We here report a fully quantum mechanical study of the main photochemical and photophysical decay routes in aqueous solution of thymine deoxy-dinucleotide (TpT(-) and TpTNa) and of its analogue locked in C3-endo puckering, characterizing five different representative backbone conformers and discussing the chemical physical effects modulating the yield of the different photoproducts. Our approach is based on time-dependent DFT calculations, using the last generation M052X functional, whereas solvent effects are included by means of the polarizable continuum model. Especially when at least one of the sugars adopts C3-endo puckering, a barrierless path on the bright pi pi* excitons leads to the S-1/S-0 crossing region corresponding to the formation of cyclobutane pyrimidine dimer. Charge transfer excited states involving the transfer of an electron from the 5' Thy toward the 3' Thy are involved in the formation of the oxetane intermediate in the path leading to 6-4 pyrimidine pyrimidinone adducts. A non-negligible energy barrier is associated with this latter pathway, which is possible only when one of the two nucleotides adopts C2-endo puckering. Monomer-like decay pathways, involving pi pi* or n pi* excited states localized on a single base, are shown to be operative also for loosely stacked bases.