The current understanding of the photochemistry of isolated canonical nucleobases is that they are very photostable thanks to rapid internal conversion from the excited state to the electronic ground state. This deactivation mechanism protects the building blocks of the genetic code from possible photochemical damage by efficiently converting electronic excitation to heat that can be safely dissipated to the environment. This process corresponds to excited state lifetimes of the order of single picoseconds or less. By contrast, many derivatives, isomers, and analogues of the canonical nucleobases have much longer excited state lifetimes, with competing and potentially hazardous photochemical pathways. These excited state dynamics are a subtle function of molecular structure. Although the picture is far from complete, there are some indications that heterosubstituted purines (e.g. guanine and isoguanine ) may be more photostable than the homosubstituted purines (2,6-diaminopurine and xanthine(15, 16)), while purines with a single ligand (adenine, 2AP,(18) 2-oxopurine, and hypoxanthine ) appear to be more photostable than most purines with two ligands. Martinez-Fernandez et al. in a thorough computational study on the photostability effects of position and oxo/amino substitution for purines with a single ligand, found that amino or oxo substitution at C 2 has little effect on the photodynamics, while C 6 functionalization leads to photostability. However, these general trends do not include details, such as the role of the solvent. A well-known example of this structure dependence is the difference between adenine (6-aminopurine), and its isomer 2-aminopurine (2AP). 2AP has a strong fluorescence signature in aqueous solution, while adenine does not. Adenine in the gas phase has an excited-state lifetime of the order of single picoseconds or less depending on the exact measurement technique, with similar lifetimes reported in solution. The lowest excited singlet state of adenine exhibits nπ * character and significant 1 nπ * / 1 ππ * mixing after initial excitation. Adenine undergoes aromatic ring puckering associated with C 2 -H tilting and out-of-plane deformation at the C 2 N 3 position to reach an S 1 /S 0 conical intersection (CI) with a barrier of about 0.1 eV, enabling relaxation to the electronic ground state. In contrast, Lobsiger et al. found isolated 2AP to have an excited-state lifetime of 150 ps when excited near the excitation threshold, which is significantly longer than in the case of isolated adenine. Furthermore, these authors observed another decay component with a lifetime of more than 2 μs, attributed to a triplet state populated by intersystem crossing. The lifetime increases significantly upon microhydration near the amino group in the C 2 position, i.e., up to a hundredfold compared to the isolated molecule. This increase presumably corresponds to the fluorescence decay noted for solvated 2AP. The excited-state dynamics also depends on the irradiation wavelength, especially near excitation threshold (origin transition) where dynamics can be slow. In that case the dynamics can be hypersensitive to any barriers, which can modulate competing relaxation pathways. For instance, at higher excitation energies, adenine can also relax via N-H bond breaking, which occurs in the 1 πσ * state. For isolated 2AP, the lifetime of 150 ps decreases by about half after exciting with ~1500 cm -1 excess energy, relative to the origin. Structural motifs do not always predict trends in potential energy surfaces and effects of substitutions do not necessarily add up. Potential energy surfaces are subtly influenced by molecular structure, and the photochemistry of 2,6DAP (see Scheme 1) cannot be simply understood as a combination of the pathways available for 2AP and 6AP. Therefore, to shed more light on the photodynamics of amino substituted purines, we have investigated the excited-state properties of isolated 2,6-dAP which has common structural features with both 2AP and adenine. The photochemistry of 2,6-dAP is also of interest since it has been suggested as a prebiotically relevant nucleobase; it was discovered in meteorite samples, and it could be synthesized from the same starting materials and under the same conditions as the canonical nucleobases.(40) More recently, 2,6-dAP was shown to entirely replace adenine in the genomes of numerous bacterial viruses (bacteriophages). In addition, its remarkable electron-donating properties were suggested to enable a potentially prebiotic version of DNA repair in UV-rich environments. Because of these prebiotic and biological implications, thorough characterization of the photodynamics of 2,6-dAP is warranted. Here, we focus on studying both the biologically relevant 9H tautomer, and the 7H tautomer. In previous work , we found 6.3 and 8.7 ns lifetimes for the isolated 9H and 7H tautomers at their respective origins. These lifetimes stand out from those in both adenine (< 1 ps) and 2AP (150 ps and 2 μs). Now, to gain further insight, we investigated the dynamics of isolated 2,6-dAP in the picosecond time regime and found an additional fast decay pathway with different barriers for each tautomer. We discuss potential mechanisms that could explain the observed lifetimes based on quantum chemical calculations. By comparing our experimental and theoretical results with data previously acquired for adenine and 2AP, we attempt to reconcile the excited state dynamics with properties of the molecular structure.