Breakdown of the mirror image symmetry in the optical absorption/emission spectra of oligo(para-phenylene)s

Abstract

The absorption and emission spectra of most luminescent, pi-conjugated, organic molecules are the mirror image of each other. In some cases, however, this symmetry is severely broken. In the present work, the asymmetry between the absorption and fluorescence spectra in molecular systems consisting of para-linked phenyl rings is studied. The vibronic structure of the emission and absorption bands is calculated from ab initio quantum chemical methods and a subsequent, rigorous Franck-Condon treatment. Good agreement with experiment is achieved. A clear relation can be established between the strongly anharmonic double-well potential for the phenylene ring librations around the long molecular axis and the observed deviation from the mirror image symmetry. Consequences for related compounds and temperature dependent optical measurements are also discussed.

A femtosecond fluorescence study of vibrational relaxation and cooling dynamics of UV dyes

“…As mentioned before, the structural rearrangement of pTP is associated to a planarization, mainly involving the torsional motion of the phenyl rings. 39 The effect of such a hot low frequency mode on the emission spectrum of pTP corresponds to a strong broadening and a related blue shift of the band, consistent with the distortion of the early transient spectra of pTP. Accordingly, the early dynamics of pTP (t o 200 fs in Fig.…”

“…39 It is tempting to assume that the early dynamics observed in the first moment of pTP and PPO are triggered by such structural changes. In the case of PPO, the time zero spectrum resembles the mirror image of the absorption (broader than emission), suggesting that the structural change has still not occurred.…”

“…This reflects an important conformational change upon excitation. 39 In the ground state, the weak single C-C bonds connecting the phenyl rings imply the presence of low frequency librational modes. The related potential is characterized by an anharmonic double minimum well, with minima at an angle of B301 with respect to the planar configuration.…”

Excitation Migration along Oligophenylenevinylene-Based Chiral Stacks: Delocalization Effects on Transport Dynamics

“…Work is now in progress for extending to conjugated materials the application of these models, which up to now has been mostly limited to the description of light-harvesting phenomena by photosynthetic systems. 40 …”

Hole-vibronic coupling in oligothiophenes: impact of backbone torsional flexibility on relaxation energies

“…As is clear from figure 7, despite a significant contribution to the relaxation energy from the torsional degrees of freedom, the linear vibronic model cannot explain the lack of vibrational structure in the UPS spectra of oligothiophenes. Thus, other mechanisms, such as strong mode mixing, quadratic vibronic coupling (Gierschner et al 2002) and/or anharmonicity of the PES (Heimel et al 2005), are contributing to the broadening of the UPS bands. As an illustration, in figure 8 we show how the results obtained for T 4 from a linear vibronic model (displaced oscillators) evolve when a quadratic vibronic coupling (distorted oscillator) to a vibrational mode u 0 that changes from 10 to 100 cm K1 upon ionization is added to the linear vibronic model.…”