Symmetry Breaking in an Excited Quadrupolar Acridine-Dione Derivative Driven by Hydrogen Bonding

Abstract

acridine-1.9(2H,5H)dion) with quadrupolar motif has been synthesized and its stationary and transient spectra have been measured. Stationary absorption and fluorescence spectra as well as nonstationary spectra show no signs of symmetry breaking (SB) in aprotic solvents, even of high polarity. The specific features of SB are revealed in alcohol solvents through a considerable red shift of stationary fluorescence spectra and the appearance of a new excited state absorption band in transient absorption spectra. SB is due to the formation of asymmetric strong hydrogen bonds, mainly on one side of the molecule. An unexpected regularity of symmetry breaking is found in mixtures of aprotic dimethylformamide and protic methanol, where methanol acts as a fluorescence quencher. It is revealed that there is no quenching as long as the methanol concentration is less than the critical value of 9 M. This leads to the conclusion that SB in such mixtures is possible only if the concentration of the protic solvent exceeds a certain threshold value.

Effect of Symmetry Breaking in Excited Quadrupole Molecules on Transition Dipole Moment

“…Let us start with a qualitative discussion. First, the reduction in the square of the transition dipole moment with increasing λ or , pictured in Figure , fully agrees with the experimentally observed decrease in the emission moment with an increase in the solvent polarity. − Second, the scale of the predicted changes in the transition dipole moment due to symmetry breaking is close to that detected in the experiments (a decrease in the emission dipole moment by about 20% and 30% was reported in refs and and in ref , respectively). Third, the observed reduction in the instantaneous emission dipole moments in polar solvents occurring in a time interval of several picoseconds , also agrees with model considered here.…”

“…So, the effect of SB on the emission transition dipole moment is expected to be significant only in the dyes with strong SB degree, when the excitation is practically fully localized on one branch of the dye. It has been shown that the formation of H-bonds between solvent molecules and the dye can greatly increase the degree of SB. ,,, Indeed, an effect of SB on the transition dipole moment of a larger scale (0.21) was found with a dye in protic solvents, where a hydrogen bond is likely to trigger SB …”

“…46,48,50,51 Indeed, an effect of SB on the transition dipole moment of a larger scale (0.21) was found with a dye in protic solvents, where a hydrogen bond is likely to trigger SB. 67 ■ CONCLUSIONS…”

“…Moreover, this research is initiated by recent experimental works. First, the effect of SB on the emission dipole moments in polar solvents is detected using nonstationary and stationary fluorescence spectroscopy. − A significant reduction in the emission dipole moment is found to occur in a time interval of several picoseconds. The magnitude of the reduction increases with the solvent polarity.…”

Watching Excited-State Symmetry Breaking in Multibranched Push–Pull Molecules

“…As mentioned above, the attempts to visualize ES-SB using UV–vis TA were generally not really successful because no clear spectral signature could be detected. − , The spectral dynamics observed in the UV–vis region is often dominated by the red shift of the stimulated emission band. This shift together with the frequent overlap of the stimulated emission band with positive ESA features seriously complicates the determination of its area and, thus, the detection of ES-SB.…”