An investigation of the chemiluminescent properties of 2,6-diphenylimidazo[1,2-a]pyrazin-3(7H)-one derivatives (1), having substituted phenyl groups, is described. Among the derivatives 1, the 6-[4-(dimethylamino)phenyl] derivatives (1a,d–f) gave a high quantum yield (ΦCL ≥ 0.0025) in diglyme/acetate buffer, which is a model reaction condition for the Cypridina bioluminescence. Their efficient chemiluminescence is mainly caused by the electronic effect of the substituent at C6. In particular, the electron-donating 4-(dimethylamino)phenyl group at C6 of 1a,d–f plays an essential role in increasing the chemiexcitation efficiency (ΦS) by the charge transfer-induced luminescence (CTIL) mechanism. The results provide useful information for designing new Cypridina luciferin analogues showing efficient chemiluminescence.

To clarify the acyl substitution effect on the fluorescent property of Cypridina oxyluciferin analogs, which have the 4-(dimethylamino)phenyl group at C5, we compared the spectroscopic properties of benzamide 1b and its 2,6-dimethylphenyl derivative 1c in various solvents with those of acetamide 1a. Similarity of the fluorescence wavelengths of 1a–c indicates that the π-electronic conjugation in the excited singlet (S1) state is terminated at the acyl group and that the benzamide moiety in 1b has little effect on modulating the florescence color. The similar fluorescence solvatochromism of 1a–c reveals that their S1 states have a similar intramolecular charge transfer character. The fact that 1a–c have various fluorescence quantum yields and lifetimes depending on solvent polarity and hydrogen bonding interactions in solutions provides information about the decay processes competing with the fluorescence process in the S1 states. Among 1a–c, benzamide 1b exhibits the most sensitive variation of the fluorescence intensity depending on the solvent used. Similarity of the fluorescence spectra of 1a–c adsorbed in NaY zeolites was also observed, which indicates that the S1 states of 1a–c give mobile conformers in the NaY supercages. The relationship between the molecular structures of 1a–c and their spectroscopic properties will provide a guide for designing a new fluorophore based on Cypridina oxyluciferin.

Spectroscopic properties of amino-analogs of luciferin and oxyluciferin were investigated to confirm the color modulation mechanism of firefly (beetle) bioluminescence. Fluorescence solvatochromic character of aminooxyluciferin analogs indicates that the bioluminescence of aminoluciferin is useful for evaluating the polarity of a luciferase active site.

Nitrogen-rich heterocycles, bis(pyrazino[2′,3′:4,5]imidazole)-fused 1,2,5,6-tetrahydro-1,4,5,8,9,10-hexaazaanthracenes (BPI-HAAs) were prepared by conventional Pd(OAc)2/BINAP-catalyzed C–N coupling reactions of 5-aryl-3-bromoaminopyrazines. The BPI-HAA core is a planar structure with aromaticity, and this heterocycle exhibits red fluorescence and moderate electron-accepting characteristics.

New boron-containing fluorophores, difluoro[amidopyrazinato-O,N]boron (APB) derivatives, were prepared from amidopyrazines. The fluorescence properties of APB were successfully modulated by an aryl substitution at the C8 position.

To establish the reaction mechanism of the high-quantum-yield bioluminescence in Cypridina (Vargula), we investigated the chemiluminescence of 6-aryl-2-methylimidazo[1,2-a]pyrazin-3(7H)-ones (1H) as Cypridina luciferin analogues in DMSO–1,1,3,3-tetramethylguanidine and in diglyme–acetate buffer. We found that the chemiluminescence of 1H with an electron-donating aryl group, such as a 4-(dimethylamino)phenyl, 3-indolyl or 3-(1-methyl)indolyl group, gave a high quantum yield (ΦCL) in diglyme–acetate buffer. This indicates that the reaction mechanism producing this high ΦCL involves the chemiexcitation of a neutral dioxetanone intermediate possessing an electron-donating aryl group to the singlet excited state of neutral acetamidopyrazine (the light emitter). In addition, we investigated the fluorescence of acetamidopyrazines and performed DFT calculations for neutral dioxetanones and the transition states (TS) of the dioxetanone's decomposition. The results made it clear that the electron-donating aryl group gives the TS and the singlet-excited acetamidopyrazine (S1) a strong intramolecular charge transfer (ICT) character, and their similar ICT character leads to the ICT TS → S1 route in the charge transfer-induced luminescence (CTIL) mechanism for efficient chemiexcitation. The reaction mechanism of the chemiluminescence of 1H can explain the highly efficient chemiexcitation of Cypridina bioluminescence.

To elucidate the ionic structure of the excited state of light-emitter coelenteramide in aequorin bioluminescence, the fluorescent properties of phenolate anions of coelenteramide analogues were investigated. Fluorescence of phenolate anion in non-polar solvents was observed by electronic excitation of a 1:1 hydrogen-bonded complex of a coelenteramide analogue with a hydrogen-bond donor molecule such as n-butylamine. In polar solvents, the phenolate anion was directly generated using a base, and its fluorescence was studied. These results confirm that the singlet-excited state of phenolate anion of coelenteramide has an intramolecular CT character, and that its fluorescence emission wavelength changes depending upon solvent polarity. The fluoro-substituent effect on the fluorescent property of phenolate anions was also clarified to help in explaining the bioluminescent property of fluorinated semi-synthetic aequorin. These results consistently support the assignment that the phenolate anion is the ionic structure of the excited light-emitter in BFP during AQ bioluminescence.