1,7- And 1,6-Regioisomers of Diphenoxy and Dipyrrolidinyl Substituted Perylene Diimides: Synthesis, Separation, Characterization, and Comparison of Electrochemical and Optical Properties

1,7- And 1,6-regioisomers of N,N′-dioctyl-di(2,4-di-tert-butylphenoxy)perylene diimide (1,7-5 and 1,6-5) and N,N′-dioctyl-dipyrrolidinylperylene diimide (1,7-8 and 1,6-8) dyes are prepared. These 1,7- and 1,6-regioisomers are successfully isolated from the regioisomeric mixture using conventional methods of separation and subsequently characterized, unambiguously, by 300 MHz 1H NMR spectroscopy. This is the very first time when 1,6-regioisomers of diphenoxy and dipyrrolidinyl substituted perylene diimides are obtained in pure form. Optical and redox properties of these 1,6-regioisomers are examined extensively and compared with respective 1,7-regioiosmers. The optical and electrochemical characteristics of diphenoxy substituted isomers, 1,7-5 and 1,6-5, were found to be virtually the same. However, quite unexpectedly, crucial differences were observed in the properties of regioisomers of dipyrrolidinyl substituted PDIs, 1,7-8 and 1,6-8. Differential pulse voltammetry revealed that 1,7-8 has better electron-donating ability compare to that of 1,6-8. Pronounced differences were observed in the optical properties too. In addition to the lowest energy absorption band at around 700 nm, 1,6-8 exhibits another strong absorption band at ca. 560 nm, and consequently, covers larger part of the visible region relative to that of 1,7-8. Steady-state emission and fluorescence lifetime studies, carried out in solvents of different polarities, revealed that regioisomer 1,6-8 has inherently lower fluorescence quantum yields and lifetimes compared to that of 1,7-8. This fundamental information on the redox and optical properties of 1,6- and 1,7-regioisomers of diphenoxy and dipyrrolidinyl substituted PDIs will be of value especially for material chemists to develop more efficient systems and devices from bay-functionalized perylene diimides.