Effects of Naphthalene-to-Azulene Isoelectronic Structural Reconstruction on Electronic and Optical Properties of...

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 (rP) and X-PDI (X-rPDI) are studied for the first time, adopting polarization-consistent optimally tuned range-separated hybrid (OT-RSH) in toluene. All X-rPDIs, including rP 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-rPDI compared to their respective PDI analogues, which, in competition with varied exciton binding energy in X-rPDIs produce red- and blue-shifted lowest excited singlet (S₁) and triplet (T₁ > 1.0 eV), respectively. Optically forbidden S₁ in rP and all X-rPDIs, with closely lying optically bright S_n suggests fluorescence turn-off. Similar ππ^* excitonic characters and lesser chalcogen contributions yield relatively smaller intersystem crossing (ISC) rates for X-rPDIs than X-PDIs. The rate increases down the chalcogen group for both X-rPDIs 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 rP and O-rPDI compared to their pristine analogues. Further, while S-rPDI shows ∼6 orders smaller rate than S-PDI, both Se-rPDI and Se-PDI display remarkably high ISC rates (∼10¹²-10¹³ s^-1). Importantly, Se-rPDI with moderately high energy T₁ 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.