Comparison of core-hole excitation spectra of organic donor/acceptor molecules in the vapor and condensed phases: p-Nitroaniline, 2-amino-6-nitronaphthalene, and 1-amino-4-nitronaphthalene

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Abstract
Vapor phase N1s and O1s core-hole spectra of the organic donor/acceptor molecules 2-amino-6-nitronaphthalene (2,6-ANN) and 1-amino-4-nitronaphthalene (1,4-ANN) are presented and compared to condensed phase results. 2,6-ANN and 1,4-ANN are found to exhibit broad single peak N1s(NO2) signals in the gas phase which become split into resolvable components in the solid. The spectral differences noted on going from the vapor phase to the solid parallel previous results on p-nitroaniline (PNA). Unlike the situation encountered in PNA (and 2,6-ANN), however, the O1s(NO2) spectrum of 1,4-ANN also indicates a moderately increased splitting as well as an apparent decrease in shake-up intensity on going from the vapor to the condensed phase. Spectral differences between the two phases are addressed within the CNDO/S(CI) equivalent-core approximation including up to doubly excited ‘‘singlet-coupled’’ configurations as a means of elucidating the apparent core-hole induced intermolecular coupling. Comparison computations are presented on PNA, 2,6-ANN, and 1,4-ANN. X-ray crystallographic measurements on 2,6-ANN indicate an intermolecular donor/acceptor pairing scheme as in PNA. Dimer models are therefore used to computationally address solid state interactions. The N1s(NO2) spectral differences observed in PNA, 2,6-ANN, and 1,4-ANN on going from the vapor to the condensed phase are attributed to strong core-hole induced intermolecular orbital mixing. Explicit inclusion of doubly excited configurations in both the monomer and dimer (solid) computations is essential to achieve theoretical assignments of the N1s(NO2) and O1s(NO2) spectra consistent with experiment. The results are analyzed in terms of dominant single and doubly excited contributions.