Two molecules, 1-hydroxypyrene-2-carbaldehyde (HP) and 1-methoxypyrene-2-carbaldehyde (MP) were explored. We investigated their photophysical properties, using experimental transient absorption and theoretical density functional theory/time-dependent density functional theory (DFT/TDDFT). HP and MP have similar geometric conformations but exhibit entirely different photophysical properties upon excitation into the S₁ state. In contrast to traditional excited state intramolecular proton transfer (ESIPT) in molecules that exhibit either single or dual fluorescence, HP has an unusual non-fluorescent property. Specifically, the ultrafast ESIPT process occurs in 158 fs and is followed by an intersystem crossing (ISC) component of 11.38 ps. In contrast to HP, MP undergoes only an 8 ps timescale process, which was attributed to interactions between solute and solvent. We concluded that this difference arises from intramolecular hydrogen bonds (IMHBs), which induce drastic structural alterntion upon excitation.

The effects of atomic electronegativity (O, S and Se atoms) on the excited-state intramolecular proton transfer (ESIPT) properties of three compounds (3-HF, 3-HTF and 3-HSeF) were systematically studied using the density functional theory (DFT) and time-dependent DFT (TD-DFT) methods. Furthermore, the antioxidant activities of the three compounds were evaluated using different parameters from the perspective of biological activities. The calculated results indicate that the generation of ESIPT is attributed to the excited-state intramolecular charge transfer owing to the change in electron density distribution on the frontier molecular orbitals and natural bond orbital (NBO) charge distribution of the O₁ and O₂ atoms at the S₀ and S₁ states. Moreover, 3-HTF shows the greatest red shift of the O₁–H₁ stretching peak and the lowest energy barrier at the S₁ state among the three compounds, implying that 3-HTF would have the strongest intramolecular hydrogen bond at the S₁ state, which is most beneficial for the occurrence of ESIPT. In addition, the decreased ionization potentials and energy gaps from 3-HF to 3-HTF and 3-HSeF means that the antioxidant activity of the compound would be enhanced along with the lowered atomic electronegativity. Interestingly, the lower ionization potentials and energy gaps of the compounds in keto forms suggest that the compound (3-HTF) easy to carry out ESIPT reaction would exhibit efficient antioxidant activity, which would establish the relationship between ESIPT reaction and antioxidant activity of the compound and provide a new notion for synthesizing more efficient antioxidants in experiments.

We investigated the photoelectric properties of two indoline dyes (CA2 and CA3) with different conjugated length spacers, and the same cyanoacrylic acid acceptor, and global evaluation models were developed based on the experimental results, the developed normal model, the transient spectrum results, and the theoretical normal model. First, the properties of two dyes were studied through ultraviolet-visible spectroscopy, energy levels, and molecular descriptors. The observations agreed well with calculations carried out through basis-set correction. Second, the short-circuit current density (J_sc) and open-circuit voltage (V_oc) curve were estimated by using the developed normal model (DNM) based on the transient spectrum, molar extinction coefficient, adsorption capacity, and recombination rate. The values of the DNM method (J_sc = 16.24 mA cm⁻², V_oc = 0.59 V, η% = 7.81 for CA2) were closer to the experimental results (J_sc = 17.27 mA cm⁻², V_oc = 0.59 V, η% = 6.57) compared with the normal model (J_sc = 21.66 mA cm⁻², V_oc = 0.50 V and η% = 8.67), which was evaluated by using the electron-injection lifetime, excited-state lifetime, fluorescence lifetime, energy difference between the TiO₂ conduction band (E_CBE), and the electron difference between the donor and the recipient (n_c) for J_sc and V_oc. The DNM could be used to evaluate J_sc, V_oc, and η from considerations of the relationship between the macroscopic properties and microscopic parameters, and provided an effective method to improve the molecular design and performance tuning by decreasing factors that affect the energy conversion unfavorably.

The multiple proton transfer reactions of 3-hydroxypyridine-(H ₂ O) ₃ have been demonstrated, and a perfect proton transfer cycle has been revealed in the ground and excited states.