Azadipyrromethene-based near-IR dyes with styryl substituents at the pyrrolic positions for organic photovoltaic applications

Abstract

Azadipyrromethene dyes are attractive building blocks for the development of near-IR dyes, with applications ranging from biosensing, photodynamic therapy and solar energy conversion. In particular, Zn(II) bis(2,8-diphenylethynyl 1,3,7,9 tetraphenylazadipyrromethene) complexes are promising n-type non-fullerene electron acceptors for organic photovoltaics. Here, the triple bond in phenylethynyl pyrrolic substituents is replaced with a double bond. The UV–Vis absorption spectra were not significantly different, indicating that both the double and triple bond similarly extend the conjugation of ADP. Interestingly, the Zn(II) complex with styryl substituents (Zn(WS6)2) showed emission in the near IR, with a λmax of 773 nm. The BF2+ chelate also emitted in the near IR with a λmax of 815 nm, but the emission was very weak. Cyclic voltammetry revealed that double bond compounds were easier to oxidize than un-substituted ADP analogues, and unlike with triple bond, were not easier to reduce. In organic photovoltaics, the Zn(WS6)2 did not perform as well as the Zn(WS3)2 analogue, showing unfavorable film morphology when blended with P3HT and low blend electron mobility of 2 × 10−8 cm2V−1s−1 by SCLC, compared to 1 × 10−7 cm2V−1s−1 in neat film. The hole mobility of Zn(WS6)2 in neat films is 6 × 10−5 cm2V−1s−1, higher than its electron mobility, suggesting its potential as p-type material as well. Further work in finding an appropriate n-type polymer acceptor is required to confirm this hypothesis. Using a triple bond at the pyrrolic positions is therefore better than a double bond for the development of n-type materials based on ADP dyes.