A new iterative approach for the synthesis of oligo(phenyleneethynediyl) derivatives and its application for the preparation of fullerene-oligo(phenyleneethynediyl) conjugates as active photovoltaic materials

Disymmetrically substituted oligo(phenyleneethynediyl) (OPE) derivatives were prepared from 2,5-bis(octyloxy)-4-[(triisopropylsilyl)ethynyl]benzaldehyde (5) by an iterative approach using the following reaction sequence: i) Corey-Fuchs dibromoolefination, ii) treatment with an excess of lithium diisopropylamide, and iii) a metal-catalyzed cross-coupling reaction of the resulting terminal alkyne with 2,5-diiodo-1,4-bis(octyloxy)benzene (3) (Schemes 2 and 3). Reaction of the OPE dimer 8 and trimer 13 thus obtained with N-methylglycine and C-60 in refluxing toluene gave the corresponding C-60-OPE conjugates 16 and 17, respectively (Scheme 4). On the other hand, treatment of the protected terminal alkynes 8 and 13 with Bu4N followed by reaction of the resulting 9 and 14 with 4-iodo-N,N-dibutylaniline under Sonogashira conditions yielded 10 and 15, respectively (Schemes 2 and 3). Subsequent treatment with N-methylglycine and C-60 in refluxing toluene furnished the C-60-OPE derivatives 18 and 19 (Scheme 4). Compound 9 was also subjected to a Pd-catalyzed cross-coupling reaction with 3 to give the centrosymmetrical OPE pentamer 20 (Scheme 5). Subsequent reduction followed by reaction of the resulting diol 21 with acid 22 under esterification conditions led to bis-malonate 23. Oxidative coupling of terminal alkyne 14 with the Hay catalyst gave bis-aldehyde 24 (Scheme 6). Treatment with diisobutylaluminium hydride followed by dicylcohexylcarbodiimide-mediated esterification with acid 22 gave bis-malonate 26. Finally, treatment of bis-malonates 23 and 26 with C-60, I-2, and 1,8-diazabicylco[5.4.0]undec-7-ene (DBU) in toluene afforded the bis[cyclopropafullerenes] 27 and 28, respectively (Scheme 7). The C-60 derivatives 16-19, 27, and 28 were tested as active materials in photovoltaic devices. Each C-60-OPE conjugate was sandwiched between poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate)-covered indium tin oxide and aluminium electrodes. Interestingly, the performances of the devices prepared from the N,N-dialkylaniline-terminated derivatives 18 and 19 are significantly improved when compared to those obtained with 16, 17, 27, and 28, thus showing that the efficiency of the devices can be significantly improved by increasing the donor ability of the OPE moiety.