Photoexcimerisation of pure acriflavine dye in water and alcohol

Abstract

The light emission properties of acriflavine have been extensively utilised in its applications as dye laser active media, as photosensitizer, as fluorescent and FRET (Fluorescence Resonance Energy Transfer) agent for sensing mechanism etc. However, studies on the photochemical and photophysical properties of acriflavine and its aggregates are very limited and little is known about its emission characteristics in different media. The present study was undertaken to acquire a comprehensive understanding of the aggregation process and emission characteristics of the solution of pure acriflavine dye in water and alcohol, namely the propanol, methanol, ethanol and butanol. This study includes confirming the photoexcimerisation of acriflavine by concentration dependent UV–Visible spectroscopy, steady state fluorescence spectroscopy and time resolved fluorescence spectroscopy at room temperature. The present study shows the monomer and excimer fluorescence emission of acriflavine in the range 495–528 nm and 549–587 nm respectively (monomer and excimer emission peak wavelength varies for different solvents). The nature of the excimer emission band of acriflavine around 575 nm for all the solvents has been tested through temperature dependent steady state fluorescence spectroscopy and also from time resolved fluorescence spectra it has been found that the excimer band does not originate from any triplet excited state. The pH dependent emission spectra for fixed concentration of acriflavine in the solvents reveals red shifts in the monomer and the excimer fluorescence maxima. The excimerisation of acriflavine in water, propanol and methanol is found as static process from excited dimers, whereas in ethanol and butanol it is the result of dynamic diffusion process between a photoexcited and a ground state monomer. The excimerisation process of the acriflavine has been quantified by evaluating the monomer-excimer equilibrium concentration i.e., the critical concentration in the cited solvents. At the critical concentration the monomer and the excimer are found to be fluoresced with equal intensity. The acriflavine solutions specifically at the critical concentration in the cited solvents will enhance its photophysical and photochemical applications by extending them up to the new range of excimer emission radiation at the wavelength in-between 549 nm and 587 nm. Hence, this report shows that, by using acriflavine solutions of certain concentrations in any one of the cited solvents as an active medium of a dye laser and thereby obtaining both the monomer and excimer emission band, the wavelength of the laser can be tuned from 495 nm to 587 nm.