As presented in Fig. , both tautomers of 2,6-dAP undergo structural changes after population of the S 1 state, with aromatic ring-puckering mechanisms being the most apparent structural changes that can be observed for both S 1 minima. The S 1 minimum-energy geometry of the 7H tautomer has a strongly puckered 5-membered subunit of the aromatic purine ring, which is the result of C 8 =N 7 bond twisting. Population of this minimum may be followed either by intersystem crossing to the triplet manifold of electronic states or by direct internal conversion to the electronic ground state via the S 1 (ππ*)/S 0 conical intersection (see Fig. and). Direct photorelaxation via the S 1 (ππ*)/S 0 conical intersection involves stronger puckering of the 5-membered heteroaromatic subunit and is associated with an energy barrier of over 0.4 eV. The presence of this energy barrier is consistent with the experimentally determined lifetime of 25 ns at excitation energies near the origin of absorption. This time constant is substantially shortened when the excess energy of the laser pulse reaches 1300 cm -1 , which is also in good qualitative agreement with the magnitude of computationally predicted barrier height. These considerations clearly indicate that the nanosecond time constant can be associated with a radiationless deactivation pathway. We cannot exclude the contribution from fluorescence though which occurs on a similar time-scale and could operate from the S 1 (ππ*) minimum of 7H 2,6-dAP, owing to the relatively high oscillator strength (0.117) of the radiative S 1 -S 0 transition. Due to the relatively long timescale of direct internal conversion through the S 1 (ππ*)/S 0 conical intersection, UV-excited 7H 2,6-dAP may also undergo intersystem crossing to the triplet manifold of electronic states. Notably, the energy difference between the S 1 /T 2 state crossing and the S 1 (ππ*) minimum amounts to merely 0.23 eV. Crossing to the triplet state from the S 1 minimum is only slowed down by the very low value of spin-orbit coupling (SOC) between these two electronic states of 1.92 cm-1. This very low degree of SOC is caused by the fact that the S1 and T2 states possess a very similar ππ* molecular orbital character, albeit with different molecular orbitals defining these excitations. However, we argue that this intersystem crossing could still enable non-negligible population of triplet states. It is worth noting that based on MS-CASPT2/SA-CASSCF calculations, Oliveira et al. identified a likely more efficient intersystem crossing channel involving the T2(nπ*) which is also associated with a higher S1/T2 SOC of ~10 cm-1. As can be seen from Fig. , the minimum of the T1 state can be reached in a practically barrierless manner after the intersystem crossing, via the peaked T2/T1 conical intersection. Overall, these two intersystem crossing channels, confirmed computationally, allow us to tentatively assign the ultralong microsecond time constant to the population of the T 1 state. Even though the T 1 minimum of 7H 2,6-dAP exhibits an analogous molecular orbital character to the above-discussed S 1 minimum, it is also characterized by a somewhat lower degree of puckering of the 5-membered heteroaromatic subunit and lower degree of C 8 =N 7 bond twisting.