Decoupling inter- and intra-dimer singlet fission

The rate of singlet fission in a dimer is known to be dependent on the coupling between the two chromophores that make up the dimer. A series of ethynyltetraceme dimers have been synthesized with the chromophore units at ortho-, meta- and para- configurations. Broadband femtosecond time-resolved pump-probe spectroscopy and time-correlated single photon counting has been used to observe the singlet state, the multiexciton state and the separated triplet states in these dimers in both solution and neat film phases. In solution, only the para-dimer forms separated triplets while the meta-dimer does not undergo singlet fission at all. In neat films, inter-dimer chromophore interactions lead to singlet fission in both the meta-dimer and the para-dimer. Target analysis using compartmental kinetic models allows for the spectral features of the different excited states in singlet fission to be resolved, along with the rate constants of the excited state processes involved.