Complex chi(2) spectra of air/water interfaces in the presence of charged surfactants were measured by heterodyne-detected broadband vibrational sum frequency generation spectroscopy for the first time. In contrast to the neat water surface, the signs of chi(2) for two broad OH bands are the same in the presence of the charged surfactants. The obtained chi(2) spectra clearly showed flip-flop of the interfacial water molecules which is induced by the opposite charge of the head group of the surfactants. With the sign of beta(2) theoretically obtained, the absolute orientation, i.e., up/down orientation, of water molecules at the charged aqueous surfaces was uniquely determined by the relation between the sign of chi(2) and the molecular orientation angle. Water molecules orient with their hydrogen up at the negatively charged aqueous interface whereas their oxygen up at the positively charged aqueous interface.
The electronic relaxation and isomerization mechanism of trans-azobenzene after the S 2 (ππ*) r S 0 photoexcitation were investigated in solution by steady-state and femtosecond time-resolved fluorescence spectroscopy. In the steady-state fluorescence spectrum, two bands were observed with their peaks at ∼390 nm (∼25 750 cm -1 ) and ∼665 nm (∼15 000 cm -1 ). These fluorescence bands showed good mirror images of the S 2 (ππ*) r S 0 and S 1 (nπ*) r S 0 absorption bands, so that they were assigned to the fluorescence from the S 2 (ππ*) and S 1 (nπ*) states having "planar" structures. The lifetimes of the S 2 and S 1 states were determined as ∼110 fs (S 2 ) and ∼500 fs (S 1 ) by time-resolved measurements. The quantum yield of the S 2 f S 1 electronic relaxation was evaluated by comparing the intensity of the S 2 and S 1 fluorescence, and it was found to be almost unity. This implies that almost all molecules photoexcited to the S 2 (ππ*) state are relaxed to the "planar" S 1 (nπ*) state. The present fluorescence data clarified that the isomerization following S 2 (ππ*) photoexcitation takes place after the S 2 f planar S 1 electronic relaxation and that the rotational isomerization pathway starting directly from the S 2 (ππ*) state does not exist. It was thus indicated that the isomerization mechanism of azobenzene is the inversion isomerization occurring in the S 1 state, regardless of difference in initial photoexcitation. The relaxation pathways in the S 1 state were also discussed on the basis of spectroscopic and photochemical data.
In copper(I) complex [Cu(dmphen)(2)]+ (dmphen = 2,9-dimethyl-1,10-phenanthroline), a "flattening" structural change is induced with 1MLCT excitation, which is a prototype of the structural change accompanied with Cu(I)/Cu(II) conversion in copper complexes. Femtosecond and picosecond emission dynamics of this complex were investigated in solution at room temperature with optically allowed S(2) <-- S(0) photoexcitation. Time-resolved emission was measured in the whole visible region, and the lifetimes, intrinsic emission spectra, and radiative lifetimes of the transients were obtained by quantitative analysis. It was concluded that the initially populated S(2) state is relaxed with a time constant of 45 fs to generate the S1 state retaining the perpendicular structure, and the D(2d) --> D(2) structural change (the change of the dihedral angle between the two ligand planes) occurs in the S(1) state with a time constant of 660 fs. The intersystem crossing from the S(1) state to the T(1) state takes place after this structural distortion with a time constant of 7.4 ps. Importantly, the temporal spectral evolution relevant to the structural change clearly exhibited an isoemissive point around 675 nm. This manifests that there exists a shallow potential minimum at the perpendicular geometry on the S1 surface, and the S1 state stays undistorted for a finite period as long as 660 fs before the structural distortion. This situation is not expected for the structural change induced by the ordinary (pseudo-)Jahn-Teller effect, because the distortion should be induced by the spontaneous structural instability at the perpendicular structure. This result sheds new light on the present understanding on the structural change occurring in the metal complexes.
Lipid/water interfaces and associated interfacial water are vital for various biochemical reactions, but the molecular-level understanding of their property is very limited. We investigated the water structure at a zwitterionic lipid, phosphatidylcholine, monolayer/water interface using heterodyne-detected vibrational sum frequency generation spectroscopy. Isotopically diluted water was utilized in the experiments to minimize the effect of intra/intermolecular couplings. It was found that the OH stretch band in the Imχ((2)) spectrum of the phosphatidylcholine/water interface exhibits a characteristic double-peaked feature. To interpret this peculiar spectrum of the zwitterionic lipid/water interface, Imχ((2)) spectra of a zwitterionic surfactant/water interface and mixed lipid/water interfaces were measured. The Imχ((2)) spectrum of the zwitterionic surfactant/water interface clearly shows both positive and negative bands in the OH stretch region, revealing that multiple water structures exist at the interface. At the mixed lipid/water interfaces, while gradually varying the fraction of the anionic and cationic lipids, we observed a drastic change in the Imχ((2)) spectra in which spectral features similar to those of the anionic, zwitterionic, and cationic lipid/water interfaces appeared successively. These observations demonstrate that, when the positive and negative charges coexist at the interface, the H-down-oriented water structure and H-up-oriented water structure appear in the vicinity of the respective charged sites. In addition, it was found that a positive Imχ((2)) appears around 3600 cm(-1) for all the monolayer interfaces examined, indicating weakly interacting water species existing in the hydrophobic region of the monolayer at the interface. On the basis of these results, we concluded that the characteristic Imχ((2)) spectrum of the zwitterionic lipid/water interface arises from three different types of water existing at the interface: (1) the water associated with the negatively charged phosphate, which is strongly H-bonded and has a net H-up orientation, (2) the water around the positively charged choline, which forms weaker H-bonds and has a net H-down orientation, and (3) the water weakly interacting with the hydrophobic region of the lipid, which has a net H-up orientation.
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