Solution-Phase Singlet Fission in a Structurally Well-Defined Norbornyl-Bridged Tetracene Dimer

Abstract

The photophysics of a norbornyl-bridged covalent tetracene (Tc) dimer BT1 and a monomer analogue Tc-e were studied in room-temperature nonpolar solvents. Notably in BT1, a Davydov-split band is observed in UV absorption, heralding interchromophore electronic interactions. Emission spectra indicate an acene-like vibronic progression mirroring the lowest-energy visible absorption. For BT1, this argues against excited-state excimer formation. Evidence of intramolecular singlet fission (SF) comes from a comparison of time-resolved emission decay signals collected for BT1 versus Tc-e in toluene. In BT1, the multiexcitonic (1)TT state is produced in 70 ns in 6% yield. A ratio of fission versus fusion rate constants provides an experimental measure of the SF reaction free energy at 52 meV in good agreement with previous calculations. The low SF yield corroborates our expectations that orbital symmetry effects on diabatic coupling for SF are important for dimers that cannot rely on more favorable thermodynamics.

Using Structurally Well-Defined Norbornyl-Bridged Acene Dimers to Map a Mechanistic Landscape for Correlated Triplet Formation in Singlet Fission

“…Namely, we have previously argued for BT1 that λ for the diabatic S 0 S 1 → 1 TT may be larger, of order 0.5 eV. 9 The origin of this prediction is in calculations we made using structures from DFT and TD-DFT with gradients, that predicted a significant intramolecular (inner-sphere) reorganization energy λ i = 0.43 eV (S 1 → Q). Given the current results, this may be an overestimation.…”

“…As we have previously described for related systems, the symmetry of this dimer and the fact that the S 1 ←S 0 is acene short-axis polarized, means that only the higherenergy excitonic transition in a Davydov-split pair is bright. 9,13,18 In other words, this system is an H-type aggregate with respect to the S 1 ←S 0 transition of each chromophore arm. To the blue and peaked at 444 nm is a second progression that is also observed in monomer models such as TIPS-Pentacene (TIPS-Pc).…”

Pentacene Dimers as a Critical Tool for the Investigation of Intramolecular Singlet Fission

“…Many of these syntheses rely on metal-catalyzed cross-coupling protocols such as the Suzuki and Sonogashira reactions, althoughi nnovatives ynthe-ses based on, for example, Diels-Alder cycloadditions [53] have recently appeared. Beyond pentacene, av ariety of other carbon-rich oligomeric chromophores have been explored for iSF and contribute to unraveling the underlying mechanism, these reports are outside the scope of this Minireview,a nd for current examples, see references [54][55][56][57][58][59][60][61][62][63][64][65][66][67].…”

Rational design of doubly-bridged chromophores for singlet fission and triplet–triplet annihilation

“…15,19,22,[34][35][36][37][38][39][40] As another way of controlling electronic coupling, the use of covalently-linked systems is promising because the electronic coupling can be designed at the molecular level. [41][42][43][44][45][46][47][48][49][50][51][52][53][54][55][56][57] In the previous study, we proposed several ways of tuning electronic coupling concerning SF based on the quantum interference. 52 We observed that covalently-linked systems with constructive quantum interference induce faster SF than those with destructive quantum interference.…”