Absorption and luminescence properties of 1, 10-phenanthroline, 2, 9-diphenyl-1, 10-phenanthroline, 2,9-dianisyl-1, 10-phenanthroline and their protonated forms in dichloromethane solution

TLDR: In this paper, trifluoroacetic acid is added to CH2Cl2 solutions of 1, 10-phenanthroline (1), 2, 9-diphenyl-1, 10phenanthropyl (2) and 2,9-dianisyl- 1,10-phennthroline(3) to cause strong changes in the absorption and fluorescence spectra at room temperature.

Abstract: Addition of trifluoroacetic acid to CH2Cl2 solutions of 1, 10-phenanthroline (1), 2,9-diphenyl-1, 10-phenanthroline (2) and 2,9-dianisyl-1, 10-phenanthroline (3) causes strong changes in the absorption and fluorescence spectra at room temperature. The low-energy absorption bands move to the red, while isosbestic points are maintained. The intensity of the structured, short-lived ππ* fluorescence band decreases and a new, unstructured longer-lived fluorescence band arises at longer wavelength. The number of equivalents of acid needed to obtain 50% protonation in the ground state increases in the series 3 < 2 < 1. The spectral changes can be fully reversed by addition of base. The proton affinity in the singlet excited state is higher than that in the ground state (ΔpKa≈ 14 units for 3), but the protonation and deprotonation processes are too slow to compete with the excited-state decay. In a rigid CH2Cl2 matrix at 77 K the shift of the fluorescence band upon protonation is smaller than that observed at room temperature. The effect of protonation on the phosphorescence band, which can be observed in the rigid CH2Cl2 matrix at 77 K, is very small.