Parallel versus Twisted Pentacenes: Conformational Impact on Singlet Fission

In this work, the authors placed two pentacene chromophores at the termini of a diacetylene linker to investigate the impact of excitation wavelength, conformational flexibility, and vibronic coupling on singlet fission.The results highlight the need of controlling the emergence of twisted conformations in the design of new singlet fission active molecules.

Harnessing triplet excited states (T1) in up/down converting solar energy processes in single-junction solar cells has proven a useful tool to maximize the use of the solar spectrum, introducing also ways to overcome the Shockley−Queisser efficiency limit. In this context, multiple exciton generation in semiconductor materials and singlet fission (SF) in molecular materials are promising candidates to maximize the efficiency of solar cells. Here, Papadopoulos et al. have synthesized LiDi, a diacetylene linked pentacene dimer, to assess the impact that conformational flexibility, excitation wavelength, and vibronic coupling have on the intramolecular SF (iSF) mechanism. Steady-state absorption measurements reveal vibrational splitting, inferring the existence of at least two types of conformers, in which the pentacene moieties are placed either parallel or twisted with respect to each other. Using time-resolved transient absorption spectroscopy, they have analyzed the low- and high-energy transitions of LiDi. Photoexcitation of the low-energy transitions identifies a superposed mixture of states, which transforms into a single observable species, namely a delocalized vibronically coupled/hot triplet pair 1(T1T1) deloc with a short lifetime (< 2 ps). Photoexcitation of the high-energy transitions of LiDi, on the other hand, results in the observation of a superposed mixture of states and its transformation into 1(T1T1) deloc within 1.0 ps. This state decays to the ground state within 4 ps via triplet−triplet annihilation. Lowering the temperature allows the identification of two different correlated triplet-pair excited states, with different spectroscopic features. The authors relate these states to the delocalized 1(T1T1) deloc and to a localized and vibronically decoupled 1(T1T1)loc state, respectively. Quantum dynamics simulations are consistent with an iSF dominated by the mediated-like mechanism and excitation-wavelength-dependent on short time scales (ca. < 10 ps). In addition, theory suggests that the quasi-free rotation at the diacetylene spacer exhibited by LiDi may easily lead to twisted conformations with very low SF quantum yields.

Altogether, these results emphasize the necessity of controlling conformational flexibility in the design of new SF-active materials.

Figure: Left- Differential femtosecond transient absorption spectra of LiDi in MeTHF at 80 K with ex 633 and 750 nm, respectively. The upper panel shows the respective species associated spectra of the mixed state (M) (black), the delocalized and vibronically coupled/hot 1(T1T1)deloc (red), and the localized and vibronically decoupled 1(T1T1)loc (blue) as obtained by target analysis. Right- Simulated time evolution of the population of the diabatic states involved in the iSF process of LiDi (top) and twisted LiDi (bottom) conformers after photoexcitation at 717 nm (left) and 659 nm (right) correlating with the experimentally used excitation wavelengths.