Photophysical Properties of Coumarin-30 Dye in Aprotic and Protic Solvents of Varying Polarities¶

Abstract

Experimental results on various photophysical properties of coumarin-30 (C30) dye, namely, Stokes' shift (Deltanu), fluorescence quantum yield (Phi(f)), fluorescence lifetime (tau(f)), radiative rate constant (k(f)) and nonradiative rate constant (k(nr)), as obtained using absorption and fluorescence measurements have been reported. Though in most of the solvents the properties of C30 show more or less linear correlation with the solvent polarity function, Deltaf = [(epsilon - 1)/(2epsilon + 1) - (n(2) - 1)/(2n(2) + 1)], they show unusual deviations in nonpolar solvents at one end and in high-polarity protic solvents at the other end. From the solvent polarity and temperature effect on the photophysical properties of the dye, following inferences have been drawn: (1) in nonpolar solvents, the dye exists in a nonpolar structure, where its 7-NEt(2) substituent adopts a pyramidal configuration and the amino lone pair is out of resonance with the benzopyrone pi cloud; (2) in medium to higher polarity solvents, the dye exists in a polar intramolecular charge transfer structure, where the 7-NEt(2) group and the 1,2-benzopyrone moiety are in the same plane and the amino lone pair is in resonance with the benzopyrone pi cloud; (3) in protic solvents, the dye-solvent intermolecular hydrogen bonding influences the photophysical properties of the dye; and (4) in high-polarity protic solvents, the excited C30 undergoes a new activation-controlled nonradiative deexcitation process because of the involvement of a twisted intramolecular charge transfer (TICT) state. Contrary to most other TICT molecules, the activation barrier for this deexcitation process in C30 is observed to increase with solvent polarity. A rational for this unusual behavior has been given on the basis of the solvent polarity-dependent stabilization and crossing of relevant electronic states and the relative propensity of interconversion among these states.

Mechanistic Study of the Formation of Bright White Light-Emitting Ultrasmall CdSe Nanocrystals: Role of Phosphine Free Selenium Precursors

“…Coumarin 30 displays its UV-vis absorption peak at 407 nm, and when excited at 380 nm it emits at 482 nm with a QY of 55.3% in acetonitrile. 76 All samples were prepared in toluene and the optical densities of the samples were kept to similar levels (~0.08-0.1). The emission data were collected from 400 nm to 700 nm.…”

“…The PLQYs of the CdSe(OLA) and CdSe(BA/OLA) nanocrystals were determined to be 11.2 and 26.9 %, respectively, using Coumarin-30 in acetonitrile (PLQY = 55.3 %) as the standard. 76 An ~27% PLQY is the highest value achieved for single-component, white light-emitting ultrasmall CdSe nanocrystals without any post-synthetic surface modification. 34 It is known that when semiconductor nanocrystals are synthesized in the presence of anionic surfactant, the resulting nanocrystals are metal rich.…”

Effect of H‐Bonding on the Photophysical Behaviour of Coumarin Dyes

“…(see Scheme 20.3 for molecular structures), behave quite unusually in solvents with relatively high polarity. Thus, in highly polar solvents, these molecules show a marked reduction in fluorescence quantum yields and lifetimes, and subsequently a sharp increase in their non-radiative decay rate [22,23,36,37,[39][40][41][63][64][65].…”

“…In this twisted ICT (TICT) state, total charge separation between the dialkylamino and carbonyl moieties takes place because the system is decoupled with a zero overlap of the orbitals involved in the conjugation of the ICT state (see Scheme 20.4). There are enormous numbers of studies reported in the literature to support the formation of the TICT state in coumarin dyes in high-polarity solvents [22,23,36,37,[39][40][41][63][64][65]. As the TICT state is highly polar in nature, in very high-polarity solvents the TICT state can become lower in energy than the planar ICT state.…”

“…Because of their extensive use as fluorescence probes, it is extremely important to understand the nature of interactions that take place with the coumarin dyes in different solvent systems. For about one decade or so, our research group has been actively involved in studying the photophysical properties of coumarin derivatives in different solvents, and also in different microheterogeneous media [32][33][34][35][36][37][38][39][40][41][42][43][44][45][46][47][48][49][50]. The aim of all these studies is to understand the specific interactions pertaining to these systems in homogeneous media, as well as to understand the nature of the solvation behaviour in different microheterogeneous media.…”