Effects of Naphthalene-to-Azulene Isoelectronic Structural Reconstruction on Electronic and Optical Properties of Perylenediimide and Its Chalcogenides

Naphthalene-to-azulene isoelectronic structural reconstruction in perylene (P), perylenediimide (PDI), and its chalcogenides (X-PDI, X = O, S, Se), similar to the Stone-Wales defect in graphene, may significantly alter the intrinsic electronic structure and thus poses scientific curiosity about how and to what extent their structure-function relationships change with such reconstruction. Structural, electronic, and photophysical properties for the reconstructed analogues of P (<i>r</i>P) and X-PDI (X-<i>r</i>PDI) are studied for the first time, adopting polarization-consistent optimally tuned range-separated hybrid (OT-RSH) in toluene. All X-<i>r</i>PDIs, including <i>r</i>P are found to be planar and dynamically stable, with thermodynamic formation energies comparable to those of their pristine congeners, indicating synthetic feasibility. The complex interplay of chalcogens and reconstruction produces an increased electronic gap in S/Se-<i>r</i>PDI compared to their respective PDI analogues, which, in competition with varied exciton binding energy in X-<i>r</i>PDIs produce red- and blue-shifted lowest excited singlet (<i>S</i>₁) and triplet (<i>T</i>₁ > 1.0 eV), respectively. Optically forbidden <i>S</i>₁ in <i>r</i>P and all X-<i>r</i>PDIs, with closely lying optically bright <i>S_n</i> suggests fluorescence turn-off. Similar <i>ππ</i>^* excitonic characters and lesser chalcogen contributions yield relatively smaller intersystem crossing (ISC) rates for X-<i>r</i>PDIs than X-PDIs. The rate increases down the chalcogen group for both X-<i>r</i>PDIs and X-PDIs due to gradually increased heavy-atom effects. Interestingly, reconstruction lowers the excited singlet-triplet gap and generates nonzero spin-orbit coupling, yielding ∼4 orders higher ISC rates in <i>r</i>P and O-<i>r</i>PDI compared to their pristine analogues. Further, while S-<i>r</i>PDI shows ∼6 orders smaller rate than S-PDI, both Se-<i>r</i>PDI and Se-PDI display remarkably high ISC rates (∼10¹²-10¹³ s^-1). Importantly, Se-<i>r</i>PDI with moderately high energy <i>T</i>₁ and a considerably large ISC rate, could serve as a better triplet photosensitizer than Se-PDI. These insights into the reconstruction-tailored structure-function relationships will help to design new azulene-based functional organic molecules.