Determining the ground and excited-state decomposition mechanisms of 1,2-dioxetane is essential to understand the chemiluminescence and bioluminescence phenomena. Several experimental and theoretical studies has been performed in the past without reaching a converged description. The reason is in part associated with the complex nonadiabatic process taking place along the reaction. The present study is an extension of a previous work (De Vico, L.; Liu, Y.-J.; Krogh, J. W.; Lindh, R. J. Phys. Chem. A 2007, 111, 8013-8019) in which a two-step mechanism was established for the chemiluminescence involving asynchronous O-O' and C-C' bond dissociations. New high-level multistate multi configurational reference second-order perturbation theory calculations and ab initio molecular dynamics simulations at constant temperature are performed in the present study, which provide further details on the mechanisms and allow to rationalize further experimental observations. In particular, the new results explain the high ratio of triplet to singlet dissociation products.

Four-membered cyclic peroxides are high-energy compounds often associated to cold light emission, but whose chemical and biological roles are still matters of debate. The often-dangerous synthesis of 1,2-dioxetanes, achieved around 50 years ago, has been mastered over the years to a point where some derivatives are commercially available. This fact does not imply that 1,2-dioxetanes can be conveniently prepared in the gram scale or that the synthesis of analogous 1,2-dioxetanones and the elusive 1,2-dioxetanedione are simple. Important questions on the mechanism of chemiluminescence and bioluminescence reactions are under experimental and theoretical scrutiny. The available data have contributed to relate structural and medium effects to the quantum efficiency of these compounds to produce excited states. Consequently, such peroxides have been suggested to produce biologically relevant electronically excited species in vivo in the absence of light. The connection of this hypothesis with melanin-mediated photodamage in the dark has renewed the interest in such cyclic peroxides. This review gives some insight on the synthesis, chemiluminescence mechanism, and biological relevance of 1,2-dioxetanes, 1,2-dioxetanones, and 1,2-dioxetanedione and provides practical protocols for those interested in engaging this field.

Light emission from luminol is probably one of the most popular chemiluminescence reactions due to its use in forensic science, and has recently displayed promising applications for the treatment of cancer in deep tissues. The mechanism is, however, very complex and distinct possibilities have been proposed. By efficiently combining DFT and CASPT2 methodologies, the chemiluminescence mechanism has been studied in three steps: 1) luminol oxygenation to generate the chemiluminophore, 2) a chemiexcitation step, and 3) generation of the light emitter. The findings demonstrate that the luminol double‐deprotonated dianion activates molecular oxygen, diazaquinone is not formed, and the chemiluminophore is formed through the concerted addition of oxygen and concerted elimination of nitrogen. The peroxide bond, in comparison to other isoelectronic chemical functionalities (−NH−NH−, −N−−N−−, and −S−S−), is found to have the best chemiexcitation efficiency, which allows the oxygenation requirement to be rationalized and establishes general design principles for the chemiluminescence efficiency. Electron transfer from the aniline ring to the OO bond promotes the excitation process to create an excited state that is not the chemiluminescent species. To produce the light emitter, proton transfer between the amino and carbonyl groups must occur; this requires highly localized vibrational energy during chemiexcitation.

Chemiluminescence is the emission of light as a result of a nonadiabatic chemical reaction. The present work is concerned with understanding the yield of chemiluminescence, in particular how it dramatically increases upon methylation of 1,2-dioxetane. Both ground-state and nonadiabatic dynamics (including singlet excited states) of the decomposition reaction of various methyl-substituted dioxetanes have been simulated. Methyl-substitution leads to a significant increase in the dissociation time scale. The rotation around the O-C-C-O dihedral angle is slowed; thus, the molecular system stays longer in the "entropic trap" region. A simple kinetic model is proposed to explain how this leads to a higher chemiluminescence yield. These results have important implications for the design of efficient chemiluminescent systems in medical, environmental, and industrial applications.

Determination of the ground- and excited-state unimolecular decomposition mechanisms of 1,2-dioxetanedione gives a level of insight into bimolecular decomposition reactions of this kind for which some experimental results are reported. Although a few studies have put some effort to describe a biradical mechanism of this decomposition, there is still no systematic study that proves an existence of a biradical character. In the present study, state-of-the-art high-level multistate multiconfigurational reference second-order perturbation theory calculations are performed to describe the reaction mechanism of 1,2-dioxetanedione in detail. The calculations indicate that the decomposition of this four-membered ring peroxide containing two carbonyl carbon atoms occurs in concerted but not simultaneous fashion, so-called "merged", contrary to the case of unimolecular 1,2-dioxetane and 1,2-dioxetanone decompositions where biradical reaction pathways have been calculated. At the TS of the ground-state surface, the system enters an entropic trapping region, where four singlet and four triplet manifolds are degenerated, which can lead to the formation of triplet and singlet excited biradical species. However, these excited species have to overcome a second activation barrier for C-C bond cleavage for excited product formation, whereas the ground-state energy surface possesses only one TS. Thus our calculations indicate that the unimolecular decomposition of 1,2-dioxetanedione should not lead to efficient excited-state formation, in agreement with the lack of direct emission from the peroxyoxalate reaction.

Excited-state chemistry is usually ascribed to photo-induced processes, such as fluorescence, phosphorescence, and photochemistry, or to bio-and chemiluminescence, in which light emission is originated by a chemical reaction. A third class of excited-state chemistry, however, is possible that promotes photochemical phenomena by chemienergizing certain chemical groups without light -chemiexcitation. By studying Dewar dioxetane, which can be viewed as the combination of 1,2-dioxetane and 1,3-butadiene, we show here how the isomerization * To whom correspondence should be addressed † Uppsala ‡ València ¶ UC 3 1 channel that characterizes the photo-induced chemistry of 1,3-butadiene can be reached at a later stage after the thermal decomposition of the dioxetane moiety. Multi-reference multiconfigurational quantum chemistry methods and accurate reaction-path computational strategies were used to determine the reaction coordinate that first decomposes the dioxetane, next transfers non-adiabatically the state from the ground to the excited state, and finally brings the system into the photochemical channel of the 1,3-butadiene group. With the present study, we open a new area of research within computational photochemistry to study chemically-induced excited-state chemistry that is difficult to tackle experimentally due to the short-lived character of the species involved in the process. The findings shall be of relevance to unveil "dark" photochemistry mechanisms which might operate in biological systems in conditions of lack of light to allow reactions that are typical of photo-induced phenomena.

Our study on the unimolecular decomposition of a relatively stable 1,2-dioxetanone derivative, model compound for bioluminescence processes, indicates the existence of different reaction pathways for ground and excited state formation.

A combined experimental and computational investigation conducted to understand the nature of the interactions between cobalt II/III redox mediators ([Co(bpy)3](2+/3+)) and their impact on the performance of the corresponding dye-sensitized solar cells (DSCs) is reported. The fully optimized equilibrium structures of cobalt(II/III)-tris-bipyridine complexes in the gas phase and acetonitrile solvent are obtained by the density functional B3LYP method using LanL2DZ and 6-31G(d,p) basis sets. The harmonic vibrational frequencies, infrared intensities and Raman scattering activities of the complexes are also calculated. The scaled computational vibrational wavenumbers show very good agreement with the experimental values. Calculations of the electronic properties of the complexes are also performed at the TD-B3LYP/6-31G(p,d)[LanL2DZ] level of theory. Detailed interpretations of the infrared and Raman spectra of the complexes in different phases are reported. Detailed atomic orbital coefficients of the frontier molecular orbitals and their major contributions to electronic excitations of the complexes are also reported. These results are in good agreement with the experimental electrochemical values. Marcus diagram is derived for the electron transfer reaction Co(II) + D35(+)→ Co(III) + D35 using the Co-N bond length as a reaction coordinate.