Benzoyleneurea (BenzU), a fluorescent nucleobase analog, exhibits strong solvent dependent fluorescence intensity. With the increasing of the concentration of polar and protic solvents, BenzU exhibits enhanced fluorescence. To understand the photophysical properties of BenzU, the energy gaps (ΔE) between dark $n\pi^*$ state and bright $\pi\pi^*$ state of BenzU are calculated in several solvents. The solvent dielectric constant and hydrogen bond between BenzU and protic solvent both have effects on ΔE. And the fluorescence quantum yields of BenzU are linear response to ΔE. Furthermore, we find that the more hydrogen bonds are formed between BenzU and protic solvent, the higher ΔE and fluorescence intensity are obtained.

A balance between the local excitation (LE) and charge-transfer (CT) transitions is an important issue of a density functional. In this work, we compare the performance between the global hybrid functional M062X and long-range separated functional CAM-B3LYP when they calculate the absorption energies of visible light photoswitches hemithioindigo (HTI) Z1, Z2, Z4, Z5 and Z7. The electronic excitations of HTI Z1 and Z2 are mixtures of LE and CT transitions. CAM-B3LYP succeeds to predict the same absorption energy sequence of the five compounds as the experimental results while M062X predicts a wrong sequence of HTI Z1 and Z2. Another global hybrid functional M06 is also employed for a comparison. Finally, we suggest CAM-B3LYP rather than M062X when dealing with mixed electronic transtions.

The green fluorescent protein (GFP) has been widely used in biochemical and biological fields for its strong fluorescence emitting property. The effective fluorescence emitting property lies in its chromophore P-HBI. Compared with GFP, the low fluorescence quantum yield (0.2 vs. 0.8 of GFP) of the wild blue fluorescent protein (BFP) restrict its extensive applications. In order to enhance the fluorescent quantum yield of BFP, numerous attempts have been executed to modify the molecular configuration such as introducing the intramolecular hydrogen bond (IHB) or benzene ring to the parent structure. In the present work, we employed the high-level quantum chemistry calculation method CASSCF to investigate the excited state deactivation mechanism of 4-BFP, which is a newly synthesized BFP chromophore derivative by introducing a benzene ring to the five-membered heterocycle attempting to increase the fluorescence quantum yield. By combination of the optimization of stable electronic structures, scanning of potential energy surfaces and construction of energy profile, we found that the fluorescent quantum yield could not be enhanced for the 4-BFP molecule. One minimum energy conical intersection was located between the S1 and S0 states, which could act as the gateway for the 4-BFP to funnel to the ground state by the internal conversion process. The current work could provide fundamental guide for further molecular structure modification or improvement to enhance the fluorescent quantum yield of BFP analogues.