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′′′′′′′′′′′′′′, 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′′′′′′′′′′′′′′; 5′′′′′′′′′′′′′′, 2′′′′′′′′′′′′′′′]hexadecathiophene (4) by regioselective bromination using N-bromosuccinimide and regioselective lithiation using lithiumdiisopropylamide in ethylbenzene/THF/heptane followed by reaction with trimethylstannylchloride 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.