occur. PT time of free HBO is ultrafast at 28 fs, whereas, the PT time of HBO(H 2 O) takes longer with time range of 43–193 fs depending on the initial structure. The slow PT pro- cess of HBO(H 2 O) is caused by competition between formations of intra- and intermolecular hydrogen bonds. Ó 2014 Elsevier B.V. All rights reserved. 1. Introduction The excited-state intramolecular proton transfer (ESIntraPT) is one of the most fundamental and important processes due to its photophysical properties [1,2]. The applications of ESIntraPT are found in many applications such as organic light emitting diodes [3], luminescent materials [4,5], and fluorescent probes [6,7]. Most of the ESIntraPT processes occur in molecules having a strong intramolecular hydrogen bond between the acidic proton (OAH, ANH 2 ) and the basic moiety (C@N, C@O), and the suitable geom- etry such as 2-(2 0 -hydroxyphenyl)benzoxazole (HBO) [8–15], 2-(2 0 -hydroxyphenyl)benzothiazole (HBT) [16–21], and 2- (2 0 -hydroxyphenyl)benzimidazole (HBI) [22,23]. Generally in the ground state, the enol form of these molecules is stable because the phenolic proton is not acidic enough to be deprotonated, implying that tautomerization does not occur in the ground state [14]. Upon photoexcitation, however, the proton transfer (PT) reac- tion from the excited enol occurs to give the excited keto tautomer in sub-picosecond timescale [8–15] (Scheme 1). Among these mol- ecules exhibiting ESIntraPT as shown in Scheme 1, HBO (in Fig- ure 1) is found to be an interesting system [8–15] because of its high quantum efficiency [21] compared to those of HBT and HBI. HBO comprises the hydroxyl group (OAH acting as a proton donor) and the benzoxazole group (a nitrogen atom acting as a proton acceptor). The OAH group of a proton donor and the N atom of an acceptor in HBO form an intramolecular hydrogen bond and exhibit a large Stokes shift arising from ESIntraPT [8–15]. To utilize HBO as organic light emitting and fluorescent probing materials, the complete understanding of its photophysical proper- ties of ESIntraPT in the gas phase and in solution is important [8,15,24,25]. Possible conformations of enol HBO have been identi- fied [12,24] as shown in Figure 1a: syn-HBO or I (intramolecular OAHN hydrogen bond), anti-HBO or II (intramolecular OAHO hydrogen bond), and opened HBO or III (no intramolecular hydro- gen bond) and only syn-HBO (I) can give the keto tautomer through the ESIntraPT process [4,8,12,13,24]. This process is known to occur in a femtosecond timescale [25] and the time constant of 60 ± 30 fs (in cyclohexane) of syn-HBO (I) using time-resolved spectrometry was reported [9]. It has been observed that in the presence of polar solvents, especially polar protic solvents, the for- mation of strong intermolecular hydrogen bond between HBO and solvents might play a significant role in changing its photophysical characteristics [8,15,24,25]. For solvated enol HBO, the formations of inter- and intramolecular hydrogen bonds between HBO and solvents are shown in Figure 1b: intramolecular hydrogen-bonded structure or IV (without an intermolecular hydrogen-bonded net- work), intermolecular hydrogen-bonded structure or V (water forming an intermolecular hydrogen-bonded network between the AOH and AN atom in the benzoxazole moiety), and open- ed-HBO or VI (intermolecular hydrogen bonding with water). Moreover, different PT pathways either ESIntraPT or excited-state intermolecular PT (ESInterPT) can occur depending on the initial structure. http://dx.doi.org/10.1016/j.cplett.2014.06.041 0009-2614/Ó 2014 Elsevier B.V. All rights reserved. ⇑ Corresponding author. Fax: +66 53 892277. E-mail address: naweekung@gmail.com (N. Kungwan). Chemical Physics Letters 609 (2014) 147–154 Contents lists available at ScienceDirect Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett