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Femtosecond time-resolved UV-visible absorption spectroscopy has been used to study the UV photochemistry of trans-azobenzene (t-AB) in solution at 30 °C. Photolysis of t-AB at 303 nm results in transient absorption at 370-450 nm, the decay of which can be fitted by a sum of two exponential components. The shorterlived component has a lifetime of 0.9 ( 0.2 ps in hexane, cyclohexane, and hexadecane and 1.2 ( 0.2 ps in acetonitrile; this is attributed to the S 2 (ππ*) excited state of t-AB. The longer-lived component has a lifetime which is similar to the recovery time of the ground-state absorption of t-AB at 303 nm, found to be 13 ( 1 ps in hexane, cyclohexane, and hexadecane and 16 ( 1 ps in acetonitrile. This longer-time-scale process is attributed to the internal conversion of an intermediate excited state, S † , into ground state t-AB, and this intermediate is tentatively assigned as a twisted conformer of excited t-AB on the S 2 or S 1 potential energy surface. The vibrational relaxation of hot t-AB molecules in the ground state, formed by internal conversion from S † , may also contribute to this longer-time-scale process.

The iron-carbonyl geometries in carboxymyoglobin (MbCO) and carboxyhemoglobin (HbCO) in ambient temperature solution have been investigated using picosecond time-resolved infrared spectroscopy. Polarized infrared and visible beams were used to monitor the change in infrared absorbance of the bound CO stretch bands on photodissociation of the ligand. The ratio of the change in absorbance for perpendicular and parallel relative polarizations of the photolysis and infrared probe beams is directly related to the angle between the ligand bond axis and the normal to the heme plane. Ratios, and hence the angles, have been obtained for the configurations giving rise to the principal CO We have recently suggested a new method of determining IR spectra with ps time resolution (2). In this measurement the sample is first pumped by a ps pulse of visible or UV light and the IR spectrum of the sample consisting of both photoproducts and unphotolyzed material can be recorded at variable ps delay times. The experiment can be done with polarized IR and visible beams, so that the magnitude and decay of the orientational correlation function for photoproducts and unphotolyzed material can be evaluated. For heme proteins-where the overall molecular rotation is slow-the magnitude of the polarization anisotropy is directly related to the angle between the transition dipoles for the IR and visible absorptions. In this paper we present results from a significantly improved version of the original experimental design, applied to the photodissociation of carboxymyoglobin (MbCO) and carboxyhemoglobin (HbCO). The measurements are used to define the heme-CO geometry for these heme proteins in solution. The preliminary results (2) for MbCO involved an uncertainty in the polarization anisotropy of >50%. In this study the error is reduced to -10% and allows a sharp definition of the ligand geometry of this heme protein in solution. MATERIALS AND METHODSSpectroscopic Method. The transient IR spectra were recorded using the same principles as previously described (2 The polarization experiment was carried out as follows. The upconversion signal intensity was measured both with the photolyzing beam blocked and with the photolyzing beam incident on the sample. The A/2 plate in the photolyzing beam was then rotated such that the polarization of that beam was rotated by 900, and the signal measurements were repeated.In this way, the change in IR absorbance could be measured for both parallel and perpendicular IR/photolysis laser polarizations. Measurements were taken using a time delay of 40 ps between the pump and probe lasers. A correction factor of ==5% was required to compensate for the difference in reflectance for the two polarizations of the dichroic beam Abbreviations: MbCO, carboxymyoglobin; HbCO, carboxyhemoglobin. 5062The publication costs of this article were defrayed in part by page charge payment. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. §1734 solely to indicate this fact.

Ultrafast time-resolved electronic absorption spectroscopy has been used to study the photochemistry of trans-azobenzene and trans-1, a derivative in which azobenzene is capped by an azacrown ether, on UV excitation to the S2(ππ*) state. Excitation of trans-1 results in transient absorption which decays with a dominant component of lifetime ca. 2.6 ps and in bleaching of the ground-state UV absorption band which recovers on a similar time scale. In contrast, excitation of trans-azobenzene results in transient absorption which decays with a dominant component with a shorter lifetime of ca. 1 ps, and in bleaching which recovers on a much longer time scale of ca. 18 ps. The recovery of the ground-state UV absorption band is not complete in either case, and the ultrafast data indicate that the quantum yield of trans-to-cis photoisomerization of 1 is approximately twice that of azobenzene. These observations demonstrate that the restricted rotational freedom of the phenyl groups in trans-1 has a significant effect on the excited-state dynamics and decay mechanism. The differences in lifetime and quantum yield of photoisomerization are attributed to rapid internal conversion from the S2 to S1 excited states of trans-1, which results in photoisomerization by an inversion mechanism in the S1 state, whereas fast rotation in the S2 state of trans-azobenzene populates a “bottleneck” state which delays the recovery of the ground state and which reduces the yield of photoisomerization; this “bottleneck” state is not accessible by trans-1. The results support the proposal that rotation is the dominant pathway for decay of the first-formed S2 state of trans-azobenzene but that inversion is the dominant pathway for decay of the S1 state.

The structure and bonding of the azo dye Orange II (Acid Orange 7) in parent and reduced forms have been studied using NMR, infrared, Raman, UV-visible, and electron paramagnetic resonance (EPR) spectroscopy, allied with density functional theory (DFT) calculations on three hydrazone models (no sulfonate, anionic sulfonate, and protonated sulfonate) and one azo model (protonated sulfonate). The calculated structures of the three hydrazone models are similar to each other and that of the model without a sulfonate group (Solvent Yellow 14) closely matches its reported crystal structure. The 1H and 13C NMR resonances of Orange II, assigned directly from 1D and 2D experimental data, indicate that it is present as > or = 95% hydrazone in aqueous solution, and as a ca. 70:30 hydrazone:azo mixture in dimethyl sulfoxide at 300 K. Overall, the experimental data from Orange II are matched well by calculations on the hydrazone model with a protonated sulfonate group; the IR, Raman, and UV-visible spectra of Orange II are assigned to specific vibrational modes and electronic transitions calculated for this model. The EPR spectrum obtained on one-electron reduction of Orange II by the 2-hydroxy-2-propyl radical (*CMe2OH) at pH 4 is attributed to the hydrazyl radical produced on protonation of the radical anion. Calculations on reduced forms of the model dyes support this assignment, with electron spin density on the two nitrogen atoms and the naphthyl ring; in addition, they provide estimates of the structures, vibrational spectra, and electronic transitions of the radicals.

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