Ab Initio Benchmark Study of Nonadiabatic S1-S2 Photodynamics of cis- and trans-Hexatriene.

The dynamics of the nonadiabatically coupled lowest singlet excited states of cis- and trans-hexatriene are studied theoretically, in a comprehensive electronic structure and quantum dynamical investigation. At the ground state equilibrium geometry the relevant S2 and S1 states carry the A1 (Ag) and B2 (Bu) symmetry labels, for the cis (trans) isomer. Various high-level electronic structure methods are used, including the recently reparametrized DFT/MRCI method, and the results are critically compared. Key parameters of interest are the vertical energy gap and the strength of vibronic coupling between the interacting electronic states. To estimate their influence, suitable comparison calculations are performed. The results are used as the basis for quantum dynamical calculations on the UV absorption spectrum and electronic population transfer involving the S1 and S2 states. Up to nine nonseparable degrees of freedom are included in the calculations. The experimental UV absorption spectrum in the 5-5.2 eV energy range can be very well reproduced. The time-dependent wavepacket propagations reveal a population transfer on the order of 30-50 fs, which becomes increasingly complete with more degrees of freedom included in the calculation. The results are briefly compared with analogous data for the s-trans-butadiene system treated by some of us recently.

Chimeric behavior of excited thioxanthone in protic solvents: I. Experiments.

The photophysics of thioxanthone (TX) dissolved in methanol (MeOH) and 2,2,2,-trifluoroethanol (TFE) was studied by time-resolved fluorescence and absorption spectroscopy. The spectrally integrated stimulated emission is seen to lose amplitude within ∼5-10 ps. This is much shorter than the fluorescence lifetimes of the compound (2.7 ns for MeOH and 7.6 ns for TFE). The initial reduction in amplitude is attributed to reversible intersystem crossing between the primarily excited (1)ππ* and a triplet (3)nπ* state. The latter one is energetically slightly (∼0.02 eV) above the former one. Addition of the quencher 1-methylnaphthalene (1-MN) reduces the fluorescence lifetime and yields triplet excited 1-MN, giving further evidence for the equilibrium of singlet and triplet excitations. The depopulation of these two states on the nanosecond time scale results in the rise of the lowest triplet state, a (3)ππ* state. Temperature dependencies attribute this to an activated internal conversion process between the two triplet states. Kinetic and energetic parameters derived from the experimental data will be compared with quantum chemical results in the accompanying paper [Rai-Constapel , V. , Villnow , T. , Ryseck , G. , Gilch , P. , and Marian , C. M. J. Phys. Chem. A 2014 , DOI: 10.1021/jp5099415].

Throwing light on dark states of α-oligothiophenes of chain lengths 2 to 6: radical anion photoelectron spectroscopy and excited-state theory.

In this work, we apply photodetachment photoelectron spectroscopy (PD-PES) on radical anions to access the lowest excited electronic states of neutral α-oligothiophenes nT (n = 2-6, where n denotes the number of thiophene rings) in the gas phase. Besides electron affinities, the spectra provide the energies of the T(1) and T(2) states which are otherwise difficult to investigate in neutral molecules due to spin selection rules. The assignment of the spectra is assisted by quantum chemical calculations using a combined density functional theory and multi-reference configuration interaction approach. For all α-oligothiophenes investigated in this work, the T(2) state is situated below S(1). In the gas phase, the S(1) state energies lie higher than in non-polar solution (0.2 to 0.4 eV). The geometry optimizations show that the S(0) state and especially the excited states gain planarity with increasing chain length. A non-planar structure or out-of-plane vibrational activity is needed to allow an efficient intersystem crossing (ISC) dynamics from S(1) to T(2), followed by internal conversion to T(1). Our theoretical calculations predict that in 6T a doubly excited state becomes nearly isoenergetic to S(1). This state is not observed by PD-PES, which is explained by the analysis of the calculated contributing electron configurations.

Ultrafast dynamics in thiophene investigated by femtosecond pump probe photoelectron spectroscopy and theory.

A hybrid of a time-of-flight mass spectrometer and a time-of-flight "magnetic-bottle type" photoelectron (PE) spectrometer is used for fs pump-probe investigations of the excited state dynamics of thiophene. A resonant two-photon ionization spectrum of the onset of the excited states has been recorded with a tunable UV laser of 190 fs pulse width. With the pump laser set to the first intense transition we find by UV probe ionization first a small time shift of the maxima in the PE spectrum and then a fast decay to a low constant intensity level. The fitted time constants are 80+/-10 fs, and 25+/-10 fs, respectively. Theoretical calculations show that upon geometry relaxation the electronic state order changes and conical intersections between excited states exist. We use the vertical state order S1, S2, S3 to define the terms S1, S2, and S3 for the characterization of the electron configuration of these states. On the basis of our theoretical result we discuss the electronic state order in the UV spectra and identify in the photoelectron spectrum the origin of the first cation excited state D1. The fast excited state dynamics agrees best with a vibrational dynamics in the photo-excited S1 (80+/-10 fs) and an ultrafast decay via a conical intersection, presumably a ring opening to the S3 state (25+/-10 fs). The subsequently observed weak constant signal is taken as an indication, that in the gas phase the ring-closure to S0 is slower than 50 ps. An ultrafast equilibrium between S1 and S2 before ring opening is not supported by our data.

Quantitative structure-property relationships in boron nitrides: the 15N- and 11B chemical shifts.

Nuclear Magnetic Resonance (NMR) chemical shifts(delta) for elements in solids may often be approached by ab initio cluster calculations. We employ this technique to investigate the influence of structural alterations on the 15N and 11B chemical shifts in boron nitrides--in both hexagonal and cubic modifications. Within a given class of connectivity, i.e., three- or fourfold coordinated nitrogen, for the first time, an almost linear correlation between the 15N chemical shift and N-B bond lengths could be established. Also, the 11B shifts in hexagonal boron nitride correlate with the B-N bond distance; however, the effect is less pronounced. For the value of the chemical shift (CS), the decisive property is the average bond length at the atom in focus. Variations of CS are predominantly caused by changes in the paramagnetic deshielding. Further, second-nearest neighbor effects on the shieldings at 15N nuclei are quantified by subtraction schemes. The present work is closely related to the verification of models for amorphous high-demand Si/B/N ceramics.