DUAL EMISSION OF 2-AMINO-4-METHYLPYRIMIDINE: A THEORETICAL STUDY
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DUAL EMISSION OF 2-AMINO-4-METHYLPYRIMIDINE: A THEORETICAL STUDY N. A. Shekhovtsov1,2*, K. A. Vinogradova2, E. B. Nikolaenkova3, V. P. Krivopalov3, and M. B. Bushuev2
DFT and TD-DFT calculations at the B3LYP/LANL2DZ level of theory are performed in order to explain the origins of dual fluorescence and uncover the relaxation pathways of 2-amino-4-methylpyrimidine (L). The computations are in good agreement with experimental data. A high-energy emission band in the photoluminescence spectrum of 2-amino-4-methylpyrimidine (λmax = 350 nm) is related to the fluorescence of monomer L. Comparison of calculated and experimental IR and absorption spectra reveals that in the solid state L molecules form supramolecular {N–H···N}n hydrogen bonded associates. The formation of such associates favors the excited state double proton transfer (ESDPT). Calculated 2D potential energy curves for the ground and first singlet excited states of dimeric associates L2 indicate the presence of two stable forms, L2-amino and L2-imino. The low-energy band in the emission spectrum of 2-amino-4methylpyrimidine (λmax = 445 nm) is probably associated with the fluorescence of the L2-imino form. Both fluorescence bands have a mixed π–π*, n–π* character as is evidenced by frontier orbitals involved in S1–S0 transitions. DOI: 10.1134/S0022476620100042 Keywords: dual emission, pyrimidine, hydrogen bond, photoisomerization, excited state double proton transfer, supramolecular chemistry.
INTRODUCTION Excited state proton transfer (ESPT) is one of the most important and most studied photoreactions in organic chemistry and photochemistry [1-3]. Compounds comprising proton donor and acceptor groups linked by short intra- or intermolecular hydrogen bonds favor the ESPT process. Amino or hydroxyl groups are common proton donors whereas carbonyl oxygen atoms or pyridinyl nitrogen atoms are common proton acceptors. One of the most useful consequences of such phototautomerization in ESPT compounds is their dual excitation wavelength dependent fluorescence: the high-energy emission band originates from the “normal” form while the low-energy emission band arises from a phototautomer formed as a result of the ESPT process. Such a unique property opens up prospects for designing smart luminescent materials which can be used in sensors [4], optoelectronic devices [5], biological labelling [6], information encryption/anti-counterfeiting [7].
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Novosibirsk State University, Novosibirsk, Russia; *[email protected]. 2Nikolaev Institute of Inorganic Chemistry, Siberian Branch, Russian Academy of Sciences, Novosibirsk, Russia. 3Vorozhtsov Institute of Organic Chemistry Siberian Branch, Russian Academy of Sciences, Novosibirsk, Russia. Original article submitted March 29, 2020; revised May 2, 2020; accepted May 4, 2020. 0022-4766/20/6110-1521 © 2020 by Pleiades Publishing, Ltd.
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Compared to excited state single proton transfer, excited state double proton transfer (ESDPT) is a rather rare phenomenon which occurs in a limited group of compounds [1, 8]. For instance,
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