Indole in aqueous solution is photoionized near threshold following single photon absorption from a femtosecond laser pulse at 260 nm. Transient absorption measurements are performed using a white-light continuum probe pulse. Excited state absorption of neutral indole molecules is characterized accurately in 1-propanol where photoexcitation at 260 nm does not lead to photoionization. The presence of 0.75 M carbon tetrachloride in a solution of indole/1-propanol leads to the formation of indole radical cations on a picosecond time scale. While solvated electrons are formed in aqueous indole within our time resolution of 200 fs, measurements of the transient absorbance out to 100 ps are flat and indicate that geminate recombination is insignificant on this time scale. This result contrasts sharply with the geminate recombination dynamics observed following the photoionization of neat water. This indicates that the bimolecular reaction between indole radical cations and solvated electrons is considerably slower than the diffusion limit. We suggest that geminate recombination arising from solute photoionization in polar solvents may be slower than previously thought.

High-efficiency light-driven hydrogen evolution from water was demonstrated by using poly(phenyleneethynylene) bearing negatively charged, [G3] poly(benzyl ether) dendrimeric side groups 3(L4) as photosensitizer. Three-dimensional wrapping of the conjugated backbone suppressed self-quenching of the photoexcited state, while methyl viologen (MV(2+)), a positively charged electron acceptor, was trapped on its negatively charged surface, to form a spatially separated donor-acceptor supramolecular complex. Studies with time-resolved fluorescence spectroscopy showed that the quenching rate constant (k(q) = 1.2 x 10(15) M(-1) s(-1)) is much greater than diffusion control rate constants. Upon excitation of 3(L4) in the presence of a mixture of MV(2+), triethanolamine (TEOA; sacrificial electron donor), and a colloidal PVA-Pt, hydrogen evolution took place with an overall efficiency of 13%, 1 order of magnitude better than precedent examples. Comparative studies with several reference sensitizers showed that spatial isolation of the conjugated backbone and its long-range pi-electronic conjugation, along with electrostatic interactions on the exterior surface, play important roles in achieving the efficient photosensitized water reduction.

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Laser flash photolysis (LFP) of perfluorophenyl azide and perfluoro-4-biphenyl azide produces the corresponding singlet nitrenes which were detected by their transient absorptions at 330 and 350 nm, respectively. The absolute rate constants of the fundamental processes that consume the singlet nitrenes (intersystem crossing, k isc, rearrangement, k R; reaction with pyridine, k pyr) were determined by monitoring the decay of the singlet nitrene and by the growth of its reaction products (ketenimine, triplet nitrene, or pyridine ylide). In the case of singlet 4-perfluorobiphenylnitrene in CH2Cl2 k isc = (2.2 ± 0.1) × 106 s-1, k R = 1013.2±0.2 exp[−(9400 ± 400)/RT] s-1, and k pyr = 109.06±0.15 exp[−(2400 ± 200)/RT] M-1 s-1. In the case of singlet perfluorophenylnitrene in CH2Cl2 k isc = (1.05 ± 0.05) × 107 s-1, k R = 1013.8±0.3 exp[−(8800 ± 400)/RT] s-1, and k pyr = 109.00±0.13 exp[−(1600 ± 160)/RT] M-1 s-1.

Laser flash photolysis (Nd:YAG laser, 355 nm, 35 mJ, 150 ps) of dimethyldiazirine and dimethyldiazirine-d 6 produces dimethylcarbene (DMC) and dimethylcarbene-d 6 (DMC-d 6), respectively. The carbenes were trapped with pyridine to form ylides which absorb around 364 nm. It was possible to resolve the growth of the ylides as a function of pyridine concentration in Freon-113, α,α,α-trifluoromethylbenzene, and perfluorohexane as a function of temperature. The observed rate constant (k obs) of ylide formation was linearly dependent on the concentration of pyridine in all solvents and at all temperatures. From plots of k obs versus [pyridine] it was possible to extract values of k pyr (the absolute rate constant of reaction of the carbene with pyridine) and τ, the carbene lifetime in the absence of pyridine, and their associated Arrhenius parameters. In Freon-113 and α,α,α-trifluoromethylbenzene the carbenes decay both by rearrangement and by reaction with solvent. In perfluorohexane the carbene decay appears to be predominantly unimolecular. The experimental results are compared with ab initio molecular orbital calculations. The experimentally determined barrier to disappearance of DMC in perfluorohexane (2.56 ± 0.05 kcal/mol) is much smaller than that calculated (7.4 ± 2 kcal/mol) using ab initio molecular orbital theory. The Arrhenius parameters and isotope effects indicate that the rearrangement of DMC in perfluorohexane has a large component of quantum mechanical tunneling. The activation energy for the disappearance of DMC-d 6 in perfluorohexane (5.63 ± 0.03 kcal/mol) is consistent with calculations which indicate that QMT makes only a minor contribution to the deuterated system under the conditions of this study.

To clarify the role of diazocarbonyl excited states in Wolff rearrangement chemistry, time-resolved infrared (TRIR) spectroscopy has been used to study methyl 2-diazo-(2-naphthyl) acetate (1) and the subsequently produced 2-naphthyl(carbomethoxy)carbene (2). TRIR analysis of the growth rate of the ketene rearrangement product following laser excitation of diazoester 1 demonstrates that in this case ketene is formed almost exclusively from the carbene. The absence of reaction from the diazoester excited state is likely due to the highly preferred anti relationship between the diazo and carbonyl groups in 1. In addition, the detection of IR bands from both the singlet and triplet states of spin-equilibrated carbene 2 has allowed a direct experimental estimate of the singlet/triplet energy gap in solution at ambient temperature. This work represents the first TRIR detection and study of a carbene intermediate and demonstrates the potential value of TRIR spectroscopy in structural and mechanistic aspects of carbene chemistry.

A nanosecond time-resolved infrared spectroscopic system based on a dispersive scanning spectrometer has been constructed. This is an advanced version of a similar system reported in a previous paper; the time resolution has been improved from 1 μs to 50 ns and the sensitivity from 10−4 in intensity changes to 10−6. These have been achieved by the use of a high-temperature ceramic infrared light source, a photovoltaic MCT detector, and a low-noise, wide-band preamplifier developed specifically for the present purpose. Time-resolved infrared spectra of a few samples of photochemical and photobiological interests are presented to show the capability of the system. The origin of the thermal artifacts, which have been found to hamper the time-resolved infrared measurements seriously, is shown to be due to the transient reflectance change induced by a small temperature jump. The future prospect of time-resolved infrared spectroscopy is discussed with reference to other methods including infrared laser spectroscopy and Fourier transform infrared spectroscopy.

The photochemical reaction process of bacteriorhodopsin in the nanosecond time range (-120-860 ns) was measured in the 1400-900 cm-1 region with an improved time resolved dispersive-type infrared spectrometer. The system is equipped with a newly developed detection unit whose instrumental response to a 5-ns laser pulse has a full width of the half-maximum of 60 ns. It provides highly accurate data that enabled us to extract a kinetic process one order of magnitude faster than the instrumental response. The spectral changes in the 1400-900 cm-1 region were analyzed by singular value decomposition and resolved into three components. These components were separated by fitting with 10- and 1000-ns exponential functions and a step function, which were convoluted with the instrumental response function. The components with decay time constants of 10 and 1000 ns are named K and KL, respectively, on the basis of previous visible spectroscopy. The spectral shapes of K and KL are distinguishable by their hydrogen-out-of-plane (HOOP) modes, at 958 and 984 cm-1, respectively. The former corresponds to the K intermediate recorded at 77 K and the latter to a K-like photoproduct at 135 K. On the basis of published data, these bands are assigned to the 15-HOOP mode, indicating that the K and KL differ in a twist around the C14-C15 bond.