Dynamics of excited state proton transfer in nitro substituted 10-hydroxybenzo[h]quinolines

Abstract

The ground state tautomerism and excited state intramolecular proton transfer (ESIPT) of 10-hydroxybenzo[h]quinoline (HBQ) and its nitro derivatives, 7-nitrobenzo[h]quinolin-10-ol (2) and 7,9-dinitrobenzo[h]quinolin-10-ol (3), have been studied in acetonitrile using steady state as well as time dependent spectroscopy and quantum-chemical calculations. In addition to the enol form absorbance in the range 360-390 nm, the absorption spectra of 2 and 3 exhibit a red shifted band at ∼450 nm. Chemometric data processing, based on individual band decomposition, allowed us to estimate the position of the ground state enol-keto tautomeric equilibrium (ΔG values of 1.03 and 0.62 kcal mol respectively for 2 and 3). The fluorescence stems from the keto form even if the enol form is optically excited as proven by the shape of the excitation spectra indicating that ESIPT takes place. The Stokes shift of the substituted compounds is substantially lower compared to HBQ, which follows from the fact that the substitution occurs in the formal cyclohexa-2,4-dienone moiety and leads to a decrease of the HOMO level of the keto tautomer. The pump-probe experiments show that in the nitro substituted HBQs 2 and 3 ESIPT occurs with a time constant of 0.89 ps and 0.68 ps, respectively. In both cases a mixture of the enol and proton transfer forms is optically excited. The enol form exhibits then the ESIPT and subsequently both fractions take the same relaxation path. We propose that in 2 and 3 the ESIPT path exhibits a potential energy barrier resulting in an incoherent rate governed process while in HBQ the ESIPT proceeds as a ballistic wavepacket motion along a path without significant barriers. The theoretical calculations (M06-2X/TZVP) confirm the existence of a barrier in the ground and excited states as result of the substitution.

Indirect solvent assisted tautomerism in 4-substituted phthalimide 2-hydroxy-Schiff bases

“…The M06-2× functional was selected by a benchmarking procedure, which has shown that there is good empirical relations between the relative stability of the tautomers, defined as ΔE, and the experimentally determined ΔG o values of set of tautomeric dyes in cycloxexane [52,53]. This qualitative approach shows very good results in predicting the position of tautomeric equilibria for compounds with intramolecular hydrogen bonds as well as describing ground and excited state proton transfer mechanisms [54][55][56][57][58]. Although the computationally ΔG o values should be preferable, their accuracy is under question even at high level calculations for compounds of the size used in the current study [59,60].…”

8-(Pyridin-2-yl)quinolin-7-ol as a Platform for Conjugated Proton Cranes: A DFT Structural Design

“…The M06-2X functional was selected for three reasons. First, it has shown that there is good empirical relations between the relative stability of the tautomers, defined as ΔE, and the experimentally determined ΔG o values of tautomeric azo and azomethyne dyes in cyclohexane and acetonitrile [ 50 , 58 , 59 ], and of heterocyclic tautomers [ 49 , 53 ]. Second, the use of M06-2X provides very good predictability of the E/Z isomerization ratio in some rotary switches [ 51 , 60 ].…”

“…The theoretical design of such proton transfer-based systems faces many challenges related to correct prediction of the ground state tautomerism, solvent effects and excited state dynamics upon irradiation. Recently, we have been able to accumulate knowledge about using density functional theory (DFT) calculations for correct prediction of the ground and excited state tautomerism of dyes and heterocycles [ 49 , 50 , 51 , 52 , 53 ], which gives a good opportunity to attempt the design of proton cranes. 8-(Pyridin-2-yl)quinolin-7-ol, where 7-hydroxy quinoline ( 4 ) serves as a tautomeric sub-unit, has been selected as a platform to investigate the effect of the substituents in the pyridine crane sub-unit ( Scheme 4 ).…”

A theoretical study on water-assisted excited state double proton transfer process in substituted 2,7-diazaindole-H2O complex

“…Recently, the substituent effect on proton transfer process has aroused great interest of many researchers [59][60][61][62][63]. A large number studies showed that ESPT dynamics was closely correlated with the substituent properties and substituted positions.…”