Revealing ultrafast relaxation dynamics in six-thiophene thin film and single crystal

Inverse Design of Tetracene Polymorphs with Enhanced Singlet Fission Performance by Property-Based Genetic Algorithm Optimization

The efficiency of solar cells may be improved by using singlet fission (SF), in which one singlet exciton splits into two triplet excitons. SF occurs in molecular crystals. A molecule may crystallize in more than one form, a phenomenon known as polymorphism. Crystal structure may affect SF performance. In the common form of tetracene, SF is experimentally known to be slightly endoergic. A second, metastable polymorph of tetracene has been found to exhibit better SF performance. Here, we conduct inverse design of the crystal packing of tetracene using a genetic algorithm (GA) with a fitness function tailored to simultaneously optimize the SF rate and the lattice energy. The property-based GA successfully generates more structures predicted to have higher SF rates and provides insight into packing motifs associated with improved SF performance. We find a putative polymorph predicted to have superior SF performance to the two forms of tetracene, whose structures have been determined experimentally. The putative structure has a lattice energy within 1.5 kJ/mol of the most stable common form of tetracene.

Plenty of Room on the Top: Pathways and Spectroscopic Signatures of Singlet Fission from Upper Singlet States

We investigate dynamic signatures of the singlet fission (SF) process triggered by the excitation of a molecular system to an upper singlet state S_N (N > 1) and develop a computational methodology for the simulation of nonlinear spectroscopic signals revealing the S_N → TT₁ SF in real time. We demonstrate that SF can proceed directly from the upper state S_N, bypassing the lowest excited state, S₁. We determine the main S_N → TT₁ reaction pathways and show by computer simulation and spectroscopic measurements that the S_N-initiated SF can be faster and more efficient than the traditionally studied S₁ → TT₁ SF. We claim that the S_N → TT₁ SF offers novel promising opportunities for engineering SF systems and enhancing SF yields.

Switching Pathways of Triplet State Formation by Twisted Intramolecular Charge Transfer

With the aim of constructing efficient photoelectric organic materials, a pyrido[3,2-g]quinoline derivative named LA17b has been synthesized, and its photodynamic relaxation processes in solvents and films were studied by time-resolved fluorescence and femtosecond transient absorption techniques. The steady-state fluorescence spectra show pronounced red-shift with the increase of the solvent polarity as well as in binary solvent hexane/ethanol by increasing ethanol concentration. However, the strong red-shift does not lead to quenching of the fluorescence. This is explained in terms of a twisted intramolecular charge transfer (TICT) state. The TICT state of LA17b in ethanol is highly emissive with a long fluorescence lifetime: 1.1 ns. TICT state was shown to play an important role in enhancement of intersystem crossing rate. TD-DFT calculations confirm the pathways of relaxation of locally excited state via TICT and triplet states. In films, the photodynamic properties are similar to that of LA17b in hexane and the TICT state vanishes due to the rigid environment. The obtained optical properties of this molecule suggest that it can be a promising candidate for various optoelectronic applications.