The Role of Biradicaloid Geometries in Organic Photochemistry

Davidson

Proc. Natl. Acad. Sci. U.S.A.

2016

168

218

Higher acenes have drawn much attention as promising organic semiconductors with versatile electronic properties. However, the nature of their ground state and electronic excited states is still not fully clear. Their unusual chemical reactivity and instability are the main obstacles for experimental studies, and the potentially prominent diradical character, which might require a multireference description in such large systems, hinders theoretical investigations. Here, we provide a detailed answer with the particleparticle random-phase approximation calculation. The 1 A g ground states of acenes up to decacene are on the closed-shell side of the diradical continuum, whereas the ground state of undecacene and dodecacene tilts more to the open-shell side with a growing polyradical character. The ground state of all acenes has covalent nature with respect to both short and long axes. The lowest triplet state 3 B 2u is always above the singlet ground state even though the energy gap could be vanishingly small in the polyacene limit. The bright singlet excited state 1 B 2u is a zwitterionic state to the short axis. The excited 1 A g state gradually switches from a doubleexcitation state to another zwitterionic state to the short axis, but always keeps its covalent nature to the long axis. An energy crossing between the 1 B 2u and excited 1 A g states happens between hexacene and heptacene. Further energetic consideration suggests that higher acenes are likely to undergo singlet fission with a low photovoltaic efficiency; however, the efficiency might be improved if a singlet fission into multiple triplets could be achieved.higher acenes | diradical | double excitation | particle-particle random-phase approximation | charge-transfer excitation

Section: Significancementioning

confidence: 99%

Nature of ground and electronic excited states of higher acenes

Davidson

Proc. Natl. Acad. Sci. U.S.A.

2016

168

218

Proc. Natl. Acad. Sci. U.S.A.

“…The points belonging to these subspaces or seams, that is, CIs and singlet͞triplet crossings, behave as energy funnels where the probability for nonadiabatic, nonradiative jumps is high (18,24,25). The nature of the deactivation is certainly related with the strong vibronic coupling taking place between the interacting states when the energy gap decreases, in particular in situations such as CIs (19).…”

mentioning

confidence: 99%

Adenine and 2-aminopurine: Paradigms of modern theoretical photochemistry

Serrano‐Andrés

Merchán

Borin

2006

281

365

Distinct photophysical behavior of nucleobase adenine and its constitutional isomer, 2-aminopurine, has been studied by using quantum chemical methods, in particular an accurate ab initio multiconfigurational second-order perturbation theory. After light irradiation, the efficient, ultrafast energy dissipation observed for nonfluorescent 9H-adenine is explained here by the nonradiative internal conversion process taking place along a barrierless reaction path from the initially populated 1 (* La) excited state toward a low-lying conical intersection (CI) connected with the ground state. In contrast, the strong fluorescence recorded for 2-aminopurine at 4.0 eV with large decay lifetime is interpreted by the presence of a minimum in the 1 (* La) hypersurface lying below the lowest CI and the subsequent potential energy barrier required to reach the funnel to the ground state. Secondary deactivation channels were found in the two systems related to additional CIs involving the 1 (* Lb) and 1 (n*) states. Although in 9H-adenine a population switch between both states is proposed, in 7H-adenine this may be perturbed by a relatively larger barrier to access the 1 (n*) state, and, therefore, the 1 (* Lb) state becomes responsible for the weak fluorescence measured in aqueous adenine at Ϸ4.5 eV. In contrast to previous models that explained fluorescence quenching in adenine, unlike in 2-aminopurine, on the basis of the vibronic coupling of the nearby 1 (*) and 1 (n*) states, the present results indicate that the 1 (n*) state does not contribute to the leading photophysical event and establish the prevalence of a model based on the CI concept in modern photochemistry.conical intersections ͉ DNA photophysics ͉ fluorescence quenching ͉ quantum chemistry ͉ ultrafast decay S tudies on the photostability of DNA and RNA bases after absorption of UV light have flourished since the early 1970s, when quenched DNA fluorescence at room temperature was first reported (1). Knowledge about the mechanisms that control nonradiative decay is fundamental, not only because of the extreme importance of photodamage in systems that form the genetic code but also because they have been used to establish the paradigm of the essentials of radiationless decay processes in nonadiabatic photochemistry (2). Adenine (6-aminopurine), the most studied nucleobase, maintains photostability by efficiently quenching its fluorescence, whereas a close constitutional isomer, 2-aminopurine, displays strong emission, and it is commonly used to substitute adenine in DNA as a fluorescent probe to detect protein-induced local conformational changes (3-7). Former proposals to explain low quantum yields of fluorescence and excited singlet deactivation in nucleobases by means of excited-state photoreactions or phototautomerisms were basically ruled out because of the absence of photoproducts and deuterium isotope effects in different solvents (2). Apparently more successful was the hypothesis known as proximity effect (8, 9), which explained ultrafast internal conver...

“…In the intermolecular process, the excited state can relax to a conical intersection 32,36,46,47 through partial or complete proton transfer to solvent, 34 resulting in ultrafast return to the ground state (see Scheme 3). With 2-(2′-hydroxyphenyl)benzotriazoles, such as Tinuvin P, it has been shown by laser flash photolysis that the deprotonation from the excited state occurs in basic solvents such as DMSO.…”

Section: Scheme 4: Paradigm For Photophysics Of Methyl Ethersmentioning

confidence: 99%

Twisted Intramolecular Charge Transfer States in 2-Arylbenzotriazoles: Fluorescence Deactivation via Intramolecular Electron Transfer Rather Than Proton Transfer

et al. 2002

Ultraviolet absorbers such as Tinuvin P (2-(2-hydroxy-5-methylphenyl)benzotriazole), 1, achieve their exceptional photostabilities as a result of deactivation of excited singlet states through excited state intramolecular proton transfer (ESIPT). Adding a methyl group to the 6′ position of 2-arylbenzotriazoles reveals an additional excited singlet state deactivation mechanism in this class of molecules which does not require intramolecular hydrogen bonding. Steady state fluorescence and fluorescence lifetime measurements for a series of 6′-methyl-2-arylbenzotriazoles provides compelling evidence for a twisted intramolecular charge transfer (TICT) mechanism of excited singlet state deactivation. Due to the steric requirements of the 6′-methyl group, conformations are favored in which the phenyl and triazole rings are no longer coplanar. In the case of compound 11 (2-(6-methoxy-2,3-dimethylphenyl)-2H-benzotriazole), the presence of a 2′-methoxy group enhances nonplanarity and results in large deactivation rates. Compound 12 (2-(6-methoxy-2,3-dimethylphenyl)-5-(trifluoromethyl)-2H-benzotriazole), which possesses both twist and enhanced donor/acceptor properties, undergoes the most efficient fluorescence quenching for the methoxyarylbenzotriazoles. Compounds with both a 6′-methyl and a hydroxy group on the phenyl ring exhibit diffusion controlled quenching (k q ) 2 × 10 10 M -1 s -1) by DMSO. This quenching appears to result from either partial or complete excited state proton transfer to DMSO, which enhances TICT deactivation of the singlet excited state.