Ultrafast hydrogen bond dynamics and partial electron transfer after photoexcitation of diethyl ester of 7-(diethylamino)-coumarin-3-phosphonic acid and its benzoxaphosphorin analog.

M S Wagner,R D Nikolova,H Iglev,E D Ilieva,R Kienberger,P St Petkov

doi:10.1039/c4cp05727a

Abstract

The solvation dynamics after optical excitation of two phosphono-substituted coumarin derivatives dissolved in various solutions are studied by fluorescence up-conversion spectroscopy and quantum chemical simulations. The Kamlet-Taft analysis of the conventional absorption and emission spectra suggests weakening of the solvent-solute H-bonds upon optical excitation, which is in contrast to the results gained by the quantum simulations and earlier studies reported for coumarin derivatives without phosphono groups. The simulations give evidence that the solvent reorganisation around the excited fluorophore leads to partial electron transfer to the first solvation shell. The process occurs on a timescale between 1 and 10 ps depending on the solvent polarity and leads to a fast decay of the time-resolved emission signal. Using the ultrafast spectral shift of the time-dependent fluorescence we estimated the relaxation time of the H-bonds in the electronically excited state to be about 0.6 ps in water, 1.5 ps in ethanol and 2.8 ps in formamide.

Highlights

hydrogen bonds (H-bonds) to the solute (a, donor acidity) or to accept H-bonds from the solute (b, acceptor basicity): n = n0 + sÁp* + aÁa + bÁb
We report on the solvation properties and H-bonding dynamics around two phosphono-substituted 7-aminocoumarins in the electronically excited state using steady-state and time-resolved fluorescence spectroscopy, as well as quantum chemical simulations
The conventional absorption and fluorescence spectra recorded for compounds 1 and 2 dissolved in several different solvents were analyzed using the Kamlet–Taft technique

Summary

Introduction

The absolute values of these coefficients give qualitative information on the strength of the accepted (a) or donated H-bonds (b) The change in these molecular properties during the solvation process following optical excitation can be traced by the comparison of s, a and b extracted from absorption and emission spectra. Merging these coefficients with computer simulations and time-resolved spectroscopy provides a detailed microscopic picture of the solvation dynamics in the electronically excited state.[16,23,24]. The electron transfer time and H-bonding dynamics after optical excitation are studied by femtosecond fluorescence up-conversion spectroscopy

Experimental

Computational details

Steady-state absorption and fluorescence

Time-resolved data

Findings

Conclusions