Probing intersystem crossing in multi-brominated eumelanin through transient absorption and surface hopping dynamics

Achieving intersystem crossing (ISC) through structural tuning in biological systems is an evolving area for therapeutic and materials research. Eumelanin, a natural pigment, offers huge potential for bio-inspired material design, yet remains underexplored in this regard. Herein, we report the ultrafast intersystem crossing in di-brominated (DMICE-Br2) and tri-brominated (DMICE-Br3) eumelanin model monomers through transient absorption spectroscopy and surface hopping dynamics. Femtosecond and nanosecond transient absorption experiments suggest triplet excited state populations in DMICE-Br2 and DMICE-Br3 with triplet quantum yields and rates of ISC as , and , respectively. Theoretical insights into ISC were obtained with nonadiabatic dynamics simulations using the surface hopping including arbitrary couplings method coupled to potential energy surfaces, modelled by linear vibronic coupling (SHARC/LVC). The results show that for both DMICE-Br2 and DMICE-Br3, the initial S1 population decays to the T2 and T3 states in the picosecond timescale to further undergo internal conversion to T1 within sub-ns for DMICE-Br2 and sub-ps for DMICE-Br3. The simulated and corroborate to the assignment of the ultrafast triplet excited state population observed in the experiments. The increased triplet yields and ISC rates in DMICE-Br2 and DMICE-Br3 are attributed to the enhanced heavy atom effect from additional bromine atoms. This work presents the experimental and computational evidence for ultrafast ISC in multi-brominated eumelanin monomers, with promising implications for eumelanin-inspired material design and photodynamic applications.