Theoretical insights into excited-state hydrogen bonding effects and intramolecular proton transfer (ESIPT) mechanism for BTS system

Abstract

In this work, N,N'-bis(salicylidene)-(2-(3′,4′-diaminophenyl)benzothiazole) (named as "BtS") system was studied about its excited-state intramolecular proton transfer (eSipt) process. the analyses about reduced density gradient (RDG) reveal the formation of two intramolecular hydrogen bonds in BtS system. Bond lengths and angles, infrared (IR) vibrations as well as frontier molecular orbitals (MOs) using tDDft method indicate that the strength of hydrogen bond should be enhanced in the S 1 state. Particularly, hydrogen bond O1-H2···N3 undergoes larger variations compared with O4-H5···N6, which infers that hydrogen bond O1-H2···N3 may play a decisive role in the ESIPT process of BTS system. Given the two hydrogen bonds of BTS molecule, two types of potential energy curves have been constructed, which confirms that only single proton transfer process occurs due to lower energy barrier along with O1-H2···N3 rather than O4-H5···N6. This work not only presents a reasonable explanation for previous experiment, but also clarifies the specific ESIPT mechanism for BTS system.As one of the most fundamental weak interaction, intra-as well as inter-molecular hydrogen bond is illocal in nature 1-3 . Proton transfer (PT), as the elementary class of photochemical and photophysical domains happening along with pre-existing hydrogen bond, has attracted much attention during the last few decades 4-8 . Upon photoexcitation, excited-state intra-(inter-) molecular proton transfer (ESIPT) process is the initial step of many photobiological and photochemical reactions, which is crucial in nature. Due to the transient properties, ESIPT processes have been adopted in several applications recently including molecular logic gates, UV filters, fluorescence sensors, etc. 9-18 . By the light of nature, both applied and cognitive attention have been paid to ESIPT phenomenon, which becomes a demanding subject of research [19][20][21][22][23][24][25] .Generally, ESIPT means the transfer of a hydroxyl (or amino) proton to an oxygen (or nitrogen) acceptor via pre-existing hydrogen bonds. Upon photoexcitation, an unstable position of proton is resulted from the projection of the nuclear wave function of molecule on the excited-state potential energy surface (PES) [26][27][28][29][30][31][32][33][34][35] . The driving force for ESIPT is provided by the energy gap between the initial and relaxed excited states. Demonstrating by the mirror symmetry between absorption and emission spectra, the nuclear configuration of the target molecule remains close to that of the ground state over the excited-state lifetime. The mirror symmetry should be broken up by the influence of ESIPT on the Franck-Condon factors [26][27][28][29][30][31][32][33][34][35] . The proton-transfer tautomer emits fluorescence at longer wavelength and results in larger Stokes shifts.As far as we know, single ESPT process may not be sufficient to investigate the complex hydrogen bonding behaviors in biological fields, since more and more photo-induced mutations refer to multiple protons...

A study of the photochemical behavior of terarylenes containing allomaltol and pyrazole fragments

“…The plausible route for the formation of α-hydroxy-1,2-diketone 14a is presented in Scheme 4 . Based on literature data we suppose that the photoreaction proceeds through an excited state intramolecular proton transfer (ESIPT) [ 34 – 36 ]. At first, compound 12a under UV irradiation undergoes rapid proton transfer from the excited state A* resulting in the formation of photoisomer B* followed by the conversion to intermediate C .…”

Modern Theoretical Approaches to Modeling the Excited-State Intramolecular Proton Transfer: An Overview

“…It might also be considered a lower-cost computational option for large polyatomic molecules as this protocol, compared to the dynamic ones, usually involves a relatively limited number of demanding energy-gradient calculations and facilitates further savings by allowing calculations under fixed system symmetry, if such is present. Typical outcomes of the static approach are absorption and emission vertical electronic energies [ 33 , 34 , 35 , 36 ], upper-bound estimations for possible energy barriers in the ground and electronically excited states [ 36 , 37 , 38 ], and detailed characterization of these states in terms of symmetry and orbital configuration [ 38 , 39 ]. Moreover, the number of other molecular features complementing the experimental ESIPT characterization can be determined, including, e.g., tautomers relative energies [ 35 , 40 ], atomic charges [ 41 , 42 , 43 ], and vibrational modes attribution [ 44 , 45 ].…”

“…In this case, again one needs to make a compromise between the computational cost and desired accuracy. Most common recent choices in ESIPT studies seem to be favoring the cc-pVTZ [ 91 ] basis set from the Dunning family on the one hand [ 3 , 33 , 39 , 40 , 48 ], and different variants of the Pople 6-311 G(d,p) [ 92 ] basis set, on the other [ 36 , 72 , 73 ]. The latter direction finds its support in a general study by Laurent et al [ 93 ], in which the basis-set effect on vertical excitation energy calculations was investigated.…”

“…The latter approach, particularly popular recently [ 33 , 37 , 73 , 102 ], has been applied e.g. by Wang et al to the BTS system [ 39 ] in methylene chloride, yielding very high accuracy predictions for excitation and emission wavelengths, with divergence from the experimental values measured in just a few nm. In addition to the general purpose methods listed above, state-specific PCM treatments of correlated linear response (cLR) [ 103 ] and the vertical excitation model within the unrelaxed density approximation (VEM-UD) [ 104 , 105 ] have been successfully applied to study ESIPT by Vérité et al [ 40 ], who pointed out the advantages that these approaches bring for the description of charge-transfer states in ESIPT reactions.…”

Structural and Energetic Insights on Two Dye Compounds: 1-Acetyl-2-Naphthol and 2-Acetyl-1-Naphthol

“…The most remarkable photophysical property of the ESIPT chromophores is the large fluorescence Stokes shift. The presence of this property in compounds showing ESIPT phenomena, together with the dual emission makes them quite exploited as OLED materials, spectroscopy tools, and fluorescence probes in biology [ 9 , 10 , 11 , 12 , 13 ]. However, the ESIPT process in 2-acetyl-1-naphthol has been in controversy for long, primarily because of its quantitative ESIPT conversion in most solution conditions and also due to small Stokes shift of the ESIPT products (keto type prototautomer) compared to those products in other typical ESIPT molecules [ 1 , 14 ].…”