Effect of alkyl substituents: 5,15-bis(trimethylsilylethynyl)- vs. 5,15-bis(triisopropylsilylethynyl)-tetrabenzoporphyrins and their metal complexes
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The copper(II), nickel(II), etc. complexes of 5,15-bis(trimethylsilylethynyl)tetrabenzoporphyrin (TMS-H2BP) and 5,15-bis(triisopropylsilylethynyl)tetrabenzoporphyrin (TIPS-H2BP) have been prepared from the corresponding bicycle[2.2.2]octadiene(BCOD)-fused precursors by the retro-Diels-Alder reaction. X-ray diffraction (XRD) analyses show that TMS-H2BP and its metal complexes of zinc(II) (TMS-ZnBP) and copper(II) (TMS-CuBP) adopt flat molecular conformations and form herringbone-type packing structures in the single crystalline state. TIPS-H2BP and the zinc(II) and copper(II) complexes (TIPS-ZnBP and TIPS-CuBP) are similar to the TMS derivatives in molecular conformation, but these TIPS derivatives form one-dimensional slipped-stack structures. The nickel complexes TMS-NiBP and TIPS-NiBP have U-shaped structures because of the small size of nickel(II) ion. Solution processed organic thin-film transistors of the benzoporphyrins were fabricated and TMS-H2BP showed the highest hole mobility of 0.11 cm2.V-1.s-1. Bulk heterojunction organic solar cells based on TMS- or TIPS-H2BP and their metal complexes as p-type and PC71BM as n-type materials were fabricated by solution process. Atomic force microscopy and thin-film XRD measurements indicated that the film crystallinities were increased by raising the annealing temperature over 180°C or by changing the substituents from triisopropylsilyl to trimethylsilyl. The best power-conversion efficiency (PCE) of 1.49% was achieved with TMS-ZnBP by annealing at 180°C with a moderate crystallinity and smooth surface.
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