Efficient tuning of optoelectronic properties of 1,6- and 1,7-disubstituted perylene diimides: agreement between theory and experiment

Abstract

In the literature, PDIs have been shown to demonstrate extensive applications in the area of organic electronics due to their ability to act as n-type semiconductors. PDIs have been applied in a wide range of electronic devices, including energy storage devices. To fit in these applications, we have modified the valine-terminated PDI core by substituting groups, viz. phenoxy, 4-methoxy phenoxy, and 3,4-dimethoxy phenoxy groups, at bay positions to fine-tune their HOMO–LUMO levels. In this paper, a series of conjugation-extended 1,7-, 1,6- and 1,6,7-regioisomers of Val-PDIs were synthesized, and 1,6,7-PDIs were removed by utilizing the Soxhlet extraction technique used for the first time. The 1,7- and 1,6-PDIs were separated by repeated crystallization and their structural characterization was performed using FTIR, ¹H NMR, ¹³C NMR, and HRMS. Further, the structures of as-synthesized PDIs were optimized by DFT calculations using the B3LYP approach with a 6-31+G(d,p) basis set, and the electronic transitions were visualized by analyzing natural transition orbitals (NTO). The photophysical properties of the highly soluble bay-substituted PDIs were evaluated using UV-Visible and fluorescence spectroscopic techniques in organic solvents ranging from non-polar to polar. Furthermore, their relative polarities in the ground vs. excited state were determined by employing the Lippert–Mataga relations. The electrochemical investigations of PDIs were done using cyclic voltammetry to evaluate their redox behavior. In addition to their unique structural relationship, 1,7- and 1,6-regioisomers revealed intriguing optoelectronic properties. Finally, symmetric supercapacitors (SSCs) were developed by exploiting the regioisomerically pure 1,7- and 1,6-disubstituted PDIs as electrode materials. The as-fabricated PDIs/GF electrodes were optimized in a two-electrode SC device given their electrochemical performance. Two of the PDIs, viz.M1 and D1, demonstrated good results of specific capacitances, i.e. 59.41 and 23.60 Fg⁻¹ respectively, in the SSC devices.