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.

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.

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Four binuclear rhenium(I) complexes of the form [Re(CO) 3 Cl] 2 BL, where BL is one of the bridging N 4donor ligands 2,3-di(2-pyridyl)quinoxaline, 2,3-di(2-pyridyl)-6-methylquinoxaline, 2,3-di(2-pyridyl)pyrido-[2,3-b]pyrazine, or 2,3-di(2-pyridyl)pyrido [3,4-b]pyrazine, have been studied by femtosecond time-resolved UV-visible and infrared absorption spectroscopies. Photolysis at 606 nm, within the metal-to-ligand chargetransfer (MLCT) absorption band of each complex, produces transient features with lifetimes which vary from 100 to 1800 ps, depending on the structure of the bridging ligand. These transient species are assigned to the 3 MLCT excited states of the complexes. A detailed analysis of the time-resolved infrared spectra reveals that the three principal ν(CO) bands of the ground state, which arise from two Re(CO) 3 Cl groups in similar environments, split into two sets of three ν(CO) bands in the 3 MLCT excited states. This splitting pattern is attributed to asymmetric charge distribution in the excited state, in which one rhenium center is oxidized and the other is a spectator which senses the reduction of the bridging ligand; i.e., the excited state is a class II mixed-valence state, Re + (BL -)Re. Changes in the transient spectra which occur within ca. 5 ps, and which are observed most clearly as broadening to lower wavenumber of the excited-state infrared bands, are attributed to vibrational relaxation in the excited state. The variation in excited-state lifetime with bridging ligand structure is attributed to changes in the rate constants for nonradiative decay, which correspond qualitatively with changes in the energy of the excited state in accordance with the "energy-gap law" for nonradiative relaxation.

UV−visible absorption and emission spectroscopy have been used to study the complexation of Ba2+ with a benzothiazolium styryl azacrown ether dye and two derivatives, one without an azacrown and one with an alkylsulfonate pendant to the benzothiazolium group. Studies of the thermal cis−trans isomerization on the kilosecond time scale, and of the excited state leading to trans−cis photoisomerization on the picosecond time scale, are reported which enable a quantitative analysis of the kinetics and complexation equilibria to be obtained. The photophysics are interpreted by a scheme which includes rotation around both the olefinic CC bond and adjacent C−C bonds in the excited state. A comparison of the data for this dye with those for other derivatives studied here and reported elsewhere indicates that the extent of intramolecular charge transfer in the excited state is a key factor in controlling the properties. An overall mechanism is proposed for the thermal and photochemical reactions of this dye which indicates that it is a good candidate for applications which require the photocontrolled complexation/release of barium cations in solution.