Article pubs.acs.org/JPCB
Solvent-Induced Changes in Photophysics and Photostability of Indole-Naphthyridines Barbara Golec,† Michał Kijak,† Volha Vetokhina,† Alexandr Gorski,† Randolph P. Thummel,‡ Jerzy Herbich,*,†,§ and Jacek Waluk*,†,§ †
Institute of Physical Chemistry, Polish Academy of Sciences, Kasprzaka 44/52, 01-224 Warsaw, Poland Department of Chemistry, University of Houston, Houston, Texas 77204-5003, United States § Faculty of Mathematics and Science, Cardinal Stefan Wyszyński University, Dewajtis 5, 01-815 Warsaw, Poland ‡
S Supporting Information *
ABSTRACT: Molecules that can simultaneously act as hydrogen bond donors and acceptors often exhibit completely different photophysical behavior in protic and aprotic solvents. Formation of multiple hydrogen bonds with, for example, water or alcohols, may lead to enhanced internal conversion; as a result, triplet formation efficiency can be reduced. These changes in photophysical characteristics may influence the photostability. In order to check this hypothesis, we have investigated spectroscopy, photophysics, and changes in photostability caused by interaction with aprotic and protic solvents for 2-(1′H-indol-2′-yl)-[1,5]naphthyridine and 2-(1′H-indol-2′-yl)-[1,8]naphthyridine, molecules with hydrogen bond accepting and donating functionalities. The photostability of these compounds in n-hexane, acetonitrile, and alcohols was studied in the regime of 365 nm irradiation. The photodegradation yield was found to be significantly lower in alcohols. In polar and protic solvents, the presence of two species was detected and attributed to syn and anti rotameric forms; the former are dominant in all environments.
1. INTRODUCTION Investigations of bifunctional azaaromatic compounds that simultaneously possess a hydrogen bond donor (D) and acceptor (A) moieties demonstrate that the photophysical properties of these systems may be strongly affected by formation of a hydrogen bond (HB).1 A particular class of such compounds consists of molecules in which the HB donor and acceptor are located in separate moieties, linked by a single bond. The examples include 2-(2′-pyridyl)indole (2-PI),1−5 2(2′-pyridyl)pyrrole (PP)2,3,6 and its derivatives,7,8 and 2-(1′Hpyrazol-5′-yl)pyridines (PPPs).9−11 Such topology often leads to a solvent-dependent appearance of two rotamers, syn and anti, with the two HB centers located either on the same or on the opposite sides of the molecule. These two species may reveal drastically different behavior in the excited state.1−6,11 The lower energy syn rotamer dominates in nonpolar solvents and in supersonic jets.12,13 Only if the arrangement of the proton acceptor and donor sites is favorable, the syn form may undergo excited-state intramolecular proton transfer (ESIPT).2,3,6,9−11,14−16 In polar aprotic solvents, a small fraction of the anti form can appear. In alcohols, the population of this form may become significant, owing to stabilization caused by a larger ground state dipole moment and more linear, and thus stronger hydrogen bonds with solvent molecules than in the syn species.2,5 It has been reported that the excited state in the syn conformer is rapidly depopulated in alcohols and © XXXX American Chemical Society
water because of a formation of intermolecular cyclic hydrogenbonded complexes.5,11,17 The mechanism of deactivation may involve either strongly enhanced S1 → S0 internal conversion or excited-state double proton transfer (ESDPT). ESDPT proceeds in 1:1 complexes in single picoseconds or faster. The enhanced internal conversion occurs in complexes with more than one molecule of alcohol (1:n complex, n ≥ 2) and is usually slower than ESDPT, from tens to hundreds of picoseconds.1 Thus, an indication of the dominant presence in alcohols of complexes other than 1:1 cyclic ones is the observation of quenching of the primary emission and lack of tautomeric fluorescence. A consequence of efficient depopulation of the excited state via proton transfer or hydrogen bonding is the decrease of quantum yield of other deactivation channels. Formation of multiple hydrogen bonds with, for example, water or alcohols may lead to enhanced internal conversion; as a result, the yield of triplet formation should be reduced. This in turn could influence the photostability, because photodestruction preferentially occurs from the triplet state. Special Issue: John R. Miller and Marshall D. Newton Festschrift Received: October 29, 2014 Revised: January 7, 2015
A
DOI: 10.1021/jp510846w J. Phys. Chem. B XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry B
quantum yield, the sample and reference solutions had identical absorbance values at the excitation wavelength. Fluorescence decay lifetimes in a nanosecond range were determined by time-resolved single photon counting (SPC): (i) on an Edinburgh FL 900 CDT spectrofluorometer, estimated time resolution of about 500 ps, and (ii) on a home-built SPC setup (excitation by IBH Nanoled emitting at 297 nm, pulse width
