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.

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This paper demonstrates that photodetachment photoelectron spectroscopy (PD-PES) of non-covalently bound heteromolecular anion clusters allows direct access to neutral contact charge transfer (CCT) states. The 2,2'-bithiophene.indole cluster is investigated as a model system in such a way that both chromophores have similar optical excitation cross-sections and hence the PD-PES shows the signature of both molecules. For the complexes O(2).stilbene, O(2).indole and O(2).N-methylindole the optical cross-sections of the conjugated chromophores are much larger than that of O(2). This forces most of the anion-to-neutral photoexcitation into the CCT state. The CCT states lie below S(1) for 2,2'-bithiophene.indole and O(2).stilbene or even below T(1) for O(2).indole and O(2).N-methylindole. Significant differences are found between the PD-PES of O(2) with indole and N-methylindole indicating that different collision sites may have different CCT state energies and as a result different (1)Delta(g) oxygen formation efficiencies. We discuss the possible consequences of the energetics and the geometry changes for the excited-state deactivation of tryptophan via the CCT state O(2)(-).tryptophan(+).

Photodetachment photoelectron spectroscopy (PD-PES) on molecular radical anions provides electron affinities and direct spectroscopic access to the lowest triplet states T(1) and T(2) of the corresponding neutral molecules. Herein, we apply this method to investigate trans-stilbene (SB), all-trans-1,4-diphenyl-1,3-butadiene (DPB), and all-trans-1,6-diphenyl-1,3,5-hexatriene (DPH). Our gas-phase T(1) energies agree with previous measurements on condensed crystals. The T(2) state energies are determined in this work for the first time. Experimental electron affinities, neutral excited-state transition energies and intensities are compared to theoretical data calculated by using DFT (ground states) or DFT/MRCI (excited states) methods. We give an explanation why intersystem crossing is of minor importance in all three compounds although the T(2) states are situated close below the S1 states.

In this work we show that photodetachment photoelectron (PD-PE) spectroscopy on radical anions allows direct access to dark states, such as dipole-forbidden nπ * triplet and singlet states. p-benzoquinone (BQ) was chosen as model molecule because it is experimentally and theoretically well investigated and its lowest electronic states are electric dipole-forbidden nπ * triplet and singlet states. Because of the high density of electronic states we observe a dense structure with many transitions in our PD-PE spectrum. We identify the first peak at 2.3 eV above the S 0 onset as the T 1 state origin, in good agreement with previous data. We are not able to resolve the splitting between the states T 1 (1 3 B 1g ) and T 2 (1 3 A u ) as well as the corresponding S 1 (1 1 A u ) and S 2 (1 1 B 1g ) states, but perform a tentative assignment of the singlet-triplet splitting on the basis of literature data. The fact that the spectral features in our PD-PE spectrum cover a broad energy range is taken as a strong experimental evidence for the presence of the T 3 ππ * state, which has been predicted to lie in this energy range. Our investigations on BQ show that in principle, by starting spectroscopy in radical anions most of the selection rules, valid in neutral BQ molecules, can be circumvented and nπ * states, triplet states and other dipole-forbidden states become directly accessible with bright intensity. Much higher electron energy resolution is required to allow unambiguous assignments and make PD-PES of radical anions a valuable method for the spectroscopy dark states of neutral closed-shell molecules.