Core-substituted naphthalenediimides (core-substituted NDIs) were incorporated into rod-like molecules and oligomers through reaction at the imide nitrogen positions. N,N’-Di(4-bromophenyl)-2,6-di(N-alkylamino)-1,4,5,8-naphthalenetetracarboxydiimide was synthesized in only three steps, and used as a versatile platform to prepare extended structures by reaction with thiophene substrates using Suzuki-coupling conditions. The optoelectronic properties of the new compounds were examined by UV/vis absorption spectroscopy, fluorescence spectroscopy, cyclic voltammetry and theoretical calculations. The imide substituents had little effect on the optical and electrochemical properties of core-substituted NDIs in solution. A bathochromic shift of the absorption was observed upon film formation, accompanied by quenching of fluorescence. These observations are consistent with increased inter-molecular interactions between core-substituted NDI moieties in the solid state. All compounds were tested in organic solar cells by blending with poly(3-hexylthiophene) (P3HT), and several showed a photovoltaic effect, demonstrating their potential as electron acceptors in organic solar cell. The best solar cell was observed for core-substituted NDI with 4-(thiophen-2-yl)phenyl imide substituents (5a), showing a power conversion efficiency of 0.57% and a large open circuit voltage of 0.87 V. This approach allows new structure-property relationship studies of non-fullerene acceptors in organic solar cells, where one can vary the imide substituent to optimize photovoltaic parameters while keeping the optical and electrochemical properties constant.
To study the structure-property relationships of core-substituted NDIs as acceptors for organic solar cells, a series of 2,6-dialkylamino NDI compounds with various substituents were synthesized, characterized and tested in bulk heterojunction solar cells by blending with P3HT. The imide substituents consisted of a linker connected to a thiophene group, where the linker was phenyl, methyl or ethyl. The core substituents were cyclohexylamino or 2-ethylhexylamino. While the various substituents had little effect on the opto-electronic properties in solution, they strongly affected device performance and blend morphology. Under the conditions studied, the best performance was obtained with the methyl linker combined with the cyclohexylamino core substituent, with a power conversion efficiency of 0.48% and a high open circuit voltage of 0.97V. For blends of P3HT with modified NDI non-fullerene acceptors, the methyl linker promoted larger phase separated domains than the ethyl or phenyl linkers. DFT calculations showed that the linker determines the orientation of the thiophene conjugated plane with respect to the NDI conjugated plane. That angle was 114°, 45°-61°, and 8° for the methyl, phenyl and ethyl linkers, respectively. Using thiophene at the end of the imide substituent adds a unique dimension to tune morphology and influence the molecular heterojunction between donor and acceptor.
The effect of vinylphenyl imide substitution on opto-electronic properties of core-substituted NDIs were examined by synthesizing a series of NDI molecules. 2-Ethylhexylamino (RF8 series and RF6) and 5-(2-ethylhexyl)thiophene (RF7 series) groups were used as core substitutions. The optical properties in solution and thin films, and the electrochemical properties in solution of vinylphenyl imide substituted compounds (RF7b and RF8b) were compared with other imide substitutions: Hydrogen (RF7H and RF8H), 2-ethylhexyl (RF7a and RF8a), and phenylthiophene (RF6). The low energy absorption band maximum of RF7H, RF7a and RF7b in dichloromethane was 534 nm, 524 nm and 531 nm, respectively, indicating a small effect from the imide substituents. The low absorption band maximum of 2-ethyhexylamino core substituted compounds of RF8H, RF8a and RF8b in dichloromethane were 627, 625 and 627 nm, respectively, suggesting that the optical property of the RF8 series had little dependence on the imide group and were governed by the alkylamino core substituents. The optical absorption of thin films were similar to the solution absorptions, except for RF8b with vinylphenyl imide substituents. The absorption onset of RF8b red shifted by 104 nm going from solution (659 nm) to film (763 nm). In thin films, RF8b showed intense absorption in the range of 300 – 800 nm. These results suggest that opt-electronic properties can be tuned using core substituents, and that low band gap organic small molecule materials can be designed with the right choice of core/imide substitutions using NDI.