Excited-state dynamics of porphyrin monomers by ultrafast multidimensional spectroscopy

Conjugated porphyrin arrays are currently being investigated as prototypical molecular systems for light harvesting, charge-separation, and photocatalytic processes. In their monomeric form, the light-induced excitation relaxation within the Q-band is expected to be controlled by the coupling of electronic and vibrational degrees of freedom, which can give rise to a complex interplay in the system dynamics [1,2].

Here, we combine ultrafast multidimensional spectroscopies (pump-probe and two dimensional electronic spectroscopy) with ab initio (DFT and TDDFT) calculations to unveil the fundamental mechanisms at the heart of these processes. Based on the evidence of a sub-100fs dynamics within the Q-bands observed by the ultrafast experiments, we introduce an effective model within a density-matrix approach which well reproduces the data. The simulations allow us to unveil the role played in the internal conversion process by specific vibrational modes, to disentangle the intricate interplay between adiabatic and nonadiabatic couplings on the one hand, and the coherent and incoherent excitation relaxation  on the other.

[1] L. Moretti et al., The Journal of Physical Chemistry Letters 11, 3443 (2020).

[2] N. K. Katturi et al., Optical Materials 107, 110041 (2020).