Triarylamine-perchlorotriphenylmethyl radicals (TARA-PCTM) may be viewed as open-shell mixed valence donor-acceptor compounds that exhibit strong charge-transfer (CT) bands in the visible to NIR spectral region. While open-shell molecules generally do not fluoresce at RT, we observed a surprisingly strong fluorescence from the highly polar excited CT state of the TARA-PCTM radicals in the visible and NIR spectral region. The fluorescence quantum yield is enhanced by a factor of up to 150 compared to the unsubstituted perchlorotriphenylmethyl radical. The enhancement depends on the donor strength of the TARA moiety which was tuned by small substituents (OMe, Me, Cl, CN, and NO 2 ) attached to the phenyl groups, thus forming a series of donor-acceptor species that mainly differ by the free energy difference of the excited CT state and the ground state. The reorganization parameters of the CT process were extracted by Bixon-Jortner fits to either the absorption or the fluorescence bands. The dynamics of the nonradiative back-electron transfer were investigated by time-resolved fluorescence and transient absorption spectroscopy in the ps to ns time regime. We observed a strong deviation of the back-electron transfer rate from the expected inverted Marcus behavior which might be due to anharmonic effects.

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We report an investigation of four differently substituted boroles by resonance Raman (RR) spectroscopy with the aim of gaining insight into the structural changes that occur upon electronic π -π * excitation in the five-membered BC 4 ring. Such boroles are prototypes for the theory of antiaromaticity, because the empty p z orbital at the boron interacts with the π system of the carbon backbone to delocalize the four π electrons. The magnitude of this interaction depends on the substituents at the boron atom. In all compounds, an intense band appears around 1600 cm −1 , which can be assigned to a C C stretching vibration. The selective amplification of this mode indicates an expansion of the borole ring upon electronic excitation. The resonant enhancement of another mode at around 1300 cm −1 , which can be assigned to a vibration between the boron and the substituent, gives evidence that the degree of antiaromaticity in the borole depends strongly on the interaction between the substituent at the boron atom with the empty p z orbital of the boron. When the boron is connected to a ferrocene, this band appears with high intensity, indicating a strong interaction between B and Fe. Furthermore, we studied the dependence of the excitation wavelength on the Raman intensities. In addition, we used DFT calculations to determine the vibrational wavenumbers.

The photophysics of the B (1)B(1) state of isolated cyclopropenylidene, c-C(3)H(2), has been studied by femtosecond time-resolved photoionisation and photoelectron spectroscopy. The carbene was produced by flash pyrolysis of 3-chlorocycloprop-1-ene. The bands at 266.9 nm and 264.6 nm have been investigated. The excited state deactivates in a two step process. The first time constant of less than 50 fs corresponds most likely to a nonradiative transition to the A-state, the second one on the order of 200 fs describes the internal conversion to the electronic ground state. The data are compared to those measured for the chlorinated carbene c-C(3)HCl. In the photoelectron spectrum of c-C(3)H(2) resonances were observed which can be assigned to members of a Rydberg d-series.

We report a combined gas phase and solution phase study of 9-fluorenone. The structure and dynamics of isolated fluorenone in the S3-state were studied by resonant enhanced multiphoton ionization with picosecond pulses in a free jet of molecules excited between 285 and 312 nm. Ionization was performed with a second ps-pulse at 351 nm. The electronic spectrum is structured, and the origin of the C (1)B2 ← X (1)A1 transition was observed at 32,122 cm(-1). Several vibrational fundamentals appear in the spectrum. In the gas phase we observe a biexponential decay, which suggests an internal conversion to the coupled S1/S2-state within 10-40 ps. A further decay that is assigned to intersystem crossing was found to be longer than 500 ps. In addition to the gas phase measurements, we studied the photophysics of 9-fluorenone in cyclohexane by femtosecond-time-resolved transient absorption spectroscopy and observed very similar dynamics upon excitation to the S3 state: It deactivates within 8-11 ps by internal conversion, followed by intersystem crossing within 120-150 ps, forming a long-lived triplet state. Experiments in acetonitrile, however, showed marked differences. Intersystem crossing is ineffective in polar solvents because the lowest excited singlest state is of ππ* character and does not interact with the (3)ππ*.

The ultrafast dynamics of isolated 9-fluorenone was studied by femtosecond time-resolved photoionization and photoelectron spectroscopy. The molecule was excited around 264-266 nm into the S(6) state. The experimental results indicate that the excitation is followed by a multistep deactivation. A time constant of 50 fs or less corresponds to a fast redistribution of energy within the initially excited manifold of states, i.e., a motion away from the Franck-Condon region. Internal conversion to the S(1) state then proceeds within 0.4 ps. The S(1) state is long-lived, and only a lower bound of 20 ps can be derived. In addition, we computed excited state energies and oscillator strengths by TD-DFT theory, supporting the interpretation of the experimental data.

The photophysics of two donor-substituted truxenone derivatives has been studied by femtosecond time-resolved transient absorption spectroscopy. The systems consist of a central truxenone acceptor with three triarylamine (TARA) branches which act as electron donors. Upon excitation in the visible regime an electron is transferred from the donor to the acceptor, generating a charge-separated state. This state can be probed via the characteristic absorption of the TARA radical cation around 700 nm. A second absorption band around 420 nm exhibits the same kinetics and is assigned to an absorption of the radical anion of the truxenone moiety. The back electron transfer and the recovery of the ground state can be interpreted within the framework of Marcus theory. To study the dependence of the back electron transfer on the electronic coupling, the distance between the donor and the acceptor was adjusted. Two solvents were employed, dimethylsulfoxide and dichloroethane. A biexponential decay of the bands assigned to the charge-separated state was observed, with time constants in the picosecond range. Surprisingly, the rates for electron back transfer do not follow the simple picture of the donor-acceptor distance being the determining factor. The observations are explained within a model that additionally takes steric interactions between the donor and the acceptor into account.