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 H-1 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.