The chemical and photochemical characteristics of hydrated heteropolytungstic acids of the Keggin ( T d ) structure, dissolved in three representative dipolar aprotic organic media (DP), N-methylpyrrolidinone (NMP), 1,1,3,3-tetramethylurea (TMU), and 1,3-dimethyl-2-imidazolidinone (DMEU), have been investigated. The photochemical and spectroscopic behavior of heteropoly complexes is qualitatively different in these solvents relative to the media conventionally used in polyoxometalate investigations (water, alcohols, acetonitrile, etc.). When the hydrated heteropoly acid a-H3PWI2Oa dissolves in NMP, TMU, or DMEU, crystalline complexes of the formula [(DP)2H]3[~-PW120a] (I-DP), where DP = NMP, TMU, and DMEU, can be isolated. Both the solutions and crystals of these salts are colored unlike the component chromophores DP or a-PWI2OM' alone, and irradiation of either the solutions or the crystals results in reduction of the a-PWI2Oa3-and oxidation of the DP. Although 1-DP complexes generally crystallize in a disordered manner, disorder-free crystals of one complex, 1-TMU, were obtained: monoclinic C2/c, with a = 21.3221 (87) A, b = 13.8654 (47) A, c = 24.8262 (71) A, j 3 = 100.830 (28)', V = 7208.87 (4.23) A3, and Z = 4; final R = 0.0387 (R, = 0.0324). The pronoun& and photosensitive chromophore in the visible region of 1-TMU does not derive from covalent or hydrogen-bonding interactions between the organic [(TMU),H]+ ions and the inorganic polyoxometalate anion. Characterization of solutions of 1-DP in acetonitrile and in DP by IH, "0, IlP, and lS3W NMR and electronic absorption spectroscopy establish that the formation of the intermolecular electron donor-acceptor species can be a reversible process in solution. The kinetics for the photochemical production of the reduced heteropolytungstate products (one-electron heteropoly blue) have been examined under several conditions. A general feature of this photoredox chemistry is fairly constant quantum yields throughout the low-energy region of the absorption spectrum for any given system. The quantum yields at 320 nm for 1-TMU in TMU and 1-DMEU in DMEU are 0.27 and 0.17, respectively. The above physical, chemical, and photochemical properties of the 1-DP in DP systems define appropriate conditions for picosecond spectroscopic examination of polyoxometalate photochemical processes. The evolution of absorption spectral transients on the picosecond timescale with 355-nm laser excitation of the 1-NMP in NMP and 1-DMEU in DMEU indicates that two or more distinct species are formed on a time scale (<80 ps) faster than bimolecular diffusion, one within the laser pulse (<20 ps). The picosecond spectroscopy of the above systems is distinct from that observed for a-H3PWl2Oa.nH2O, n = 7, in acetonitrile excited at 355 nm. Picosecond spectroscopic control experiments with tetracyanoethylene (TCNE) and NMP indicate that TCNE anion radical and NMP cation radical are produced in <25 ps. The chromophore of NMP cation radical cannot be observed under the conditions used in the 1-DP/DP p...

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The energies of the charge-separated P+.Qstates of four amide-linked tetratolylporphyrin-benzoquinone (PQ) molecules in dichloromethane, taken as the difference in redox potentials for formation of P+.Q and PQ-, are 1.37 f 0.02 eV.

A description is presented of a laser system that can be used for two concurrent and independent types of picosecond spectroscopic measurements. Two data-collection sybsystems, (1) a picosecond pump-probe transient absorption/emission spectrometer and (2) a streak camera system for time-dependent measurements of absorption and emission, have been developed as independent work stations within an integrated system based on a single mode-locked Nd:YAG laser oscillator which concurrently supplies each subsystem with picosecond pulses. The electronic links among the two work stations and the laser are discussed. In addition to applications involving pump-probe absorption and pulsed laser-induced emission measurements, this system can be used for picosecond-pulsed preparation-pump-probe absorption and preparation-pump emission experiments. Examples of data collected with each subsystem are presented to illustrate the complementary nature of the information that can be obtained with this versatile laser system.

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