The peroxide decomposition that generates the excited-state carbonyl compound is the key step in most organic chemiluminescence, and chemically initiated electron exchange luminescence (CIEEL) has been widely accepted for decades as the general mechanism for this decomposition. The firefly dioxetanone, which is a peroxide, is the intermediate in firefly bioluminescence, and its decomposition is the most important step leading to the emission of visible light by a firefly. However, the firefly dioxetanone decomposition mechanism has never been explored at a reliable theoretical level, because the decomposition process includes biradical, charge-transfer (CT) and several nearly degenerate states. Herein, we have investigated the thermolysis of firefly dioxetanone in its neutral (FDOH) and anionic (FDO(-)) forms using second-order multiconfigurational perturbation theories in combination with the ground-state intrinsic reaction coordinate calculated via the combined hybrid functional with Coulomb attenuated exchange-correlation, and considered the solvent effect on the ground-state reaction path using the combined hybrid functional with Coulomb attenuated exchange-correlation. The calculated results indicate that the chemiluminescent decomposition of FDOH or FDO(-) does not take place via the CIEEL mechanism. An entropic trap was found to lead to an excited-state carbonyl compound for FDOH, and a gradually reversible CT initiated luminescence (GRCTIL) was proposed as a new mechanism for the decomposition of FDO(-).

The chemiluminescence phenomenon of 3-(2'-spiroadamantyl)-4-methoxy-4-(3″-phosphoryloxy)-phenyl-1,2-dioxetane (AMPPD or m-AMPPD) has been routinely applied in clinical diagnostics. Although the AMPDD chemiluminescence immunoassay is one of the most successful immunoassays, the mechanism of AMPPD chemiluminescence remains largely unknown. The AMPPD chemiluminescence process is composed of three steps: AMPPD is enzymatically triggered to produce 3-(2'-spiroadamantyl)-4-methoxy-4-(3″-hydroxyphenyl)-1,2-dioxetane (m-AMPD); m-AMPD decomposes into the excited-state methyl m-oxybenzoate anion (m-MOB(-)); the excited-state m-MOB(-) relaxes to its ground state and emits light. Obviously, the middle step is critical for the chemiluminescence and has not been well understood because of both experimental and theoretical difficulties. We performed the first theoretical study on the chemiluminescent decomposition mechanism of m-AMPD and its para isomer, p-AMPD, using the complete active space self-consistent field and the second-order multiconfigurational perturbation methods in addition to the density functional method. This investigation revealed that (1) neither the intramolecular chemical initiated electron exchange luminescence (CIEEL) nor the concerted charge transfer (CT) mechanism can describe the decomposition of m- and p-AMPD well. Instead, their decomposition occurs according to our previously proposed mechanism of gradually reversible CT-initiated luminescence. (2) The different stabilities of the m- and p-AMPD chemiexcited states might be the basis for the large difference in their chemiluminescence efficiencies. (3) The relationship between the chemiluminescence efficiency and the position of the electron donor on the aromatic ring, the so-called "odd/even selection rule," does not fully explain the chemiluminescence efficiency of dioxetanes. The odd/even selection rule is only correct for partial dioxetanes, because it does not capture the origin of the relationship between the chemiluminescence and the donor. We revealed that the origin consists of a combination of conjugation, induction, and steric effects. On the basis of this combination of effects, we theoretically designed some 1,2-dioxetanes to guide experimentalists in the synthesis of these excellent chemiluminescent molecules.

The firefly is famous for its high bioluminescent efficiency, which has attracted both scientific and public attention. The chemical origin of firefly bioluminescence is the thermolysis of the firefly dioxetanone anion (FDO(-)). Although considerable theoretical research has been conducted, and several mechanisms were proposed to elucidate the high efficiency of the chemi- and bioluminescence of FDO(-), there is a lack of direct experimental and theoretical evidence. For the first time, we performed a nonadiabatic molecular dynamics simulation on the chemiluminescent decomposition of FDO(-) under the framework of the trajectory surface hopping (TSH) method and theoretically estimated the chemiluminescent quantum yield. The TSH simulation reproduced the gradually reversible charge-transfer initiated luminescence mechanism proposed in our previous study. More importantly, the current study, for the first time, predicted the bioluminescence efficiency of the firefly from a theoretical viewpoint, and the theoretical prediction efficiency is in good agreement with experimental measurements.

A usual strategy in both experimental and theoretical studies on bio- and chemiluminescence is to analyze the fluorescent properties of the bio- and chemiluminescence reaction product. Recent findings in a coelenteramide and Cypridina oxyluciferin model raise a concern on the validity of this procedure, showing that the light emitters in each of these luminescent processes might differ. Here, the thermal decomposition path of the firefly dioxetanone and the light emission states of the Firefly oxyluciferin responsible for the bio-, chemiluminescence, and fluorescence of the molecule are characterized using ab initio quantum chemistry and hybrid quantum chemistry/molecular mechanics methods to determine if the scenario found in the coelenteramide and Cypridina oxyluciferin study does also apply to the Firefly bioluminescent systems. The results point out to a unique emission state in the bio-, chemiluminescence, and fluorescence phenomena of the Firefly oxyluciferin and, therefore, using fluorescence properties of this system is reasonable.

Time-dependent density functional theory (TDDFT) with and without a spin-flip scheme is extensively compared in on-the-fly trajectory surface hopping molecular dynamics with a global switching (GS) algorithm. The simulation is performed for cis-trans azobenzene photoisomerization following the excitation to the S(nπ*) state that is involved in a conical intersection (CI) between ground and first excited states. This CI is found correctly to be a single-cone (artificial double-cone) structure computed by the TDDFT method with (and without) spin-flip. Nevertheless, simulated quantum yields and lifetimes are in very good agreement; 0.43 and 63 fs (0.34 and 62 fs) for cis-to-trans isomerization, and 0.11 and 2200 fs (0.13 and 1040 fs) for trans-to-cis isomerization, by TDDFT with (and without) a spin-flip scheme. Distributions of excited-state decay, hopping spots and products, as well as typical trajectories have similar patterns and behaviors with and without spin-flip. The global switching trajectory surface hopping method is demonstrated to be well suited to TDDFT on-the-fly dynamic simulation with and without spin-flip. For comparison, previous simulations with the CASSCF method and Tully's fewest-switches trajectory surface hopping method are also addressed.