Efficient Singlet‐State Deactivation of Cyano‐Substituted Indolines in Protic Solvents via CNHO Hydrogen Bonds

Abstract

The photophysical properties of indoline (I) and three of its derivatives, namely, N-methylindoline (MI), 5-cyanoindoline (CI), and 5-cyano-N-methylindoline (CMI), are studied in H-donating solvents of varying polarity. Based on measurements of fluorescence yield and lifetime, and of triplet yield and hydrated-electron formation, two distinct mechanisms of solvent-induced fluorescence quenching are evidenced. The first mechanism involves the cyano substituent and leads to an increase in the rate constant of internal conversion of one order of magnitude in ethanolic solution and of more than two orders of magnitude in water, as compared to solutions in n-hexane or acetonitrile. A similar trend had previously been observed in the case of 4-N,N-dimethylaminobenzonitrile (DMABN). The second mechanism reduces the fluorescence lifetimes of the non-cyanated derivatives in aqueous solution by one order of magnitude and is related to the formation of hydrated electrons. Neither of these mechanisms is influenced by methylation at the ring nitrogen. Quantum chemical calculations are performed on the ground and excited states of the hydrogen-bonded complexes between protic solvents and MI as well as CMI. Stable hydrogen-bonded configurations involving the CN substituent and a solvent OH group are found; these configurations are stable both in the ground and the first excited singlet states, whereas the corresponding complex at the ring amino nitrogen is stable in the ground state only. The CN--HO configuration is therefore a prime candidate for a mechanistic explanation of the observed quenching by the first mechanism. These findings may have useful applications for the design of fluorescence probes for water in biological systems.

Broadband fluorescence reveals mechanistic differences in excited-state proton transfer to protic and aprotic solvents

“…36 Actually, uorescence quenching by protic solvents has been reported for several organic dyes but the exact mechanism has not been fully elucidated. [43][44][45][46][47][48][49][50][51][52] One common factor in all cases is the presence of strong hydrogen bonds between the solute and the protic solvent. Moreover, the HB strength in most cases increases signicantly in the excited state as a result of intramolecular charge-transfer (CT) type excitation.…”

Remarkable fluorescence enhancement of benzo[g]chromen-2-ones induced by hydrogen-bonding interactions with protic solvents

“…Hydrogen bonding often affects the photodynamic properties of aromatic molecules as well as their static spectral properties. In particular, in aromatic carbonyl compounds, [1][2][3][4][5][6][7][8][9] nitrogen-containing heterocyclic compounds 10 and aromatic amines, [11][12][13] the rates of radiationless transitions (internal conversion and intersystem crossing) are remarkably affected by intramolecular and intermolecular hydrogen-bonding interactions on the carbonyl oxygen, nitrogen hetero-atom, or amino groups. We have demonstrated that aniline derivatives with an electron-withdrawing group at the ortho-or meta-position exhibit rapid internal conversion through intermolecular hydrogen-bonding interactions with protic solvent molecules.…”

Kutatómunka a BME Fizikai Kémia és Anyagtudományi Tanszékén

“…Az 5-ciano-N-metilindolin esetében igazoltuk, hogy a cianocsoport és a protikus oldószerek között hidrogénkötés alakul ki, amely gerjesztett állapotban is stabil. 23 A jelenség kioltást okoz, ami a lokális víztartalom fluoreszcenciás kimutatására ad lehetõséget. A níluskék, az oxazin 720 és a kumarin 102 esetében a sav-bázis tulajdonságok változását vizsgáltuk gerjesztett állapotban.…”