The lowest charge-transfer excited state (CT1) of electron donor–acceptor crystals or polymers is demonstrated to be a plausible precursor of free charge carriers when such materials are photoexcited. Rate constants for the dissociation of charge–transfer states are formulated for two approximate descriptions of CT1: classical ion pair and Wannier exciton. The electric field dependence of the dissociation rate constant is postulated to be given by Onsager’s 1934 theory (O-34) of ion pair dissociation. This formulation of CT1 dissociation obviates the need to invoke electron–hole ‘‘thermalization’’ lengths of 2 to 3 nm in order to explain free charge carrier formation in donor–acceptor materials.

Recent advances in the transient dc photocurrent technique for measuring excited state dipole moments, developed in our group, are discussed. A variety of approaches with detailed analyses of their advantages and disadvantages including cell design, circuit construction tricks, the data acquisition procedure, calibration, and the theoretical treatment of different conditions, are presented. Sensitivity, time resolution limitations, and newly developed features, such as the signal’s dependence on light polarization as well as charge separation at interfaces are outlined. Dipole moments of a few molecules (diphenylcyclopropenone, bianthryl, dimethylaminonitrostilbene, Coumarin 153, and fluoroprobe) suitable for calibration purpose are reported—some of them for the first time.

has had a good safety record, with the ma-

Often over the years, we have asked scientific colleagues why it is that water is blue. Common responses have included light scattering-after all the sky is blue-and coloration by dissolved impurities-Cu2+ has been a popular suggestion. However, the work described below demonstrates that water has an intrinsic color, and that this color has a unique origin. This intrinsic color is easy to see and has been seen by the authors in the Caribbean and Mediterranean Seas and in Colorado mountain lakes. Because the absorption that gives water its color is in the red end of the visible spectrum, one sees blue, the complementary color of red, when observing light that has passed through several meters of water. This color of water also can be seen in snow and ice as an intense blue color scattered back from deep holes in fresh snow. Blue to bluegreen hues also are scattered back when light deeply penetrates frozen waterfalls and glaciers.Water owes its intrinsic blueness to selective absorption in the red part of its visible spectrum. The absorbed photons promote transitions to high overtone and combination states of the nuclear motions of the molecule; i.e., to highly excited vibrations. To our knowledge the intrinsic blueness of water is the only example from nature in which color originates from vibrational transitions. Other materials owe their colors to the interaction of visible light with the

Excitation of a carotenoid (C) porphyrin (P) fullerene (C60) molecular triad yields the porphyrin first excited singlet state, C−P−C60, which decays via a sequential two-step photoinduced electron-transfer process into a C•+−P−C60 •- charge-separated state with a lifetime of 340 ns in 2-methyltetrahydrofuran solution. The transient dc photocurrent method has been used to investigate the dipole moment of the charge-separated state in tetrahydrofuran and 2-methyltetrahydrofuran. The results show formation of a giant dipole with a moment in excess of 150 D, corresponding to separated charges located on the fullerene and carotene moieties of the triad.