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.
Polarized fluorescence in NADH under two-photon excitation with femtosecond laser pulses
Polarized fluorescence decay in NADH molecules in aqueous solution under two-photon excitation by femtosecond laser pulses has been studied. The excitation was carried out by linear and circularly polarized radiation at four wavelengths: 720, 730, 740, and 750 nm. Time-dependent polarized fluorescence signals were recorded as a function of the excitation light polarization and used for determination of a set of molecular parameters, two lifetimes characterizing the molecular excited states, and the rotation correlation time τrot. The results obtained can be used to create and prove theoretical models describing the intensity and polarization of fluorescence in NADH involved in the regulation of the redox reactions in cells and tissues of living organisms.
Phase transitions in polyphenyls: Raman spectra of p-terphenyl and p-quaterphenyl in the solid state
Abstract Phase transitions in crystalline p-terphenyl and p-quaterphenyl are investigated with Raman spectroscopy in relation to three aspects of structural transitions: (i) changes in molecular and crystal symmetries. (ii) continuous versus discontinuous transitions, and (iii) order—order displacive transition versus order—disorder transition. The nature of phase transitions for these two polyphenyls is compared with that observed for biphenyl. It is found that phase transitions in polyphenyls (biphenyl, p-terphenyl, and p-quaterphenyl) involving, a ring—ring twist change crystal symmetries, but have a small effect on intermolecular interactions. The transition temperature increases with an increase in the length of the polyphenyl. The phase transition in p-quaterphenyl, like that in biphenyl, is continuous (second order, critical temperature ≈ 243 K) and destroys the inversion symmetry of the site. On the other hand,the phase transition in p-terphenyl is quasi-continuous (pseudo-second order) with a transition temperature of 175 ± 10 K. Although the present result appears to argue against an order—order displacive (soft mode) mechanism, it does not provide any direct support for an order—disorder