A combined theoretical and experimental study on molecular photonics

Abstract

A detailed account of quantum chemical procedures for estimating the rate constants of the photophysical processes at work in polyatomic organic molecules is given. The results obtained from combined experimental and theoretical research into the spectral-luminescent properties of acridine, 9-aminoacridine, 2,7-dimethyl-9-diphenylaminoacridine, and of their protonated forms are reported. The electronic absorption and fluorescence spectra of acridine have been investigated at room temperature in ethanol solution of varying pH and in other solvents of different chemical nature and polarity. The energy of excited states, the rate constants of the deactivation of the excited states, and the dipole moments obtained by quantum chemical methods for the examined compounds are presented. The findings of the investigations are discussed.

Photobase effect for just-in-time delivery in photocatalytic hydrogen generation

“…3e), the PL is much weaker and no stimulated emission regions can be seen. This is in agreement with a faster intersystem crossing and low PL quantum yield of the S 1 state expected in the non-protonated species 35,36,44 . The resulting higher triplet state then relaxes to lower lying T 1 state 36 .…”

“…This absorption range is very convenient for the excitation during the optical spectroscopy experiments. In contrast to S 2 , the energy level of S 1 state is lower in the protonated state, leading to a pronounced red-shift at pH 3 of the secondary peak at longer wavelengths 35 . As mentioned before, acridine is mostly non-protonated at pH 7.…”

“…of acridine. The sharp absorption peaks at 354 nm can be ascribed to transitions to singlet state S 2 , whose energy is largely unaffected by the changes in pH 35,44 . This absorption range is very convenient for the excitation during the optical spectroscopy experiments.…”

“…Hence, the normalized absorption spectrum in neutral conditions is identical to the spectrum acquired at pH 13, with the exception of a small peak at 430 nm due to a small fraction of the protonated species. The PL quantum yield of acridine is strongly dependent on pH, with the yield decreasing with pH 35,45 . The spectra are also different with the peak of emission of the protonated species (pH 3) at 475 nm and of the non-protonated species (pH 13) at 427 nm.…”

“…Therefore, we started with polyethylene glycol (PEG) derived nitrogen-free CDs and introduced acridine, a model photobase, into the photocatalyst. Acridine is an N-heterocyclic aromatic compound, resembling molecules likely present in the CDs 20 , with pK a = 5.5 in the ground state and pK a * = 10.7 in the excited state [34][35][36] . This means that at pH 7 it is protonated to a very small extent (3%), but becomes mostly protonated in the excited state (see details in the Supplementary Note 1).…”