Mechanical Tuning of Fluorescence Lifetime and Bandgap in an Elastically Flexible Molecular Semiconductor Crystal

Despite having superior transport properties, lack of mechanical flexibility is a major drawback of crystalline molecular semiconductors as compared to their polymer analogues. Here single crystals of an organic semiconductor are reported that are not only flexible but exhibit systematic tuning of bandgaps, fluorescence lifetime, and emission wavelengths upon elastically bending. Spatially resolved fluorescence lifetime imaging and confocal fluorescence microscopy reveals systematic trends in the lifetime decay across the bent crystal region along with shifts in the emission wavelength. From the outer arc to the inner arc of the bent crystal, a significant decrease in the lifetime of ≈1.9 ns is observed, with a gradual bathochromic shift of ≈10 nm in the emission wavelength. For the crystal having a bandgap of 2.73 eV, the directional stress arising from bending leads to molecular reorientation effects and variations in the extent of intermolecular interactions- which are correlated to the lowering of bandgap and the evolution of the projected density of states. The systematic changes in the interactions quantified using electron density topological analysis in the compressed inner arc and elongated outer arc region are correlated to the non-radiative decay processes, thus rationalizing the tuning of fluorescence lifetime.