The concept of finite temperature Wigner phase-space sampling allowing the population of vibrationally excited states is introduced and employed to study temperature effects on the absorption spectrum of 2-nitronaphtalene (2NN) and its relaxation dynamics. It is found that, despite the fact that the general deactivation mechanism of 2NN after light irradiation does not change with increasing temperature, i.e., after excitation to the singlet manifold, 2NN deactivates via internal conversion in less than 100 fs and then undergoes intersystem crossing to the triplet manifold before decaying to the lowest triplet state in less than 150 fs, the intersystem crossing rate increases at higher temperatures. This increase is attributed to the appearance of a new deactivation pathway, which is not operative at lower temperatures. The present example illustrates that appropriate initial conditions beyond the idealized temperature of 0 K are indispensable to obtain reliable excited state mechanisms, which can be then related to experimental conditions.