Light-induced dilation in nanosheets of charge-transfer complexes

We report the observation of a sizable photostrictive effect of 5.7% with fast, submillisecond response times, arising from a light-induced lattice dilation of a molecular nanosheet, composed of the molecular charge-transfer compound dibenzotetrathiaful-valene (DBTTF) and C-60. An interfacial self-assembly approach is introduced for the thickness-controlled growth of the thin films. From photoabsorption measurements, molecular simulations, and electronic structure calculations, we suggest that photostriction within these films arises from a transformation in the molecular structure of constituent molecules upon photoinduced charge transfer, as well as the accommodation of free charge carriers within the material. Additionally, we find that the photostrictive properties of the nanosheets are thickness-dependent, a phenomenon that we suggest arises from surface-induced conformational disorder in the molecular components of the film. Moreover, because of the molecular structure in the films, which results largely from interactions between the constituent p-systems and the sulfur atoms of DBTTF, the optoelectronic properties are found to be anisotropic. This work enables the fabrication of 2D molecular charge-transfer nanosheets with tunable thicknesses and properties, suitable for a wide range of applications in flexible electronic technologies.