Inhomogeneous Broadening of Electronic Spectra of Dye Molecules in Solutions

Abstract

It is well known in spectroscopy of complex organic molecules that the large width of their absorption and emission bands is largely due to the existence of a continuous set of vibrational sublevels in each electronic state. The spectroscopic properties of dye molecules in solution are in addition influenced by the surrounding medium.(1-3) For several decades it had been believed that because of the fast energy exchange between the vibrational sublevels the fluorescence spectrum of organic dye in solution was independent of the frequency of the exciting light. In 1970 it was shown (4) for the first time that apart from molecular vibrations there is another cause of the substantial broadening of electronic spectra of organic molecules in solution, namely, the fluctuations of the structure of the solvation shell surrounding the molecule. The variation of the local electric field caused by the fluctuation of the shell structure leads to a statistical distribution of the frequencies of the electronic transitions of the molecules and, therefore, to inhomogeneous broadening of the dye spectrum.This broadening was experimentally demonstrated (4)(5)(6) from the dependence of the fluorescence spectrum of a dye solution on the exciting radiation frequency at 77°K. Later, Personov et al.(7) observed inhomogeneous broadening of electronic spectra from frozen solutions of complex molecules at lower (liquid helium) temperatures. It was shown that at liquid helium temperatures discrete fluorescence spectra of complex molecules with

Surface plasmon coupled phosphorescence (SPCP)

“…These results show no noticeable spectral shift in the SPCP versus the free space emission for different polymer film thicknesses. Typically, spectral shifts in chromophore emission are ascribed to the changes in the electronic distribution of the energies of electronic transitions [33]. Since we do not observe any noticeable spectral shifts for the coupled phosphorescence with respect to free space emission, we concluded that the reported observations of SPCP are not due to changes in photophysical properties.…”

Comment on ‘temperature dependent spectroscopic and excited state dynamics of 3-hydroxychromones with electron donor and acceptor substituents’

“…The temperature-induced blue shift is well known effect observed for different organic compounds [15][16][17][18][19][20][21][22][23][24]. It is common to represent the spectral properties of such solutions by the energy level diagram of the spectral centre or solvate consisting of the dye and the nearest solvent molecules.…”

“…Solvatochromic systems are characterised by the next well documented [15][16][17][18][19][20][21][22][23][24] features of the emission bands:…”

“…It is well known that the main mechanism of intermolecular interactions in neutral polar solutions without specific interactions is the dipole-dipole electric interactions of a solute and solvent molecules. Hence, intermolecular interactions depend on the dipole moments and geometric configuration of all molecules [15][16][17][18][19][20][21][22][23].…”

“…Energy levels should take into account free energy of the solvate including not only the electronic and the oscillation energies of the dye molecule but also the energy of intermolecular interactions. Thus, one must consider the dipole-dipole interactions of a solute with the nearest solvent molecules as the strongest interactions in the commonly used polar solutions, and also the thermal fluctuations of these interactions [7,[15][16][17][18][19][20][21][22][23].…”