Absorption, fluorescence, and magic-angle pump-probe experiments characterize the solvatochromism and relaxation dynamics of three structurally related near-infrared tricarbocyanine dyes (HDITCP, IR125, and IR144) in solution. Agreement with solvatochromic theory is found in solvents where the conductivity approximately matches that predicted for complete ionic dissociation. The nonpolar solvatochromism of HDITCP and IR125 allows the polar solvatochromism of the IR144 absorption spectrum to be attributed to a specific functional group. Resonance structure arguments predict a dipole moment decrease upon electronic absorption by IR144, consistent with the observed solvatochromism. Assuming a point dipole, spherical cavity reaction field model, self-consistent feedback between the solvent and the polarizable IR144 solute accounts for 1/2 to 1/3 of the observed polar solvent shifts. A geometry change in the excited state leads to nearly nonpolar solvatochromism in the IR144 emission spectrum. Femtosecond magic-angle pump-probe transients show similar underdamped intramolecular vibrational quantum beats in all three molecules, but find solventdependent overdamped responses on a picosecond time scale in all three dyes. The quantum beat decays determine inhomogeneous vibrational dephasing times of a few picoseconds. Vibrational relaxation, nonpolar solvation, and dielectric relaxation all take place on similar time scales, but the picosecond relaxations are all slower and of larger amplitude for IR144 (polar solvatochromism) than HDITCP (nonpolar solvatochromism). In contrast to HDITCP, IR144 has a prominent solvent-dependent "coherence spike" near T ) 0 which is attributed to femtosecond polar solvation dynamics.

Polarized femtosecond pump-probe spectroscopy is used to observe electronic wavepacket motion for vibrational wavepackets centered on a conical intersection. After excitation of a doubly degenerate electronic state in a square symmetric silicon naphthalocyanine molecule, electronic motions cause a ϳ100 fs drop in the polarization anisotropy that can be quantitatively predicted from vibrational quantum beat modulations of the pump-probe signal. Vibrational symmetries are determined from the polarization anisotropy of the vibrational quantum beats. The polarization anisotropy of the totally symmetric vibrational quantum beats shows that the electronic wavepackets equilibrate via the conical intersection within ϳ200 fs. The relationship used to predict the initial electronic polarization anisotropy decay from the asymmetric vibrational quantum beat amplitudes indicates that the initial width of the vibrational wavepacket determines the initial speed of electronic wavepacket motion. For chemically reactive conical intersections, which can have 1000 times greater stabilization energies than the one observed here, the same theory predicts electronic equilibration within 2 fs. Such electronic movements would be the fastest known chemical processes.

Femtosecond two-dimensional Fourier transform (2D FT) spectra remove inhomogeneity and simultaneously provide time resolution and two dimensions (excitation, signal) of frequency resolution limited only by the sample. The time evolution of separate real and imaginary 2D FT spectra is used to study local environments around molecules in methanol. Polar solvation dynamics are extracted by comparing 2D FT spectra of structurally related dyes with and without spectral sensitivity to solvent polarity. This comparison reveals specific signatures of inertial motion and the dynamic Stokes' shift.

No abstract

Polarized femtosecond pump-probe spectroscopy is used to observe electronic wavepacket motion for vibrational wavepackets centered on a conical intersection. After excitation of a doubly degenerate electronic state in a square symmetric silicon naphthalocyanine molecule, electronic motions cause a 100 fs drop in the polarization anisotropy that can be quantitatively predicted from vibrational quantum beat modulations of the pump-probe signal. Vibrational symmetries are determined from the polarization anisotropy of the vibrational quantum beats. The polarization anisotropy of the totally symmetric vibrational quantum beats shows that the electronic wavepackets equilibrate via the conical intersection within 200 fs. The relationship used to predict the initial electronic polarization anisotropy decay from the asymmetric vibrational quantum beat amplitudes indicates that the initial width of the vibrational wavepacket determines the initial speed of electronic wavepacket motion. For chemically reactive conical intersections, which can have 1000 times greater stabilization energies than the one observed here, the same theory predicts electronic equilibration within 2 fs. Such electronic movements would be the fastest known chemical processes.