Research Article  Open Access
Theoretical Analysis of the Effect Provoked by BromineAddition on the Thermolysis and Chemiexcitation of a Model Dioxetanone
Abstract
Chemi/bioluminescence are phenomena in which chemical energy is converted into electronically excited singlet states, which decay with light emission. Given this feature, along with high quantum yields and other beneficial characteristics, these systems have gained numerous applications in bioanalysis, in biomedicine, and in the pharmaceutical field. Singlet chemiexcitation is made possible by the formation of cyclic peroxides (as dioxetanones) as thermolysis provides a route for a ground state reaction to produce singlet excited states. However, such thermolysis can also lead to the formation of triplet states. While triplet states are not desired in the typical applications of chemi/bioluminescence, the efficient production of such states can open the door for the use of these systems as sensitizers in photocatalysis and triplettriplet annihilation, among other fields. Thus, the goal of this study is to assess the effect of heavy atom addition on the thermolysis and triplet chemiexcitation of a model dioxetanone. Monobromination does not affect the thermolysis reaction but can improve the efficiency of intersystem crossing, depending on the position of monobromination. Addition of bromine atoms to the triplet state reaction product has little effect on its properties, except on its electron affinity, in which monobromination can increase between 3.1 and 8.8 kcal mol^{−1}.
1. Introduction
Bioluminescence is a widespread natural phenomenon in which living organisms convert chemical energy into light emission via biochemical reactions [1–5]. Bioluminescence can be found in organisms as different as bacteria, dinoflagellates, fungi, crustaceans, worms, insects, and fishes. Light emission from these systems is the result of enzymecatalyzed reactions, which can be divided into two main classes: luciferaseluciferin reactions [2, 5–8] and photoprotein systems [2, 9]. The luciferase enzyme is responsible for catalyzing the oxidation of its substrate, luciferin, which generates an electronically excited singlet state product. This product, generally termed oxyluciferin, subsequently relaxes to the ground state by photon emission. It should be noted that luciferin, luciferase, and oxyluciferin are only generic terms, showing significant structural differences between bioluminescent species. Moreover, luciferaseluciferin reactions are the most prevalent bioluminescent systems [1–3, 5–8].
Photoprotein systems have been found to be exclusive to marine organisms and are characterized by the formation of a stable enzymesubstrate complex [2]. Such complex is formed between the apoprotein and an oxygenated marine luciferin (2hydroxyperoxycoelenterazine). Binding of calcium ions to the photoprotein surface triggers the decomposition of the stable complex, which occurs with light emission [2].
Bioluminescence can be considered a subtype of chemiluminescence, in which light emission arises from a chemical reaction [1, 10, 11]. The efficiency of light emission of both bioluminescent and chemiluminescent reactions is described in terms of quantum yield, which is controlled by three factors [1, 10, 11]: chemical yield of the ground state reaction, chemiexcitation yield of the singlet excited state product, and finally the fluorescent quantum yield of the emitter. Typically, bioluminescent systems present significantly higher quantum yields than chemiluminescent reactions, with some reactions reaching quantum yields of 45–61% [12]. Given this efficient production of electronically singlet excited state products, relative nontoxicity of luciferin compounds, and the relatively simple chemistry of these systems, among other beneficial characteristics (as sensitivity and sensibility), several chemi/bioluminescent systems have gained numerous biomedical, pharmaceutical, and bioanalytical applications. More specifically, these systems are used in the analytical determination of ATP and other metabolites, in environmental monitoring, in bioimaging, and in biosensing, as a gene reporter, tested as alternative excitation sources in photodynamic therapy of cancer, and used in investigations of infectious diseases, among others [13–17].
The efficient formation of singlet excited state products, necessary for the use of these systems in the many applications referred above, is only made possible by the formation of cyclic peroxide intermediates during the different chemi and bioluminescent reactions [18–28]. Within the large number of different chemi and bioluminescent systems, these peroxide intermediates can take the form of dioxetanes (I), dioxetanones (II), or dioxetanedione (III) [18–28], which can be seen in Scheme 1. These cyclic peroxides are responsible for chemi and bioluminescence as their thermolysis provides a route for a thermally activated ground state reaction to produce singlet excited state products [18–28]. This chemiexcitation process is thought to arise from crossing points between the ground state and excited state potential energy surfaces (PES) on the reaction coordinate.
It should be noted that while chemi and bioluminescent systems are better known for their production of singlet excited states, experimental studies have shown that more structurally simple dioxetanes and dioxetanones have the ability to produce triplet excited states [1, 10, 25–27]. While no experimental results are found for more complex chemi/bioluminescent systems, different theoretical studies have found pathways for triplet chemiexcitation in the thermolysis of dioxetanone rings in several systems [1, 11, 18, 19, 24, 28, 29]. This production of triplet excited states can be problematic for the several practical applications based on the formation of lightemitting singlet states, as triplet states are very easily quenched, and their formation will not be detected with the luminescent and fluorescent approaches typically used to detect chemi/bioluminescence. Moreover, triplet states are more reactive and are able to produce harmful reactive species (as singlet oxygen), which can lead to some problems when using these systems in biological samples.
While the formation of triplet state (instead of singlet ones) is not desired in the typical applications of chemi/bioluminescence, such states can have important roles in other applications. One such example is upconversion by triplettriplet annihilation, which typically proceeds as follows [30–32]: a sensitizer molecule is photoexcited and undergoes intersystem crossing to a triplet state. Subsequently, the sensitizer transfers its energy to an emitter molecule via fast triplettriplet energy transfer, which stores the energy in the lowest triplet state of the emitter molecules. Then, two emitters interact and triplettriplet annihilation occurs, which brings one emitter molecule to an excited singlet state while quenching another to its ground state. The emitter then emits light via fluorescence, at a higher energy than that of the photons initially absorbed by the sensitizer. Triplettriplet annihilation has been already applied with success in several research fields, as in luminescence bioimaging [33], photovoltaics [34, 35], and photoinduced drug release [36].
Another useful application of triplet excited states is on the field of photocatalysis [37–39]. In this field, photosensitizers are used to mediate photochemical reactions by absorbing light and using that energy to activate ground state reactants toward some specific chemical reactions. One common method of photoactivation is via energy transfer from the longerlived triplet state of the photosensitizer to the substrate [37–39]. Another photoactivation pathway involves an electron transfer from or to the photoexcited sensitizer [37, 38].
Given this, if one can shift the spin of the chemi/bioluminescent products, from lightemitting singlet to triplet states, one can open the door for new types of applications for chemi and bioluminescent systems, as in upconversion processes by triplettriplet annihilation and photocatalysis. One way to facilitate intersystem crossing to triplet states is to introduce heavy atoms (as bromine and iodine) into the molecular structures, the so called “heavy atom effect” [40–42]. Thus, the objective of this work is to model theoretically the effect of brominesubstitution in a model dioxetanone (IV, Scheme 1) and, more specifically, in its thermolysis and triplet chemiexcitation steps. To our knowledge, this is the first theoretical study trying to understand the possible role of the “heavy atom effect” in the reactions of dioxetanone molecules and in what way it affects triplet chemiexcitation. To this end, a methodology combining density functional theory (DFT) based and multireference methods was used.
2. Theoretical Methodology
All calculations were made with the Gaussian 09 program package [43], with no solvent effects. DFT methods (particularly longrangecorrected hybrid exchangecorrelation density functionals) have been gaining traction in the study of chemi/bioluminescent reactions, given their ability to provide quite accurate qualitative pictures for these systems [18–21, 24, 28, 44–47]. In this study was used the CAMB3LYP longrangecorrected density functional [44], which provides good estimates for and local excitations and charge transfer and Rydberg states [45]. Moreover, this functional was already used with success in the study of different dioxetanones [20, 21].
Geometry optimizations and frequency calculations were made with the CAMB3LYP functional, with the 631G(d,p) basis set being used for all atoms except bromine, for which the LanL2DZ basis set was used. The combination of 631G(d,p) and LanL2DZ basis set was termed Basis Set1 (BS1). A restricted (R) approach was used for closedshell species, while an unrestricted (U) species was used for openshell structures. The U approach was used with a brokensymmetry technology, which mixes the HOMO and LUMO, making an initial guess for a biradical.
The thermolysis reaction of the model dioxetanones was studied by performing intrinsic reaction coordinates (IRC) [48], at the CAMB3LYP/BS1 level of theory, which assessed if the obtained transition states connected the desired reactants and products. The transition states were located by using the STQN method [49]. In this work was used the QST3 variant, which requires three molecular specifications: the reactants, the products, and an initial structure for the transition state. The Cartesian coordinates of these transition state structures, used in the IRC calculations, can be found in Tables and of Supplementary Material available online at https://doi.org/10.1155/2017/1903981.
The energies of the geometry optimizations, IRC, and QST3obtained structures were reevaluated by single point calculations with the CAMB3LYP density functional and the 631+G(d,p) basis set for all atoms, except for bromine. For this atom, the LanL2DZdp basis set was used, which includes polarization and diffuse functions. The combination of 631+G(d,p) and LanL2DZdp basis set was termed Basis Set2 (BS2). Thus, the energies of the singlet ground state () and first triplet state () were both calculated at the CAMB3LYP/BS2 level of theory.
The spinorbit coupling between and states was calculated by using the CASSCF method [50]. The LanL2DZ basis set was used for all atoms. The active space consisted of two electrons on two orbitals. These were single point energy calculations made on the DFTcomputed IRC or QST3obtained structures.
3. Results and Discussion
We started this work by analyzing the thermolysis reaction of unsubstituted dioxetanone IVa and monobrominated species IVb, whose energetic profiles are presented in Figures 1(a) and 1(c), respectively. In Figures 1(b) and 1(d) are presented important geometric parameters: the bond lengths of O_{1}O_{4} and C_{2}C_{3} (Scheme 1). It should be noted that the iminocyclopentadienyl moiety was based on the scaffold of azaBODIPY [40–42], which are molecules capable of producing triplet states upon photoexcitation, when they are functionalized with heavy atoms.
(a)
(b)
(c)
(d)
Analysis of the geometric parameters shows that the thermolysis of both species occurs via a stepwise mechanism. The reaction begins by O_{1}O_{4} bond breaking, while the length of C_{2}C_{3} remains constant. Only after O_{1}O_{4}, does the length of C_{2}C_{3} increases, subsequently leading to its cleavage. Analysis of the () value for the transition state of both IVb (~0.57) and IVa (~0.55) showed that these structures have a biradical character. Given this, we can ascribe a stepwisebiradical mechanism for the thermolysis of both dioxetanone species, which is in line with previous theoretical studies of such molecules [18, 20, 21, 24, 28, 29].
While a stepwisebiradical mechanism is usually found in the decomposition of these cyclic peroxides, it can be further subdivided: the biradical is formed due to an electron transfer from an electronrich moiety to the dioxetanone, thereby forming a radical cation and a radical anion, respectively; the biradical is formed due to the homolytic cleavage of the O_{1}O_{4} bond. In this case, both molecules appear to undergo thermolysis via homolysis, as the electron spin density of the transition state resides only on the O_{1} and O_{4} heteroatoms (Figures 2(c) and 2(d)). This finding is in line with the limited charge transfer found between the dioxetanone and iminocyclopentadiene moieties, as demonstrated in Figures 2(a) and 2(b). The atomic charges were measured within the Natural Population Analysis (NPA) scheme. The finding that these molecules undergo a homolysisbased stepwisebiradical thermolysis can be attributed to the absence of an ionizable group, as seen before in the theoretical analysis of such molecules [18, 20, 21, 24, 28, 29].
(a)
(b)
(c)
(d)
Both species have similar activation energies of 24.7 kcal mol^{−1} for IVa and 24.5 kcal mol^{−1} for IVb. These energies were calculated at the CAMB3LYP/BS2 level of theory, with thermal corrections calculated at the CAMB3LYP/BS1 level of theory. Considering that no solvents effects were considered and that these species are only model dioxetanones, the obtained activation parameters compare well with the experimentally obtained ones for several cyclic peroxides (within ~20.0 kcal mol^{−1}) [51–53].
So far, the main conclusion is that the addition of a bromine atom affects slightly the decomposition of the model dioxetanone, as there are only very minor differences between the thermolysis of species IVa and IVb. In fact, both species present the same characteristics as other dioxetanone without an ionized group [18–21, 24, 28, 29].
In the same vein, we have found a pathway for triplet chemiexcitation for species IVa and IVb (Figures 1(a) and 1(c), resp.), in line with other theoretical works on different dioxetanones, dioxetanes, and dioxetanedione [11, 18, 19, 24, 28, 29]. Upon starting the reaction, the energetic difference between and states was large for both molecules (about 48 kcal mol^{−1}). However, from the reactant onward, the energy of the state decreased significantly to a point in both species where the  energy gap is low enough to allow chemiexcitation: 2.5 kcal mol^{−1} (at 0.32 amu^{1/2} bohr) for IVa and 2.6 kcal mol^{−1} (at 0.32 amu^{1/2} bohr) for IVb. In conclusion, by analyzing the and energetic profiles, our results indicate that both IVa and IVb species are capable of triplet chemiexcitation. Moreover, the very similar  energy gap suggests a triplet chemiexcitation of similar magnitude, thereby indicating a small effect provoked by the addition of a bromine heavy atom.
It should be noted, however, that intersystem crossing is a process formally forbidden in nonrelativistic quantum theory, and, so, inferring singlettriplet transition probabilities from energy gaps is not sufficiently accurate [40, 54]. So, to assess the efficiency of intersystem crossing, we must take into account the spinorbit coupling (SOC) between and [40, 54]. These were computed at the multireference CASSCF level of theory, and the SOC values for IVa and IVb are presented in Table 1. The CASSCFcomputed  energy gaps (3.1 kcal mol^{−1} for IVb and 2.9 kcal mol^{−1} for IVa) are in line with the DFTcomputed ones gap (2.5 kcal mol^{−1} for IVb and 2.6 kcal mol^{−1} for IVa). Once again, it appears that the addition of a bromine heavy atom has little effect, as the SOC values for IVa (5.3 cm^{−1}) and IVb (6.1 cm^{−1}) are very similar, despite the SOC being higher for the monobrominated species.
 
obtained during the IRC calculations, corresponding to points of lower S_{0}T_{1} energy gaps: 2.5 kcal mol^{−1} (at 0.32 amu^{1/2} bohr) for IVa and 2.6 kcal mol^{−1} (at 0.32 amu^{1/2} bohr) for IVb. state structures obtained with the QST3 method. 
So far, it does appear that the addition of a bromine atom has only a limited effect on the triplet chemiexcitation of model dioxetanone IV. However, it might be possible that this lack of effect is due to the position of brominesubstitution in the cyclopentadiene moiety and not due to a general absence of the “heavy atom effect” in this molecule. To test this hypothesis, we have calculated transition state structures (with the QST3 method, at the CAMB3LYP/SD1 level of theory) for species IVb, IVc, and IVd. The Cartesian coordinates of these structures can be found in Tables – of Supplementary Material. At those DFTcomputed structures, CASSCF single point energy calculations were made to obtain the SOC values (Table 1). The CASSCFcomputed  energy gaps are similar to all species (between 10.8 and 11.4 kcal mol^{−1}), further indicating that the position of the bromine atom has little effect on the value of the  energy gap. However, this is not true for the SOC values. These increased more significantly, from 7.5 to 11.5 cm^{−1}, with IVd presenting SOC values more than double the ones presented previously by IVa. Thus, these results indicate that the addition of heavy atoms can indeed increase the efficiency of intersystem crossing and, so, the formation of triplet state products, but this effect is controlled by the position in which the heavy atom is inserted.
Having analyzed the effect of monobromination on the thermolysis and triplet chemiexcitation of model dioxetanone IV, we have studied some properties of the reaction product (species Va–d, Scheme 1). The Cartesian coordinates of these structures can be found on Tables – of Supplementary Material. These properties are the electron affinity (EA), ionization energies (IE), and  energy gaps (present in Table 2). The EA and IE were computed vertically, with single point energy calculations on the structure, with as the reference state. All V species present very high IEs, with a limited effect provoked by monobromination, which limits the use of these species as electron donors in photocatalysis. On the contrary, the EA values are more suitable for the use of these species as electron acceptors. Moreover, the addition of bromine atoms can significantly improve the EA of V up to 8.8 kcal mol^{−1}, depending on the location of the substitution on the cyclopentadiene ring. As for  energy gaps, these were computed adiabatically. Our calculations have indicated that monobromination has a limited effect on the  energy gaps, except for species Vc, which decreased the gap by 3.5 kcal mol^{−1}.

4. Conclusion
Chemi and bioluminescence are phenomena in which chemical energy is converted into light emission, via chemical and biochemical reactions. Given this feature, coupled to high quantum yields, relative nontoxicity of the reaction substrates, and the relatively simple chemistry, among other beneficial characteristics (as sensitivity and sensibility), several chemi/bioluminescent systems have gained numerous biomedical, pharmaceutical, and bioanalytical applications.
The efficient formation of lightemitting singlet excited states, needed for the applications referred above, is made possible by the formation of cyclic peroxides (as dioxetanes or dioxetanones). Their thermolysis provides a route for a thermally activated ground state reaction to produce singlet excited state products. However, both experimental and theoretical studies have demonstrated that the thermolysis of these species is also able to produce triplet states. While the formation of such states is not desired in the typical applications of chemi/bioluminescence, efficient production of triplet states might open the door for the use of chemi/bioluminescent systems as sensitizers in the fields of photocatalysis and upconversion processes by triplettriplet annihilation, among others.
Given this, the objective of this theoretical study was to assess the effect induced by heavy atom substitution (in this case, bromine) on the thermolysis and triplet chemiexcitation of a model dioxetanone. Our calculations indicated that monobromination has little effect on the thermolysis reaction, with little effect on the energetics of the reaction and on the variation of other parameters (as selected bond lengths and electron spin and charge density). However, the addition of bromine atoms can increase the spinorbit coupling of the and , thereby increasing the efficiency of intersystem crossing. Nevertheless, this effect is dependent on the position of the brominesubstitution. Study of the reaction product showed a general limited effect provoked by monobromination on the ionization energies and  gaps. On the contrary, monobromination improves the electron affinity of the product, with the degree of improvement being controlled by the position of monobromination.
Conflicts of Interest
The authors declare that they have no conflicts of interest.
Acknowledgments
This work was made in the framework of Project PTDC/QEQQFI/0289/2014, which is funded with national funds by FCT/MEC (PIDDAC). The project is also cofunded by “Fundo Europeu de Desenvolvimento Regional” (FEDER), through “COMPETEPrograma Operacional Fatores de Competitividade” (COMPETEPOFC). This work was also made in the framework of the Project Sustainable Advanced Materials (NORTE0100145FEDER000028), funded by “Fundo Europeu de Desenvolvimento Regional (FEDER),” through “Programa Operacional do Norte” (NORTE2020). Acknowledgment to Project POCI010145FEDER006980, funded by FEDER through COMPETE2020, is also made. The Laboratory for Computational Modeling of Environmental PollutantsHuman Interactions (LACOMEPHI) is acknowledged. Luís Pinto da Silva also acknowledges a postdoctoral grant funded by Project Sustainable Advanced Materials (NORTE0100145FEDER000028).
Supplementary Materials
In the Supplementary Material are found cartesian coordinates of important geometries, at the CAMB3LYP/BS1 level of theory.
References
 L. Pinto Da Silva and J. C. G. Esteves Da Silva, “Firefly chemiluminescence and bioluminescence: efficient generation of excited states,” ChemPhysChem, vol. 13, no. 9, pp. 2257–2262, 2012. View at: Publisher Site  Google Scholar
 A. S. Tsarkova, Z. M. Kaskova, and I. V. Yampolsky, “A tale of two luciferins: fungal and earthworm new bioluminescent systems,” Accounts of Chemical Research, vol. 49, no. 11, pp. 2372–2380, 2016. View at: Publisher Site  Google Scholar
 J. Vieira, L. P. Da Silva, and J. C. G. E. Da Silva, “Advances in the knowledge of light emission by firefly luciferin and oxyluciferin,” Journal of Photochemistry and Photobiology B: Biology, vol. 117, pp. 33–39, 2012. View at: Publisher Site  Google Scholar
 E. P. Coutant and Y. L. Janin, “Synthetic routes to coelenterazine and other imidazo[1,2a]pyrazin3one luciferins: essential tools for bioluminescencebased investigations,” Chemistry, vol. 21, pp. 17158–17171, 2015. View at: Publisher Site  Google Scholar
 S. V. Markova and E. S. Vysotski, “Coelenterazinedependent luciferases,” Biochemistry (Moscow), vol. 80, no. 6, pp. 714–732, 2015. View at: Publisher Site  Google Scholar
 S. M. Marques and J. C. G. Esteves Da Silva, “Firefly bioluminescence: a mechanistic approach of luciferase catalyzed reactions,” IUBMB Life, vol. 61, no. 1, pp. 6–17, 2009. View at: Publisher Site  Google Scholar
 S. Tu and H. I. X. Mager, “Biochemistry of bacterial bioluminescence,” Photochemistry and Photobiology, vol. 62, no. 4, pp. 615–624, 1995. View at: Publisher Site  Google Scholar
 D. E. Desjardin, A. G. Oliveira, and C. V. Stevani, “Fungi bioluminescence revisited,” Photochemical & Photobiological Sciences, vol. 7, pp. 170–182, 2008. View at: Publisher Site  Google Scholar
 O. Shimomura and F. H. Johnson, “Peroxidized coelenterazine, the active group in the photoprotein aequorin,” Proceedings of the National Academy of Sciences of the United States of America, vol. 75, pp. 2611–2615, 1978. View at: Google Scholar
 M. Matsumoto, “Advanced chemistry of dioxetanebased chemiluminescent substrates originating from bioluminescence,” Journal of Photochemistry and Photobiology C: Photochemistry Reviews, vol. 5, no. 1, pp. 27–53, 2004. View at: Publisher Site  Google Scholar
 I. Navizet, Y. J. Liu, N. Ferré, D. RocaSanjuán, and R. Lindh, “The chemistry of bioluminescence: an analysis of chemical functionalities,” ChemPhysChem, vol. 12, pp. 3064–3076, 2011. View at: Google Scholar
 K. Niwa, Y. Ichino, S. Kumata et al., “Quantum yields and kinetics of the firefly bioluminescence reaction of beetle luciferases,” Photochemistry and Photobiology, vol. 86, no. 5, pp. 1046–1049, 2010. View at: Publisher Site  Google Scholar
 A. Roda, M. Guardigli, E. Michelini, and M. Mirasoli, “Bioluminescence in analytical chemistry and in vivo imaging,” TrAC  Trends in Analytical Chemistry, vol. 28, no. 3, pp. 307–322, 2009. View at: Publisher Site  Google Scholar
 M. B. Gu, R. J. Mitchell, and B. C. Kim, “Wholecellbased biosensors for environmental biomonitoring and application,” Advances in Biochemical Engineering/Biotechnology, vol. 87, pp. 269–305, 2004. View at: Publisher Site  Google Scholar
 C. M. Magalhães, J. C. G. Esteves da Silva, and L. Pinto da Silva, “Chemiluminescence and Bioluminescence as an excitation source in the photodynamic therapy of cancer: a critical review,” ChemPhysChem, vol. 17, pp. 2286–2294, 2016. View at: Publisher Site  Google Scholar
 K. E. Luker and G. D. Luker, “Bioluminescence imaging of reporter mice for studies of infection and inflammation,” Antiviral Research, vol. 86, no. 1, pp. 93–100, 2010. View at: Publisher Site  Google Scholar
 L. Cevenini, M. M. Calabretta, A. Lopreside et al., “Exploiting nanoluc luciferase for smartphonebased bioluminescence cell biosensor for (anti)inflammatory activity and toxicity,” Analytical and Bioanalytical Chemistry, vol. 408, pp. 8859–8868, 2016. View at: Google Scholar
 L. Pinto da, C. M. Silva, and J. C. G. Esteves da Silva, “Interstate crossinginduced chemiexcitation mechanism as the basis for imidazopyrazinone bioluminescence,” ChemistrySelect, vol. 1, pp. 3343–3356, 2016. View at: Google Scholar
 L. P. Da Silva and J. C. G. E. Da Silva, “Interstate crossinginduced chemiexcitation as the reason for the chemiluminescence of dioxetanones,” ChemPhysChem, vol. 14, no. 5, pp. 1071–1079, 2013. View at: Publisher Site  Google Scholar
 B.W. Ding, P. Naumov, and Y.J. Liu, “Mechanistic insight into marine bioluminescence: photochemistry of the chemiexcited cypridina (sea firefly) lumophore,” Journal of Chemical Theory and Computation, vol. 11, no. 2, pp. 591–599, 2015. View at: Publisher Site  Google Scholar
 B. W. Ding and Y. J. Liu, “Bioluminescence of firefly squid via mechanism of single electrontransfer oxygenation and chargetransferinduced luminescence,” Journal of the American Chemical Society, vol. 139, pp. 1106–1119, 2017. View at: Google Scholar
 L. F. M. L. Ciscato, F. H. Bartoloni, A. S. Colavite, D. Weiss, R. Beckert, and S. Schramm, “Evidence supporting a 1,2dioxetanone as an intermediate in the benzofuran2(3H)one chemiluminescence,” Photochemical and Photobiological Sciences, vol. 13, no. 1, pp. 32–37, 2014. View at: Publisher Site  Google Scholar
 S. Schramm, I. Navizet, P. Naumov et al., “The light emitter of the 2coumaranone chemiluminescence: theoretical and experimental elucidation of a possible model for bioluminescent systems,” European Journal of Organic Chemistry, vol. 4, 2016. View at: Publisher Site  Google Scholar
 L. Pinto Da Silva and J. C. G. Esteves Da Silva, “Mechanistic study of the unimolecular decomposition of 1,2dioxetanedione,” Journal of Physical Organic Chemistry, vol. 26, no. 8, pp. 659–663, 2013. View at: Publisher Site  Google Scholar
 W. Adam and W. J. Baader, “Effects of methylation on the thermal stability and chemiluminescence properties of 1,2dioxetanes,” Journal of the American Chemical Society, vol. 107, no. 2, pp. 410–416, 1985. View at: Publisher Site  Google Scholar
 S. P. Schmidt and G. B. Schuster, “Kinetics of unimolecular dioxetanone chemiluminescence. Competitive parallel reaction paths,” Journal of the American Chemical Society, vol. 100, no. 17, pp. 5559–5561, 1978. View at: Publisher Site  Google Scholar
 W. Adam, “Thermal generation of electronic excitation with hyperenergetic molecules,” Pure and Applied Chemistry, vol. 52, no. 12, pp. 2591–2608, 1980. View at: Publisher Site  Google Scholar
 H. Isobe, S. Yamanaka, S. Kuramitsu, and K. Yamaguchi, “Regulation mechanism of spinorbit coupling in chargetransferinduced luminescence of imidazopyrazinone derivatives,” Journal of the American Chemical Society, vol. 130, no. 1, pp. 132–149, 2008. View at: Publisher Site  Google Scholar
 L. Yue, Y.J. Liu, and W.H. Fang, “Mechanistic insight into the chemiluminescent decomposition of firefly dioxetanone,” Journal of the American Chemical Society, vol. 134, no. 28, pp. 11632–11639, 2012. View at: Publisher Site  Google Scholar
 T. F. Schulze and T. W. Schmidt, “Photochemical upconversion: present status and prospects for its application to solar energy conversion,” Energy & Environmental Science, vol. 8, pp. 103–125, 2015. View at: Publisher Site  Google Scholar
 X. Cui, J. Zhao, Y. Zhou, J. Ma, and Y. Zhao, “Reversible photoswitching of triplettriplet annihilation upconversion using dithienylethene photochromic switches,” Journal of the American Chemical Society, vol. 136, no. 26, pp. 9256–9259, 2014. View at: Publisher Site  Google Scholar
 K. Xu, J. Zhao, D. Escudero, Z. Mahmood, and D. Jacquemin, “Controlling triplettriplet annihilation upconversion by tuning the PET in aminomethyleneanthracene derivatives,” Journal of Physical Chemistry C, vol. 119, no. 42, pp. 23801–23812, 2015. View at: Publisher Site  Google Scholar
 Q. Liu, T. Yang, W. Feng, and F. Li, “Blueemissive upconversion nanoparticles for lowpowerexcited bioimaging in vivo,” Journal of the American Chemical Society, vol. 134, no. 11, pp. 5390–5397, 2012. View at: Publisher Site  Google Scholar
 J. S. Lissau, J. M. Gardner, and A. Morandeira, “Photon upconversion on dyesensitized nanostructured ZrO_{2} films,” Journal of Physical Chemistry C, vol. 115, no. 46, pp. 23226–23232, 2011. View at: Publisher Site  Google Scholar
 A. Monguzzi, D. Braga, M. Gandini et al., “Broadband upconversion at subsolar irradiance: triplettriplet annihilation boosted by fluorescent semiconductor nanocrystals,” Nano Letters, vol. 14, no. 11, pp. 6644–6650, 2014. View at: Publisher Site  Google Scholar
 S. H. C. Askes, A. Bahreman, and S. Bonnet, “Activation of a photodissociative ruthenium complex by triplettriplet annihilation upconversion in liposomes,” Angewandte Chemie—International Edition, vol. 53, pp. 1029–1033, 2014. View at: Publisher Site  Google Scholar
 M. H. Shaw, J. Twilton, and D. W. C. MacMillan, “Photoredox catalysis in organic chemistry,” Journal of Organic Chemistry, vol. 81, no. 16, pp. 6898–6926, 2016. View at: Publisher Site  Google Scholar
 M. Kozlowski and T. Yoon, “Editorial for the Special Issue on Photocatalysis,” Journal of Organic Chemistry, vol. 81, no. 16, pp. 6895–6897, 2016. View at: Publisher Site  Google Scholar
 A. Iyer, S. Jockusch, and J. Sivaguru, “A photoauxiliary approach—enabling excited state classical phototransformations with metal free visible light irradiation,” Chemical Communications, vol. 53, no. 10, 2017. View at: Publisher Site  Google Scholar
 B. C. De Simone, G. Mazzone, J. Pirillo, N. Russo, and E. Sicilia, “Halogen atom effect on the photophysical properties of substituted azaBODIPY derivatives,” Physical Chemistry Chemical Physics, vol. 19, pp. 2530–2536, 2017. View at: Publisher Site  Google Scholar
 N. Adarsh, M. Shanmugasundaram, R. R. Avirah, and D. Ramaiah, “AzaBODIPY derivatives: enhanced quantum yields of triplet excited states and the generation of singlet oxygen and their role as facile sustainable photooxygenation catalysts,” Chemistry, vol. 18, no. 40, pp. 12655–12662, 2012. View at: Publisher Site  Google Scholar
 B. Kuçukoz, G. Sevinç, E. Yildiz et al., “Enhancement of two photon absorption properties and intersystem crossing by charge transfer in pentaarylborondipyrromethene (BODIPY) derivatives,” Physical Chemistry Chemical Physics, vol. 18, pp. 13546–13553, 2016. View at: Google Scholar
 M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A. Robb, J. R. Cheeseman et al., Gaussian 09, Revision A.02, Gaussian, Wallingford, Connecticut, USA, 2009.
 T. Yanai, D. P. Tew, and N. C. Handy, “A new hybrid exchangecorrelation functional using the Coulombattenuating method (CAMB3LYP),” Chemical Physics Letters, vol. 393, no. 1–3, pp. 51–57, 2004. View at: Publisher Site  Google Scholar
 C. Adamo and D. Jacquemin, “The calculations of excitedstate properties with timedependent density functional theory,” Chemical Society Reviews, vol. 42, no. 3, pp. 845–856, 2013. View at: Publisher Site  Google Scholar
 C. G. Min, L. Pinto da Silva, X. K. Esteves da Silva et al., “A computational investigation of the equilibrium constants for the fluorescent and chemiluminescent states of coelenteramide,” ChemPhysChem, vol. 1, pp. 117–123, 2017. View at: Google Scholar
 L. Pinto da Silva, C. M. Magalhães, D. M. A. Crista, and Esteves da Silva J. C. G., “Theoretical modulation of singlet/triplet chemiexcitation of chemiluminescent imidazopyrazinone dioxetanone via C8substitution,” Photochemical & Photobiological Sciencies, 2017. View at: Publisher Site  Google Scholar
 K. Fukui, “The path of chemical reactions—the IRC approach,” Accounts of Chemical Research, vol. 14, no. 12, pp. 363–368, 1981. View at: Publisher Site  Google Scholar
 C. Peng, P. Y. Ayala, H. B. Schlegel, and M. J. Frisch, “Using redundant internal coordinates to optimize equilibrium geometries and transition states,” Journal of Computational Chemistry, vol. 17, no. 1, pp. 49–56, 1996. View at: Google Scholar
 M. Klene, M. A. Robb, M. J. Frisch, and P. Celani, “Parallel implementation of the CIvector evaluation in full CI/CASSCF,” Journal of Chemical Physics, vol. 113, no. 14, pp. 5653–5665, 2000. View at: Publisher Site  Google Scholar
 F. A. Augusto, G. A. De Souza, S. P. De Souza Jr., M. Khalid, and W. J. Baader, “Efficiency of electron transfer initiated chemiluminescence,” Photochemistry and Photobiology, vol. 89, no. 6, pp. 1299–1317, 2013. View at: Publisher Site  Google Scholar
 M. Matsumoto, Y. Ito, M. Murakami, and N. Watanabe, “Synthesis of 5tertbutyl1(3tertbutyldimethylsiloxy)phenyl4,4dimethyl2,6,7 trioxabicyclo[3.2.0]heptanes and their fluorideinduced chemiluminescent decomposition: Effect of a phenolic electron donor on the CIEEL decay rate in aprotic polar solvent,” Luminescence, vol. 17, no. 5, pp. 305–312, 2002. View at: Publisher Site  Google Scholar
 M. Tanimura, N. Watanabe, H. K. Ijuin, and M. Matsumoto, “Intramolecular Chargetransferinduced decomposition promoted by an aprotic polar solvent for bicyclic dioxetanes bearing a 4(benzothiazol2yl)3 hydroxyphenyl moiety,” Journal of Organic Chemistry, vol. 76, no. 3, pp. 902–908, 2011. View at: Publisher Site  Google Scholar
 C. M. Marian, “Spinorbit coupling and intersystem crossing in molecules,” Wiley Interdisciplinary Reviews: Computational Molecular Science, vol. 2, no. 2, pp. 187–203, 2012. View at: Publisher Site  Google Scholar
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Copyright © 2017 Luís Pinto da Silva et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.