Theoretical Investigations of Excited State Proton Transfer in Selected Molecular Systems

Abstract

Excited state proton transfer (ESPT) plays a key role in various chemical and biological systems, including DNA where it plays a role in the deactivation mechanism. Compared to the ground state, molecules typically have low PT barriers on the excited state, so that the process occurs on ultrafast timescales. Another typical aspect is fluorescence from the PT tautomer that has a large Stokes shift with respect to the normal form [1,2]. This has led to the design and application of various molecules in areas like imaging cells, sensing, white light emitting materials, etc. In this thesis, the dynamics of ESPT on ultrafast timescales in selected molecular systems has been investigated. The first is excited-state intramolecular proton transfer (ESIPT) in 2- (oxazol-2 yl)-3-hydroxychromone (OHC), a derivative of the well-known 3 hydroxychromone [3,4] and flavone family. OHC has three low-energy conformers: OHC-A and B feature OH···O H-bond within the chromone ring but differ by 180o torsion of the oxazole ring, while OHC-C exhibits OH···N bonding between the chromone and oxazole rings and is the lowest energy conformer. The former pair has a bright S1 state but a close lying S2 dark state, while in the latter, S2 is the first bright state. With fewest switches surface hopping (FSSH) dynamics, we have investigated the dynamics and PT propensity of the conformers following photoexcitation as well as the anticipated fluorescence behaviour of the PT tautomers [5]. Protic solvents like water may play an active role by serving as a proton relay, affecting the PT barrier and hence PT propensity [6, 7]. Studies of water-assisted proton transfer have shown that the impact of water on proton transfer is system-dependent. In 3-hydroxyflavone, one water molecule facilitates PT, but a second hinders it [6]. In 8-hydroxyquinoline, the barrier drops drastically with one water, but rises again with the second [7]. For 2-(1H-pyrazol-5-yl)pyridine, proton transfer is absent in the isolated molecule but becomes feasible in the presence of one water molecule [8]. We have studied how one water molecule bridging the H-bonding sites in OHC-A, B and C affects the PT. By means of FSSH simulations, we have analysed the variety of dynamics in the OHC-water dimers. This includes near-concerted and stepwise double proton transfer as well as single proton transfer, influenced by the intermolecular motions of the OHC-water dimers. Molecules or dimers containing more than one H-bond open up the possibility of proton transfer at each site [8]. In this thesis, we revisit the excited state dynamics of [2,2’-bipyridyl]- 3,3’ diamine, or BP(NH2)2, that shows two H-bonds between amine NH and pyridine N atoms. While an experimental study [9] suggested ultrafast double proton transfer (DPT), subsequent computations and simulations [10, 11] suggested that only single proton transfer takes place. The present study complements the previous efforts; exploring the dynamics in detail, we find that some of the ultrafast dynamics may indeed be as previously theoretically anticipated. Chapter 1 of this thesis provides a brief overview of different photo excited processes including ESPT in different molecular systems. Chapter 2 briefly describes the theoretical methods used in this thesis, including different electronic structure methods and Fewest Switch Surface Hopping (FSSH) dynamics used to simulate and analyse the excited state dynamics. Chapter 3 explores the excited state energy landscape and ESIPT dynamics of the three conformers of the OHC molecule. OHC-B and C have an energy difference comparable to room temperature thermal energy, while the energy of OHC-A is slightly higher. OHC-A and B have lower PT barriers on the first bright state than OHC-C. Similar to previous theoretical results for 3-hydroxychromone [3,4], we also observed a conical intersection (CI) at short rOH distances. Additionally, another CI is located close to the PT minimum in the case of OHC-A and B. ESIPT dynamics of the three conformers are studied using FSSH simulations, which reveals the PT propensities, average PT time and fluorescence properties of the conformers [7]. Chapter 4 investigates the ESPT dynamics of OHC in the presence of single water molecule. FSSH dynamics of OHC-A and B-H2O dimer revealed diverse proton transfer pathways, including double proton transfer (DPT) and single proton transfer (SPT) events. DPT involves proton relay via water between donor and acceptor oxygen atoms. Two distinct types of DPT were observed: donor-first, which follows a near-concerted pathway, and acceptor-first, which proceeds via both sequential and near-concerted pathways. SPT, characterized by one-sided proton transfer, was sometimes associated with activation through O–H torsional motion. For OHC-A.H2O, the presence of water slightly enhances the PT propensity as compared to OHC-A, while OHC-B.H2O it is slightly reduced. For OHC-C.H2O, the PT propensity remains similar to OHC-C. Additionally, we see the possibility of intramolecular PT in this case. Overall, the nature of the excited states, PT propensity and the excited state dynamics are highly dependent on the orientation and movement of the water molecule during the dynamics of the OHC conformers with one water molecule. Chapter 5 presents the excited state potential profile and proton transfer dynamics of BP(NH2)2. Experimental work by Toele and Glassbeek using femtosecond fluorescence upconversion technique identified two emission bands, both decaying in 250 fs [9]. Previous theoretical studies by Ortiz-Sanchez et al.[10] and Chaihan and Kungwan[11] have tried to understand the origins of these two bands and excited state potential energy landscape using different methods. Our calculations revealed two ground state conformers with C2 and Ci symmetry, while only the Ci form exists in the S1 state. Our computational results show that SPT is feasible but DPT is unlikely, as previous theory work also found. Our FSSH simulations suggest that the 250 fs timescale seen in the experiment may indeed arise from the twisting of the bipyridyl rings over this timescale after SPT, as suggested in one of the earlier studies [10]. Chapter 6 summarizes the main findings of the thesis and highlights potential directions for future work. 1. A Weller, Naturwissenschaften 42, 175 (1955). 2. P K Sengupta and M Kasha, Chem. Phys. Lett. 68, 382 (1979). 3. A Perveaux, M Lorphelin, B Lasorne, D Lauvergnat. Phys. Chem. Chem. Phys. 19, 6579 (2017). 4. P Nag, S R Vennapusa. 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