Oligothiophene/graphene supramolecular ensembles managing light induced processes: Preparation, characterization, and femtosecond transient absorption studies leading to charge‐separation

摘要

Advances  in  organic  synthetic  chemistry  combined  with  the  exceptional  electronic  properties  of  carbon  allotropes,  particularly graphene, is the basis to design and fabricate novel electron donor-­‐acceptor ensembles with desired properties for technological applications. Thiophene-­‐based materials, mainly thiophene-­‐containing polymers, are known for  their  notable  electronic  properties.  In  this  frame  moving  from  polymer  to  oligomer  forms,  new  fundamental  information would help to the better understanding of their electrochemical and photophysical properties. Furthermore, a successful  combination  of  their  electronic  properties  with  those  of  graphene  is  a  challenging  goal.  In  this  work  two  oligothiophene compounds consists of three and nine thiophene-­‐rings, abbreviated as 3T and 9T, respectively, were synthesized and noncovalently associated with liquid phase exfoliated few-­‐layered graphene sheets (abbreviated as eG), forming donor-­‐acceptor 3T/eG and 9T/eG nanoensembes. Markedly, intra-­‐ensemble electronic interactions between the two  components  in  the  ground  and  excited  states  were  evaluated  with  the  aid  of  UV-­‐Vis  and  photoluminescence  spectroscopy. Furthermore, redox assays revealed an one-­‐electron oxidation of 3T accompanied by one-­‐electron reduction due  to  eG  in  3T/eG,  while  two  reversible  one-­‐electron  oxidations  of  9T  accompanied  by  one-­‐electron  reduction  of  eG  9T/eG. The electrochemical band gap for 3T/eG and 9T/eG ensembles were calculated and verified that the negative free-­‐energy change for the charge-­‐separated state of 3T/eG and 9T/eG via the singlet excited state of 3T and 9T respectively, were  thermodynamically  favorable.  Finally,  results  of  transient  pump-­‐probe  spectroscopic  studies  at  the  femtosecond  time scale were supportive of charge transfer type interactions in the 3T/eG and 9T/eG ensembles. The estimated rates for intra-­‐ensemble charge separation were found to be 9.52 x 109 s-­‐1 and 2.2 x 1011 s-­‐1, respectively, for 3T/eG and 9T/eG in THF, revealing moderate to ultrafast photoinduced events in the oligothiophene/graphene supramolecular ensembles