Mesomeric and twisted intramolecular-charge-transfer states as a key to polarity-dependent fluorescence of donor-acceptor-substituted aryl pyrenes

Computational study by the AM1 method has been performed for pyrene-based donor-acceptor-substituted systems, with the aim to elucidate the origin of their polarity-dependent fluorescence governed by mesomeric and twisted internal-charge-transfer (MICT and TICT, resp.) states. Using theoretical methods, principal relationships have been established between the constitution of arylpyrene derivatives (donor-acceptor strength of substituents, the substitution pattern, sterical hindrance, inclusion of additional aryl spacers between the donor and acceptor moieties, etc.) and environmental effects (solvent polarity and external electric field strength), and the properties of the MICT and TICT states (energy, localization, dipole moment, allowedness). These relationships have been compared to the experimental fluorescence properties. The substituent-induced donor-acceptor difference has been varied in a continuous way in both directions by employing point charges in the molecular surrounding (“sparkles”). A remarkable feature of the phenylpyrene molecule has thus been revealed: it can exist in two MICT and two TICT states, the CT states in each pair being oppositely polarized and much the same in energy. It is shown, moreover, that the quantum-chemically calculated trends in MICT and TICT energies in the families of related compounds can be qualitatively judged from simple MO considerations including the analysis of frontier MO energies and shapes for the isolated molecular subunits. The approach employed is, therefore, applicable as a first-step tool in the design of compounds with the desired features of polarity-sensitive fluorescence.