Excimer Formation Inhibits the Intramolecular Singlet Fission Dynamics: Systematic Tilting of Pentacene Dimers by Linking Positions.

The role of excimer formation in inhibiting or enhancing the efficiency of the intramolecular singlet fission (iSF) process has been a subject of recent debate. Here, we investigated the effect of excimer formation on iSF dynamics by modifying its configuration by connecting pentacenes at various positions. Hence, pentacene dimers having slip-stacked (2,2' BP, J-type), oblique (2,6' BP), and facial (6,6' BP, H-type) configurations were synthesized by covalently linking pentacenes at positions 2,2', 2,6', and 6,6', respectively, with an ethynyl bridge, and their ultrafast excited-state relaxation dynamics were characterized. Femtosecond time-resolved transient absorption spectra revealed that the efficiency of iSF dynamics decreased from slip-stacked (182%) to oblique configuration (97%),whereas in the 6,6' BP with facial configuration, strong electronic coupling led to the formation of excimers that decayed nonradiatively without formation of correlated triplet pairs. These studies reveal the formation of excimers by strong intrapentacene electronic coupling upon ultrafast excitation, preventing the efficient iSF process.

Planarity and Length of the Bridge Control Rate and Efficiency of Intramolecular Singlet Fission in Pentacene Dimers.

Singlet fission (SF) improves the power conversion efficiency of optoelectronic devices by converting high-energy photons into two triplet excitons. SF dynamics and efficiency (Φ) are controlled by various factors. Here, the effect of planarity and length of the bridge in pentacene dimers on the intramolecular SF (iSF) process was investigated by synthesizing the dimers connected by bridges having fluorene (FL-PD, planar), methyl-substituted biphenyl (MBP-PD, twisted), and diphenyl acetylene (DPA-PD, longer) groups and characterizing their excited-state relaxation dynamics using nanosecond and femtosecond pump-probe spectroscopy. Transient absorption studies reveal that iSF dynamics of FL-PD having a planar bridge are ∼787 times faster (187 ps) and exhibit higher Φ (198%) by feasible electronic coupling, compared to MBP-PD possessing a twisted bridge showing a low Φ of ∼16%. However compared to FL-PD, iSF dynamics of DPA-PD with an increase of bridge length are slower by an order (1.09 ns) and show comparable Φ of 185% through extended conjugation. Thus, the planarity and length of the bridge in pentacene dimers control the rate and efficiency of the iSF process.