Designing Small Molecule Photoswitching Scaffolds Inspired from Popular Organic Photovoltaics Materials

Parag Das,¹ Nathan J. Grinalds,² Cory T. Kornman,¹ Ion Ghiviriga,¹ Khalil A. Abboud,¹ Łukasz Dobrzycki,¹ Jiangeng Xue,² Ronald K. Castellano*¹

1 Department of Chemistry, University of Florida, Gainesville, US
2 Department of Materials Science and Engineering, University of Florida, Gainesville, US

Small molecule photoswitches have emerged as versatile tools to control chemical and biological processes at the nanoscale level using light as an external stimulus and can have a wide range of applications within the materials sciences and chemical biology. Dicyanorhodanine (RCN) is a popular electron acceptor unit within the organic photovoltaics (OPV) community due to its essential role in the development of light-absorbing materials. Previously our group reported the Z/E photoisomerization behavior of RCN-functionalized oligothiophenes upon selective excitation in solution and also in the solid state. Recently, we also have demonstrated the well-controlled photochemical behavior along with facile photoswitching of donor-acceptor molecules functionalized with 1,1-dicyanomethylene-3-indanone (INCN), another popular electron-accepting unit, upon selective photoirradiation spanning the UV and visible regions. Here, one model compound was shown to undergo tandem electrocyclization and sigmatropic rearrangement upon only 454 nm excitation. The results clarified recent literature reports of photoisomerization related photodegradation of INCN-containing molecules using broad spectrum irradiation sources.

Our current work introduces novel RCN-pyrrole scaffolds which offer highly efficient photoswitching and excellent thermal stability. Given the molecular design, both the Z and E isomers are available from synthesis and can be isolated using chromatographic techniques. Inter- and intramolecular hydrogen bonding interactions present in the E and Z isomers, respectively, are proposed to be crucial features for the bistable nature of these molecules. Upon extending the π-conjugation of the pyrrole unit, a significant red shift is observed in solution, with absorption peak maxima at 534 nm and 492 nm for the E and Z isomer, respectively. One model chromophore has been shown to undergo bidirectional quantitative isomerization using excitation at 404 nm and 595 nm. Comparator molecules have been synthesized and studied to show the effect of hydrogen bonding on the switching properties. Complementary ground- and excited-state computational studies have also been used to elucidate their excellent photoswitching behavior. Near-quantitative reversible photoswitching and photopatterning within polystyrene films have also been achieved. Our collective work shows that RCN and INCN, the terminal units of popular OPV materials, are highly efficient photoswitching motifs, and should be considered for future functional materials development.