Antibatic photovoltaic response in zinc-porphyrin-linked oligothiophenes

We present the synthesis of oligohexylthiophenes starting from 3, 4'-dihexyl-[2, 2']bithiophene (1) 3, 4', 4", 4"'-tetrahexyl-[2, 2'; 5', 2"; 5", 2"']quarterthiophene (2), 3, 4', 4", 4''', 4''",4''''",4"""4'"'"-octahexyl-[2, 2'. 5', 2"; 5", 2'''; 5''', 2''''; 5'''', 2'''''; 5''''', 2''''''; 5'''''', 2''''''']octithiophene (3) and 3, 4', 4'', 4''', 4"'', 4''''', 4'''''', 4''''"', 4'''''''', 4'""'''', 4""'''''', 4"''''''''', 4'''''''''''', 4''''''''''', 4"""""""-hexadecahexyl-[2, 2'; 5', 2"; 5", 2'''; 5"', 2''''; 5'''', 2'''''; 5'''", 2''''"; 5'''''', 2'''''"; 5"''''', 2''''""; 5"""''', 2"""""; 5""""", 2"""""'; 5"""""', 2""""""; 5"""""" 2""""""'; 5""""""', 2"""""""; 5""""""", 2"""""""']hexadecathiophene (4) by regioselective bromination using N-bromosuccinimide and regioselective lithiation using lithiumdiisopropylamide in ethylbenzene/THF/heptane followed by reaction with trimethyl-stannylchloride and subsequent palladium catalysed Stille coupling. We further synthesised 5,15-bis(3, 4', 4", 4"', 4", 4""', 4""", 4"""'-octahexyl-[2, 2'; 5', 2" 5", 2'"; 5"', 2""; 5"", 2""'; 5""', 2"""; 5""", 2"""']octithiophene-5-yl)-10, 20-bis(3, 5-ditertbutylphenyl)zinc(II)porphyrin (5) from trimethyl(3, 4', 4", 4'", 4"", 4""', 4""", 4"""'-Octahexyl-[2, 2'; 5', 2"; 5"; 2'"; 5"', 2""; 5"", 2"'"; 5""', 2"""; 5""", 2"""']octithiophene-5-yl)stannane (3-SnMe3) and 5, 15-dibromo-10, 20-bis(3, 5-ditertbutylphenyl)zinc(II)porphyrin (6) by Stille coupling. All the products were characterised by size exclusion chromatography (SEC), NMR, MALDI-TOF and elemental analysis and purified by preparative SEC before subjecting them to photophysical studies. UV-vis and emission spectroscopy were used to determine quantum yields and energy transfer. The photon balance was established and used to rationalise the photovoltaic behaviour of 4 and 5. While 4 gave rise to photovoltaic devices giving a moderate photovoltaic response that was symbatic with the absorption spectrum, 5 showed a photovoltaic response that was antibatic with a part of the absorption spectrum of the zinc-porphyrin constituent. We ascribe this behaviour to efficient internal conversion of the energy absorbed by (and the energy transferred to) the zinc-porphyrin constituent. (c) 2004 Elsevier B.V. All rights reserved.