Multiemissive arylpyrrolidines: novel mechanochromic fluorophores investigated by multiscale calculations, electronic spectroscopy, and XRD

Multiemission and mechanochromism are appealing properties in the fields of sensors and biosensors, imaging, and smart materials. These features are further enhanced by Aggregation-Induced Emission (AIE) combined with organic Room Temperature Phosphorescence (RTP). Despite in many aromatic luminophores emission is quenched by aggregation, here we present two novel mechanochromic organic compounds, namely para- and meta- nitrophenyl-pyrrolidine, which display distinct emissive behaviour upon grinding. By joining experimental and theoretical efforts one can enlighten the interplay of molecular stacking in the solid state with the light emission properties. The multiemissivity of such aryl-pyrrolidines allows for a selection of the photoluminescence by simply modifying the aggregation state or the crystallinity of the sample. The mechanochromic behaviour emerges from the emission spectra of crystalline and ground meta-nitrophenyl-pyrrolidine. Driven by this experimental evidence, XRD analyses were carried out to characterize the different crystalline phases fostering the hypothesis of an organic Crystallization-Induced Emission (CIE) related phenomenon. To corroborate the conclusions drawn from the UV-vis and XRD analyses we carried out a thorough computational investigation with a multiscale approach which allowed us to unveil the nature of the CIE. The clusters of interacting aryl-pyrrolidines were retrieved by simulating the aggregation of such compounds with different classical Molecular Mechanics (MM) Molecular Dynamics (MD) solvation boxes and comparing them with the information on crystal packing derived from XRD data. The force-filed parameters of both the solutes and the solvents were obtained by Density Functional Theory (DFT). The excitation states of the nitrophenyl-pyrrolidines aggregates (extracted from the MM-MD solvation box) were computed with DFT and time-dependent DFT methods to enlighten the multiemissive mechanochromic behaviour of these novel organic RTP compounds.