Robert Wilbrandt

Laser-induced absorption and fluorescence studies of photochromic Schiff bases

Abstract Three photochromic Schiff bases: N-salicylideneaniline (SA), N-salicylidene-1-naphthylamine (SN), and N,N′-bis-(salicylidene)- p -phenylenediamine (BSP), were studied in acetonitrile by means of steady-state and time-resolved absorption and fluorescence spectroscopy, as well as semiempirical quantum chemical calculations. In all these molecules, the transient absorption and two-step laser-induced fluorescence spectra of long-lived transients are remarkably similar. The photochromic species is tentatively assigned to the non-hydrogen bonded form of the proton transfer reaction product with a considerable contribution of zwitterionic character.

The lowest triplet state of 1,3,5-hexatrienes : Quantum chemical force field calculations and experimental resonance Raman spectra

Theoretical and Raman spectroscopic studies are presented of E and Z‐1,3,5‐hexatriene and their 3,4‐ and 2,5‐dideuteriated analogs in ground and excited triplet states. The T1 potential energy surface is calculated from extended SCF‐LCAO‐MO‐CI theory. Energy minima and equilibrium geometries are determined in T1 . Frequencies and normal modes of vibration are calculated for the minima of the T1 and S0 states. Energies of higher triplet levels are computed and oscillator strengths for the transitions from T1 to Tn are determined. The displacements in equilibrium geometries between the T1 and the Tn level corresponding to the strongest T1→Tn transitions are calculated and are used to estimate the intensities of the resonance Raman spectra of the T1 state under the assumption of a predominant Franck–Condon scattering mechanism. The results indicate that the planar E and Z forms of hexatriene and its analogs are the only ones contributing substantially to the T1→Tn absorption and the T1 resonance Raman spectr...

Photophysics of trioxatriangulenium ion. Electrophilic reactivity in the ground state and excited singlet state

Trioxatriangulenium (TOTA+. 4,8,12-trioxa-4,8,12,12c-tetrahydro-dibenzo[cd,mn]-pyrenylium) is a closed shell carbenium ion, which is stable in non-nucleophilic polar solvents at ambient temperatures In alcohols, small quantities of the leuco ether are formed in a reversible reaction. The physical and chemical properties of the excited singlet state of the trioxatriangulenium (TOTA+) carbenium ion are investigated by experimental and computational means The degeneracy of the lowest excited states is counteracted by Jahn-Teller-type distortion, which leads to vibronic broadening of the long wavelength absorption band. A strong fluorescence is observed at 520 nm (tau(fl) = 14.6 ns, phi(fl) = 0.12 in deaerated acetonitrile). The fluorescence is quenched by 10 aromatic electron donors predominantly via a dynamic charge transfer mechanism, but ground state complexation is shown to contribute in varying degrees Quenching is also observed in the presence of halide ions Quenching rate constants are derived from lifetime measurements while charge transfer (CT) complex formation constants follow from the steady-state Stern-Volmer plots. CT-complex formation with three discogenic triphenylenes is studied separately. Phosphorescence spectra, triplet lifetimes. and triplet-triplet absorption spectra are provided. In the discussion, TOTA+ is compared to the unsubstituted xanthenium ion and its 9-phenyl derivative with respect to the excited state properties.

Resonance Raman spectra and quantum chemical vibrational analysis of the C7H7⋅ and C7D7⋅ benzyl radicals

Time‐resolved resonance Raman (RR) spectra of the benzyl radical and its perdeutero isotopomer are presented. The radicals are created by laser flash photolysis (λ=266 nm) of benzylchloride in solution. The spectra are excited at a wavelength of 315 nm, in resonance with the intense D3(2 2A2)←D0(1 2B2) transition. Twenty Raman bands, both polarized and depolarized, are observed. The RR spectra are analyzed through quantum chemical force field for π electrons (QCFF/PI) and ab initio calculations of equilibrium geometries, vibrational frequencies, Franck–Condon factors, and vibronic interactions. Polarized intense bands are assigned to totally symmetric a1 modes, and a number of depolarized bands to fundamentals of b1 modes. The observed activity of b1 modes suggests vibronic coupling, which is confirmed theoretically by calculations of vibronic interactions between the D3(2 2A2) and D5(4 2B2) states. The results from semiempirical and ab initio calculations are compared with experiment. The contributions o...

Vibrational analysis of (SCN)2 and the transient (SCN)2

The vibrational spectra of thiocyanogen and the transient radical anion (SCN)2− are interpreted in detail through molecular orbital and normal coordinate calculations. The results support the assignment of (SCN)2− to the anion of thiocyanogen and indicate a substantial weakening of the S–S and C≡N bonds in going from the parent molecule to its radical anion.

Time-resolved resonance Raman spectra of trans- and cis-stilbene in the lowest excited triplet state

Abstract Time-resolved resonance Raman spectra are obtained of the lowest excited triplet state T 1 produced from each of the trans and cis isomers of stilbene in methanol at room temperature, and from trans-stilbene in glycerol at 203 K. The room-temperature spectra are found to be identical for the two isomers. The spectra are compared with preliminary results of a quantum chemical force field calculation of frequencies of a g modes a planar T 1 trans-stilbene. The T 1 spectra are tentatively ascribed to a planar or near-planar trans geometry. Implications for the stilbene T 1 potential energy surface are discussed.

Time-resolved resonance raman spectrum of all-trans-diphenylbutadiene in the lowest excited singlet state

Abstract The resonance Raman spectrwn of all-trans-diphenylbutadiene in its lowest excited S 1 state excited in resonance with the S 1 → S n absorption band at 650 nm in non-polar solvents is reported. Three vibrational bands at 1572, 1481 and 1165 cm −1 are observed. A possible assignment of the the 1481 cm −1 band to the CC double-bond stretching mode may support the lowest S 1 state being of 1 A g *− symmetry.

Resonance Raman spectra of the transient Cl− and Br− radical anions

Abstract The resonance Raman spectra of the short-lived radical anions Cl 2 − and Br − in aqueous solution are reported. The observed wavenumbers of 279 cm −1 for Cl − and 177 cm −1 for Br − are about 10% higher than those published for the corresponding species isolated in solid argon matrices. Under the assumption of a Morse potential. harmonic frequencies and anharmonicity constants are calculated to be 286.4 and 3.83 cm −1 for Cl − and 187.2 and 5.17 cm −1 for Br − . These values lead to extrapolated dissociation energies of 0.66 and 0.21 eV, respectively. These results are compared with those from matrix-isolation studies at low temperatures and work in the gas phase.

A quantum chemical anaylsis of the time‐resolved resonance Raman spectrum of 2,5‐dimethyl–2,4‐hexadiene in the T1 state

Theoretical calculations are presented on 1,3‐butadiene and 2,5‐dimethyl–2,4‐hexadiene (tetramethyl–butadiene or TMB) in ground and excited triplet states. The T1 potential energy surfaces are calculated from extended self‐consistent‐field–linear combination of atomic orbits–molecular orbital–configuration interaction (SCF‐LCAO‐MO‐CI) theory. Energy minima and equilibrium geometries are determined in T1. Frequencies and normal modes of vibration are calculated for planar geometries in S0 and T1 and for a geometry in T1 twisted 90° around one of the formal C■C double bonds. Energies of higher triplet levels are computed and oscillator strengths for the transitions from T1 to Tn are determined. The displacements in equilibrium geometries between the T1 and Tn levels corresponding to the strongest T1→Tn transitions are calculated and are used to estimate the intensities of the resonance Raman spectra in T1 under the assumption of a predominant Franck–Condon scattering mechanism. For TMB, the calculated reson...

Vibrational spectroscopic and quantum chemical studies of the trioxatriangulenium carbocation

Abstract Raman and infrared (IR) spectra of the stable carbocation trioxatriangulenium (TOTA) are reported. In a theoretical effort, ab initio and density function theory (DFT) methods are used to determine the vibrational spectra and the optimized geometry of TOTA. The theoretical results are compared with the experimentally obtained spectra and with the previously reported X-ray crystal structure. It is concluded that DFT calculations based on the non-local exchange-correlation functionals (UB-LYP) yields the most accurate description of the vibrational spectra and the geometry of TOTA. As a result of the calculations, it appears that the positive charge in the TOTA carbocation is substantially delocalized.