Pulling the levers of photophysics: how structure controls the rate of energy dissipation.

The process of internal conversion is at the heart of many chemical reactions by being responsible for the conversion of electronic excitation energy into vibrational energy. The ultrafast process from higher electronic states assures that it can rival competing pathways. This also implies that nuclear motion in only a very few vibrational degrees of freedom are involved in internal conversion—if more were to take part the rate of energy dissipation would simply not be able to compete with the rate of photochemical reaction or radiative transition. This is inherently different from thermalized reactions where the total number of degrees of freedom of the system influences the rate of reaction because of the statistical distribution of energy. Such statistical behavior is not necessarily observed by ultrafast, excited-state reactions. A general picture of the structural parameters that control the rate of an internal conversion process leading to energy dissipation is not easily deducible. Herein, we demonstrate how the rate of such an internal conversion process can change by more than an order of magnitude for related molecules as a consequence of minor structural variations. We disentangle the complex process and identify one specific vibrational mode involved through the use of the techniques time-resolved mass-spectrometry (TRMS) and time-resolved photoelectron spectroscopy (TRPES) on four molecules: 2methylcyclobutanone (2-MeCB), 2-methylcyclopentanone (2-MeCP), 3-methylcyclopentanone (3-MeCP), and 3-ethylcyclopentanone (3-EtCP). These four molecules are structurally similar to cyclobutanone (CB), cyclopentanone (CP), and cyclohexanone (CH) investigated in our previous works (Figure 1). Following excitation to S2, (n,3s) state, the temporal evolution of the ion currents presented in Figure 2 closely resemble that of the (n,3s) photoelectron peak also given in the figure for 2-MeCB and 2-MeCP. Consequently, the decay in the ion yield can be used as a measure of the lifetime of the (n,3s) state. A similar relationship was observed for the unsubstituted cycloketones. The ion currents reveal a set of timescales for the (n,3s)!(n,p*) transition, that is, S2!S1, and thereby for the conversion of part of the electronic energy of the system into vibrational energy, ranging over more than an order of magnitude from 0.37 0.01 ps for 2-MeCB to 5.79 0.16 ps for 3-MeCP (Table 1). A clear grouping of the timescales is seen in terms of the ring size. For a particular