Emission of Acridine and its Aggregates Isolated in the Neon Matrix

Abstract

The results of experimental studies of fluorescence and phosphorescence of acridine in the low-temperature inert neon matrix, at 7 K, are reported. It is found that the low-temperature inert matrix of neon affects the energy levels of acridine molecules very weakly even as compared. with nonpolar (aprotic) and non-reactive solvent (e.g. hexane) and that there are different sites for acridine molecules in the neon matrix. However, the observed fluorescence spectra are strongly dependent on the excitation wavelength and besides the different (monomer) sites other emitting species are also contributing to the observed fluorescence emission of acridine in the neon matrix. Clear-cut evidence of the formation of singlet excimers of acridine in the neon matrix demonstrates itself as a very broad and structureless fluorescence spectrum with a relatively large shift from the origin of monomer (site) fluorescence which is characterized by a very distinct vibrational structure. The phosphorescence emission was observed only for the monomers. The observed differences in the low-energy part of excitation spectra of phosphorescence and fluorescence are discussed in terms of the close-lying excited singlet states of n, π* and π, r* character (mixed by the vibronic coupling) and tentatively interpreted as dne to the formation of resonance acridine dimers. Their fluorescence spectrum is slightly shifted toward lower energies from the origin of monomer (site) fluorescence.

A Theoretical Insight into the Photophysics of Acridine

“…A pronounced interchange in the bond alternation of the benzenic rings is expected to have a significant influence in the energy of the state and large vibrational progressions along the C−C stretching modes can be predicted. Finally, the geometrical changes in the B 2 π → π*(L b ) states are small, although a slight expansion of the molecule takes place, consistent with the results of vibrational spectroscopy, showing intense transitions and short Franck−Condon progressions. , The behavior of the L a and L b states is similar to that found in the equivalent states of analogous compounds. ,, …”

“…As regards the emission spectroscopy of acridine, most of the studies have been devoted to the description of the relative position of the low-lying singlet and triplet states and the definition of the photophysical paths and their dependence on the environmental conditions. ,, The intense fluorescence reported in protic solvents with large lifetimes has been attributed to the presence of a π → π* state as the lowest singlet state in polar environments. On the other hand, the low fluorescence quantum yield in the vapor and nonpolar solvents, ,, has been explained by a more stable n → π* state and the predominance of nonradiative processes such as intersystem crossings and internal conversions in these media.…”

“…On the other hand, the low fluorescence quantum yield in the vapor and nonpolar solvents, ,, has been explained by a more stable n → π* state and the predominance of nonradiative processes such as intersystem crossings and internal conversions in these media. The influence of the state reversal and the effect of the vibronic coupling (the so-called proximity effect , ) on the final photophysics of acridine is a controversial issue of current interest. ,,, The effects of temperature, protonation, and gas or crystal matrices on the fluorescence process have also been studied ,,,,, and will be discussed below. The triplet−triplet spectra of acridine in different solvents and the study of the formation rate of the lowest triplet state 7-9 have been reported as indirect ways to determine the radiationless mechanisms in the corresponding emission spectra.…”

Normal Mode Analysis of Vibrations of Jet-Cooled Acridine Dimer

“…The normal modes analysis of the vibrations of ACD molecule and of its direr was performed utilizing a semiempirical ΡΜ3 method [9] which, despite some obvious deficiencies, is generally considered as suitable for handling the vibrational problems in weakly bound molecular and hydrogen-bound complexes and which we have utilized earlier for optimization of the ground-state geometry of acridine direr and calculations of its molecular orbitals, electronic states, and relevant transition dipole moments [4].…”

“…Α preliminary analysis of the observed vibronic structure of LIF excitation spectra of (ΑCD)2 [2] was based on a comparison with the data available from the observations of the ground-state IR and Raman spectra in liquid and in 8olid phase [6,7] and from the analysis of the vibronic structure of electronic absorption and phosphorescence spectra in low-temperature crystalline and amorphous matrices [8,9]. This comparative analysis has shown that the observed structure of LIF excitation spectrum can be separated into two sets of vibrational modes -the low-frequency vibrational modes of clear intermolecular origin inherent to (ΑCD)2 and the vibrational modes which presumably are of intramolecular origin.…”