A DFT-Based Theoretical Study on the Photophysics of 4-Hydroxyacridine: Single-Water-Mediated Excited State Proton Transfer Bijan Kumar Paul, Subrata Mahanta, Rupashree Balia Singh, and Nikhil Guchhait* Department of Chemistry, UniVersity of Calcutta, 92 A. P. C. Road, Kolkata 700009, India ReceiVed: September 18, 2009; ReVised Manuscript ReceiVed: December 30, 2009 Study of intra- and intermolecular hydrogen-bonding interaction and excited state proton transfer reaction has been carried out in 4-hydroxyacridine (4-HA) and its hydrated clusters theoretically. Density functional theory [B3LYP/6-311++G(d,p)] has been exploited to calculate structural parameters and relative energies of different conformers of 4-HA and its hydrates. The substantial impact of solvent reaction field on hydrogen- bond energies, conformational equilibrium, and tautomerization reaction in aqueous medium have been realized by employing Onsager and PCM reaction field methods, and the stability of the conformers of 4-HA is found to be profusely modulated by the electrostatic influence of the solvent. A deeper insight into the nature of H-bonding in 4-HA and its hydrated clusters has been achieved under the provision of natural bond orbital and atoms in molecule analysis. Elucidation of potential energy curves for proton transfer reaction reveals that an intrinsic and two-water-molecule-assisted proton transfer (PT) reaction in 4-HA is hindered by high energy barrier in the S 1 surface, whereas single-water-assisted PT reaction is practically rendered barrierless. At the same time, the appreciably high barrier height of the ground state potential energy curve in all the cases unambiguously rules out the possibility of ground state proton transfer reaction. 1. Introduction Proton transfer (PT) reactions and hydrogen bonds in heterocycles play a vital role in many chemical and biological systems. 1-5 An interesting type of proton transfer in aqueous solution is one in which one or more solvent water molecules can mediate the process by serving as a bridge that connects the donor and acceptor sites. These water molecules stabilize the transition state and thereby substantially lower the classical energy barrier for proton transfer reaction. Such phenomenon has been postulated in the action of enzymes (e.g., carbonic acid anhydrase 6 ) as well as in other tautomerization reactions. 7,8 The phenomenon of solvent-assisted tautomerization has grabbed attention over the years both from experimental and theoretical perspectives. 9-15 Simons and co-worker 16 have found that the activation energy barrier of formamide is lowered by 26.0 kcal/ mol by adding simply one water molecule. Because of the light mass of the proton, quantum mechanical tunneling is very important in these reactions, and the shape of the potential energy surface (PES) has an influence on the tunneling prob- ability. Truong and co-workers have calculated tunneling probabilities and rate constants for the double proton transfer in water-assisted tautomerization and found that the tunneling effect is very large, which lowers the barrier by about 4.60 kcal/ mol. 17 Double proton transfer, quite naturally by virtue of its potential relevance to biological systems 6 and diagnosis of many chemical reactions, 7,8 has long been an active topic of research for theoreticians as well as experimentalists. 18-25 Recently, Li et al. 15 used HF, DFT, and CASSCF methods and showed that 8-hydorxyquinoline (8-HQ) can show more facile excited state proton transfer (ESPT) in the presence of a single water molecule compared to that in the absence of the same. In the present study, we are interested in the proton transfer reaction in 4-hydroxyacridine (4-HA), a molecule very similar to 8-HQ and 7-hydroxy-1-indanone (7HIN). 16-18 The molecule acridine is an aza-polycyclic aromatic hydrocarbon found in the partial combustion of fossils fuels and tobacco and also detected in motor vehicle exhaust emissions, cigarette smoke, shale oil, coal tar, and coal liquefaction products. 26 Our target molecule, 4-HA, is a substituted product of the parent molecule, acridine. A detailed photophysical study and characterization of 4-HA is demanding due to its wide range of potential applications as a good chelating agent in analytical chemistry. The cyclopal- ladated complex of 4-HA is known to have anticancer activity, 27 and metal complexes of 4-HA are known to serve as the intercalators for DNA, 27 etc. Very recently, we have reported a spectral study of 4-HA 24 and shown that photophysical proper- ties are reasonably distinct from other studied related molecules having a six-membered intramolecular hydrogen-bond (IMHB) ring between the proton donor and acceptor systems. 11-13 The molecule 4-HA, having a five-membered IMHB ring imparts a strain to the hydrogen-bonded site, and hence, the hydrogen bond is weak in nature. However, the most interesting observa- tion is that the molecule 4-HA shows a solvent-water-mediated ESPT reaction. 24 The principal objectives of the present work is thus to carry out a detailed theoretical analysis of the photophysics of 4-HA with particular attention being inclined along a solvent-water-assisted ESPT reaction. Density functional theory has been implemented to calculate the ground-state structural parameters, relative energies of different conformers of 4-HA, and tautomers of hydrates of 4-HA. Atoms in molecule (AIM) and natural bond orbital (NBO) analysis have been performed to get a better understanding of the hydrogen-bonding interaction. The ground- and excited-state potential energy surfaces (PESs) have been evaluated along the proton transfer coordinate for the bare molecule and its hydrated clusters to ascertain the feasibility of the process. Several transition state structures proposed for the tautomerization processes have also been examined carefully. * To whom correspondence should be addressed. Fax: +91 33 2351 9755. E-mail: nguchhait@yahoo.com. J. Phys. Chem. A 2010, 114, 2618–2627 2618 10.1021/jp909029c  2010 American Chemical Society Published on Web 02/01/2010