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

Abstract

We present a femtosecond broad-band fluorescence up-conversion study of the vibrational relaxation dynamics of two UV chromophores, 2,5-diphenyloxazole (PPO) and para-terphenyl (pTP), pumped with a large excess of vibrational energy (42000 cm À1 ). The band narrowing of the transient fluorescence spectrum reflects a biphasic cooling process in a few hundreds of fs and a few ps. In the sub-ps regime, our data suggest a structural rearrangement in the excited state, followed by thermalization of the excess energy. These dynamics affect the fluorescence spectra of PPO and pTP in different ways. In PPO, the damping of a low frequency vibrational wavepacket and a significant sub-ps narrowing of the band characterize the vibrational relaxation. In pTP, the latter is faster and appears as a red shift with distortion of the band in o200 fs.

Polychromatic femtosecond fluorescence studies of metal–polypyridine complexes in solution

“…The high local temperature of the molecule after IVR should lead to a structureless fluorescence at early times, as opposed to the vibrational structure that shows up in absorption. Such a broadening of bands at time zero and their narrowing at later times was for example clearly visible in our fluorescence up-conversion study of the 2,5-diphenyloxazole (PPO) dye in cyclohexane [21,30]. This does not occur here because the 1 MLCT lifetime is too short to allow for the manifestation of cooling.…”

“…This occurs in the same fashion in Fe-and Rucomplexes and, for the latter, it is influenced neither by the ligands, nor by the solvent. Furthermore, this behaviour appears to be quite general, since it has been observed in other metal complexes [35,36], in the case of organic dyes [30,[37][38][39], and even of chromophores in proteins [19,40]. The ultrafast IC may involve conical intersection among the manifold of electronic states, but also and as mentioned above, the excess energy is redistributed among lower frequency modes that are optically silent.…”

“…The above observations have a direct incidence on the functioning of DSSCs, since injection from the Franck-Condon accessed state [16,17] must compete with the ultrafast departure from it due to intramolecular processes such as IC and IVR on a sub-10 fs time scale. This aspect is discussed in a forthcoming paper presenting fluorescence up-conversion studies of DSSCs [41].…”

Femtosecond broadband fluorescence upconversion spectroscopy: Spectral coverage versus efficiency

“…The more accurate and precise the observation of an instantaneous emission spectrum, the more forceful will be conclusions about the underlying couplings and processes. [1][2][3] In general, if transient fluorescence band shapes are to be used in a quantitative way, then their photometric accuracy must be ensured. As a corollary, transient spectra of the same sample, but from different laboratories, should be identical.…”

Site-specific incorporation of a fluorescent terphenyl unnatural amino acid

“…25 Moreover, these moieties have been found to be sensitive to environmental conditions including solvent. 26 Consequently, the terphenyl fluorophore has been utilized in a variety of applications including two-photon laser scanning microscopy, 27,28 femtosecond fluorescence spectroscopy, 29 and a-helical secondary structure investigations. 30 These studies have facilitated the expanded knowledge of charge transfer, isomerization and protein dynamics within biological systems.…”