Polarized two-photon fluorescence excitation studies of pyrimidine

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Abstract
Two-photon fluorescence excitation studies are reported for pyrimidine (1, 3-diazine) dissolved in a hexane solution at room temperature. Quantitative two-photon absorption strengths for the three lowest electronic absorption bands are measured relative to the benzene B2u ← A1g two-photon transition. The B1(nπ*) ← A1 excitation profile matches the one-photon absorption band closely, indicating an allowed two-photon transition. However, the B2(ππ*) two-photon intensity is vibronically induced by a 1600 cm−1 b2 vibration, even though transitions into this state are formally two-photon allowed. In addition, polarization analysis is used to uncover a new transition in pyrimidine in a region of overlapping absorptions. This new transition is assigned to the lowest A2(nπ*) state. If this assignment is correct, the A2(nπ*), B1(nπ*) splitting is 1.4 eV. Spectroscopic CNDO and INDO calculations of the two-photon absorption strengths are also reported.