ON THE INTERSYSTEM CROSSING MECHANISM OF [1]BENZOTHIENO[3,2- b][1]BENZOTHIOPHENE CONTAINING SYSTEMS

Probably one of the most interesting processes involving spin should be intersystem crossing, a spin-forbidden transition at first approximation, singlet-exciton generated by absorption of a photon converts into a triplet-exciton, or in other cases it proceeds in the opposite direction; from triplet to a singlet in what is called reverse intersystem crossing. To provide clarity about the underlying physical processes, we have developed three experimental setups for the study of intersystem crossing on these systems: Firstly, solutions of [1]Benzothieno[3,2-b][1]Benzothiophene (BTBT) and octyl chain substitutions at the 2 and 7 position and with various probing techniques as well as quantum modelling through Density Functional Theory ab-initio calculations. Secondly, then we proceed to use electron-withdrawing and donating substituents at such positions. Thirdly, we managed to insert 2,7-Dioctyl[1]Benzothieno[3,2-b][1]Benzothiophene(C8-BTBT) inside the cavity of a synthetic bioprotein that aided in controlling aggregation and finally with a couple of thin films of C8-BTBT. In BTBT solutions, we observe highly efficient intersystem-crossing (ISC) through a spin-vibronic mechanism. This is the first demonstration of such a mechanism in thienoacenes molecules. Then through the introduction of electron-withdrawing and electron-donating substituent groups, we saw how the ISC got impacted. We examine the rate of ISC, showing that the introduction of heavy atoms to increase spin-orbit coupling did not yield higher rates of ISC in a counter-intuitive way and which may need to be reviewed by theoreticians. We propose that the mechanism responsible for unusually fast ISC in such materials is due mainly to the role of out of plane vibrations and vibrations originating from the side chains, for their overall interaction play an instrumental role in activating the spin forbidden transition. In the final experiments of aggregates and thin films of C8-BTBT, we found that even in the case of small aggregates intersystem crossing is effectively deactivated as relaxing channel, which our investigation suggests is due to changes of the intermolecular forces that makes crystal packing possible for they inactivate those vibrational modes present in solution.