Title: The effect of hydrogen bond strength on emission properties in 2-(2’-hydroxyphenyl)imidazo[1,2-a]pyridines
Author: Anton J. Stasyuk Patrick Bultinck Daniel T. Gryko
Michał K. Cyran´ski
Reference: JPC 9988
To appear in: Journal of Photochemistry and Photobiology A: Chemistry
Received date: 9-5-2015
Revised date: 3-8-2015
Accepted date: 16-8-2015
Please cite this article as: Anton J.Stasyuk, Patrick Bultinck, Daniel T.Gryko, Michal
K.Cyran´ski, The effect of hydrogen bond strength on emission properties in 2-(2’hydroxyphenyl)imidazo[1,2-a]pyridines, Journal of Photochemistry and Photobiology
A: Chemistry http://dx.doi.org/10.1016/j.jphotochem.2015.08.013
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The effect of hydrogen bond strength on emission properties in 2-(2'-hydroxyphenyl)imidazo[1,2a]pyridines
Anton J. Stasyuk,a,b Patrick Bultinck,c Daniel T. Gryko*d and Michał K. Cyrański*b a Warsaw University of Technology, Noakowskiego 3, 00-664, Warsaw, Poland b University of Warsaw, Faculty of Chemistry, Pasteura 1, 02-093 Warsaw, Poland. Tel: +48 22 8220211, E-mail: email@example.com c Department of Inorganic and Physical Chemistry, Ghent University, Krijgslaan 281 (S3), 9000
Ghent, Belgium d Institute of Organic Chemistry Polish Academy of Sciences, Kasprzaka 44/52 01-224, Warsaw,
Poland. Fax: +48 22 632 66 81; Tel: +48 22 343 30 63, E-mail: firstname.lastname@example.org
Highlights The emission maxima of derivatives of 2-(2'-hydroxyphenyl) imidazo[1,2-a]pyridine depend on the strength of the hydrogen bond. Improved predictions of the fluorescence characteristics can be achieved using the
QTAIM approach. 2 Easily calculated HOMA indexes based just on geometry make it possible to predict the emission wavelength of 2-(2'-hydroxyphenyl) imidazo[1,2-a]pyridine derivatives.
The correlation between the structures and molecular properties on the one hand and the fluorescence properties on the other for a set of 2-(2'-hydroxyphenyl)imidazo[1,2-a]pyridines exhibiting excited-state intramolecular proton transfer (ESIPT) was analyzed via comparison of the optical data and computational results. Ab initio calculations were used to characterize the electronic transitions of 2-(2'-hydroxyphenyl)imidazo[1,2-a]pyridine and its three derivatives both in non-polar and polar solvents by using DFT and TDDFT methods and the IEFPCM model. The calculations involving geometry optimization in the excited state allow to precisely predict the emission wavelength from the excited keto state. We constructed and studied the correlations between the positions of the emission maximum for the compounds considered and the hydrogen bond energies values for ρ(r) and 2ρ(r) at the hydrogen bond critical point and the aromaticity of chosen structural fragments. It was found that the fluorescence properties of
ESIPT molecules could be manipulated by directed modification of their chemical structure, most importantly by the introduction of electron-withdrawing functionalities into their proton donor moieties. The electron-withdrawing –CN group hypsochromically shifts the emission from the keto excited state by ~60 nm and strengthens at the same time the intramolecular hydrogen bond.
Keywords: hydrogen bond, DFT-calculations, fluorescence, ESIPT, imidazo[1,2-a]pyridine 3 1. Introduction.
Proton transfer is an important and fundamental process in chemistry, biology and related fields.1-8 Proton transfer gains in importance if it is associated with a photonic excitation. In the last few decades, there has been an increasing interest in design and characterization of compounds that exhibit the excited-state intramolecular proton transfer (ESIPT)9-12 phenomenon because of their numerous practical applications e.g. such as UV-photostabilizers,13-17 fluorescent solar concentrators18-20 and last but not least laser dyes.21-24 Typical compounds that display ESIPT possess a skeleton of benzoxazoles,25-30 benzothiazole,31-37 flavones,21,38,39 10hydroxybenzo[h]quinoline40-43 or 2-pyridil-pyrazoles.44,45 Due to the large Stokes shift they have found applications as probes for solvation dynamics,46 fluorescent sensors,47-50 probes for biological environments51 and recently organic light emitting devices.52,53 Perhaps the most spectacular example of practical application of ESIPT is the use of 2-(2'-hydroxy-5methylphenyl)benzotriazole (TINUVIN P) as stabilizer in plastics.54-57 Most of them form a sixmembered ring based on a strong intramolecular hydrogen bond interaction between a hydrogen atom donor group (e.g. –OH, -NH2) and an acceptor (most often =N-, >C=O) although five–58-60 or seven-membered61-62 rings are also known.
It has been widely accepted that the driving force for the ESIPT reaction stems from the significant difference between the acidity and basicity of the involved groups.63-67 It is important to note that ESIPT molecules are mostly stable in the enol form (EnS0) in the ground state S0. An excitation from this state with UV irradiation leads to the so-called Franck-Condon S1 excited state (EnS1), which leads via proton transfer to the tautomer species S1 often named the keto form (KeS1). Thus the absorption EnS0 → EnS1 and the emission KeS1 → KeS0 results in a drastically and anomalously large Stokes shift (see Scheme 1). 4
The dynamics of the ESIPT reaction is determined by the shape of the double well potential energy surface (PES),67-70 and depends on the system. The PES can be barrierless along the reaction coordinate or the reaction may proceed either as proton tunneling, or as part of intramolecular vibration redistribution.66,68,71