Resveratrol is a naturally occurring compound that belongs to the class of phytoalexins, antimicrobial chemical substances produced by plants to combat infection by a pathogen. It is well-known for its antioxidant properties and is commonly found in grapes and peanuts. Although its most stable isomeric form is trans, upon exposure to ultraviolet (UV) radiation, resveratrol undergoes photoisomerization, originating its cis configuration. The experimentally determined molar yield of cis-resveratrol generated from trans-resveratrol excitation (in a hydroalcoholic solution exposed to solar irradiation) was approximately 60% and underwent significant degradation after 150 minutes. This interconversion was detailed in several computational and experimental works, demonstrating that the isomerization process is consistently observed for trans-resveratrol, in both solution and solid phases. In addition to isomerization, photoexcited resveratrol can undergo reactions that result in byproducts (Figure ). Photoreactions are observed when transresveratrol is in a liquid environment but, unlike photoisomerization, they are not detected in the solid phase. An example of such a photoreaction is the cyclization of trans-resveratrol. After UV irradiation in water and ethanol solutions, the cyclized photoproduct 2,4,6-trihydroxy-phenanthrene (THP) was detected. Another photoproduct of trans-resveratrol excitation is (E)-4-(6,8dihydroxy-naphthalen-2-yl)but-3-en-2-one, known as resveratrone. Yang et al. This product was proposed to be formed from the cis isomer, which can undergo a pericyclic ring closure, followed by a photoinduced 4e -cyclization and by a series of tautomerization steps. The experimental generation of THP and resveratrone by exposing hydroalcoholic solutions of trans-and cis-resveratrol to solar radiation shows that when trans-resveratrol is used as the precursor, the formation of photoproducts is higher (25.4% for THP and 11.7% for resveratrone) compared to their formation from the excitation of the cis isomer (16.5% for THP and 6.2% for resveratrone). This findings suggest that trans-resveratrol is a more effective precursor for these photoproducts under solar light exposure and that THP is produced in higher amounts than resveratrone, regardless of the isomer initially excited. The formation of THP is influenced by environmental factors, such as UV-light exposure conditions and solvent environment, which can be mitigated by using photo-protective agents in the formulations. Yet, Francioso et al. showed that THP exhibits cytotoxic and genotoxic effects, which were attributed to a possible ability to oxidize DNA. Trans-resveratrol is well known for its antioxidant capacity and ability to selectively quench singlet oxygen ( 1 O2), a reactive species implicated in various oxidative stress-related diseases. The ability of quenching singlet oxygen is attributed to specific structural characteristics. While primarily recognized as an antioxidant, resveratrol's potential to generate singlet oxygen remains debated, although several studies have reported singlet oxygen generation upon photoexcitation. In this process, the excited molecule transfers energy to molecular oxygen ( 3 O2), converting it into singlet oxygen ( 1 O2), a key mechanism in photodynamic therapy (PDT). However, it's believed that it's not resveratrol that can generate singlet oxygen but certain of its derivatives. Fotiou et al. experimentally concluded that trans-resveratrol excitation under UV radiation can generate reactive oxygen species, such as 1 O2 and peroxynitrite (ONOO⁻). However, their study does not clarify the structure of the compound directly responsible for the photosensitization. Zhao et al. proposed that trans-resveratrol, after excitation, undergoes cyclization and oxidation to form THP -which would be the specie implicated in the generation of singlet oxygen -which will then react with trans-resveratrol through a [4+2] cycloaddition. The generation of singlet oxygen by the THP compound was not experimentally confirmed by Zhao et al. but was assumed based on results observed for other phenanthrene-like compounds. Lagunes et al. attribute the formation of ¹O₂ and its subsequent reactions with various compounds present in wine, to changes in its taste and aroma. On the other hand, Monsour et al. demonstrated the formation of photoproducts and quenching of singlet oxygen but revealed that neither resveratrol nor its derivatives could act as photosensitizer for singlet oxygen production. Understanding the ability of trans-resveratrol and its photoproducts to yield singlet oxygen is essential as it directly influences the applicability of these compounds in therapeutical applications. The present study uses quantum chemistry methods to investigate the photophysical properties of transresveratrol and its derivatives cis-resveratrol, THP, and resveratrone to elucidate the potential of these molecules to yield singlet oxygen. We assess three aspects of the reaction sequence schematically illustrated in Scheme 1: i) the absorption band overlap between trans-resveratrol precursor and the derivatives (i.e., the difference between h1 and h2 in reactions 1 and 2); ii) the competition between fluorescence (reaction 2a) and intersystem crossing (ISC) (reaction 2b); iii) the energetic availability to excite a singlet oxygen state (reaction 3). By exploring these properties, we can better understand the role of trans-resveratrol in singlet oxygen generation. The possibility of using trans-resveratrol as a photosensitizer to produce highly reactive oxygen species makes it attractive for photodynamic therapy, broadening its potential beyond its current use as an antioxidant. Scheme 1. Relevant reactions to singlet oxygen generation after photoexcitation of trans-resveratrol. The derivatives investigated in this work are trans-resveratrol itself plus cis-resveratrol, THP, and resveratrone.