Two-color two-photon excited fluorescence of 2-methyl-5-tert-butyl-p-quaterphenyl (DMQ): ab initio calculations and experimental determination of the molecular parameters.
The paper presents experimental and theoretical studies of two-photon excitation dynamics in 2-methyl-5-tert-butyl-p-quaterphenyl (DMQ) dissolved in cyclohexane/paraffin. Experimentally, a two-color two-photon (2C2P) excitation by two femtosecond laser pulses at 800 and 400 nm has been used in combination with the time-resolved detection of polarized molecular fluorescence. The fluorescence decay was found to be two-exponential, resulting in the molecular excited state lifetime of 753 ± 10 ps and the rotational correlation time of 724 ± 45 ps. Control over the excited and fluorescent photons polarization has been used for determination from experiment of seven independent molecular parameters. The experimental data were analyzed on the basis of the recent theoretical approach [Shternin, P. S., Gericke, K.-H., and Vasyutinskii, O. S. Mol. Phys. 2010, 108, 813-825] supported by ab initio computations of the DMQ electronic structure and transition dipole moments. The results obtained imply that the two-photon absorption tensor S is mostly diagonal and that the Szz tensor component onto the molecular long axis gives the major contribution of 93%. However, it was also found that a number of different symmetry two-photon transitions related to the dipole moment components dxdz and dydz are excited in the conditions of our measurements.
Conformational analysis of p-terphenyl by vibrational spectroscopy and density functional theory calculations
Abstract The structures, vibrational frequencies, and infrared and Raman intensities of p -terphenyl have been calculated by the density functional theory method at the B3LYP/6-311+G** level. On the basis of the results, the infrared and Raman spectra of p -terphenyl in solution and in the melt have been analyzed. The observed spectra have been attributed to two stable rotational isomers around the inter-ring CC bonds: a helical conformer ( D 2 symmetry) and an alternately twisted conformer ( C 2h symmetry). These two isomers coexist in solution and in the melt. The 518-cm −1 infrared band and the 375-cm −1 Raman band of the helical conformer are correlated with the 483-cm −1 infrared band and the 329-cm −1 Raman band of the twisted conformer, respectively. All of these bands have been assigned to the mixtures of out-of-plane boat deformations and in-plane rotational displacements. The frequencies of these modes are sensitive to the conformation of p -terphenyl.
ORDER-DISORDER PHASE TRANSITION IN P-TERPHENYL AND P-TERPHENYL:TETRACENE DOPED CRYSTALS AS STUDIED BY RAMAN SPECTROSCOPY
Abstract Phase transition in crystalline pure p-terphenyl and p-terphenyl: tetracene doped crystals was studied with Raman spectroscopy, for temperatures from 295 to 10 K. In particular, the torsional Raman vibrational mode with a “hard-core frequency” of 230.8 cm −1 was investigated in its frequency and bandwidth dependence upon temperature. The results were analyzed based on an order-disorder model allowing the determination of the activation energies and orientational correlation times of the molecular diffusive process in the monoclinic (above 193 K) and triclinic (below 193 K) phases of the crystals. The activation energy is observed to decrease from the monoclinic to the triclinic phase, whereas the orientational correlation times increase, both in the undoped and the doped crystals. The doping of p-terphenyl with tetracene appears to affect the activation energy and the orientational correlation times in a different way in each phase.