Theoretical study on intramolecular hydrogen transfer involving amino-substituted perylenequinone

Abstract

2-Amino-perylenequinone (2-NH2-PQ) and 3,10-diamino-perylenequinone (3,10-diNH2-PQ) were used as model compounds for investigating the intramolecular hydrogen transfer (IHT) reactions involving amino-substituted perylenequinonoid photosensitizers (PQP). A semi-empirical MO method (AM1) was employed to calculate the theoretical values associated with the IHT process. It was found that amino-substitution enhanced the entropy of a specific isomer of PQ, causing it to be more stable than other possible isomers. Amino-substitution raised the HOMO energy levels (EHOMO) of PQ, by increasing its electron-donating ability. It was also found that placing an NH2 group in the 2-position lowered the energy barriers to IHT in PQ in the ground state and the singlet excited state, and 3,10-diNH2-substitution increased the IHT energy barriers in PQ in both states. The energy barrier for the double IHT was almost two times the energy barrier for the single IHT, making the double IHT a less important process. The IHT energy barrier for NH2-PQ decreased in the singlet excited state, while the energy barrier decrease from the excited state to ground state was larger for 2-NH2-PQ than for 3,10-diNH2-PQ. The charge on the hydrogen atom and the molecular dipole moment varied during the IHT process, and the IHT energy barriers were governed by the differences in the charges on the hydrogen atom in the transition state and initial state. The dipole moment for the molecular singlet excited state changed considerably during the IHT process, indicting that IHT would be accelerated by strongly polar solvents. Like the PQ case, there was no correlation between changes in the charge on the hydrogen atom and dipole moment changes in the IHT process.