Conjugated porphyrin arrays: synthesis, properties and applications for functional materials

Conjugated porphyrin arrays that possess delocalised electronic networks have, for the most part, been assembled by using alkene or alkyne type bridging units or by directly connecting individual porphyrin chromophores with multiple bonds to form fused porphyrin arrays.

Diketopyrrolopyrrole-porphyrin conjugates as broadly absorbing sensitizers for dye-sensitized solar cells

A series of four new push-pull zinc porphyrin-based dyes was synthesised for hybrid photovoltaic solar cells with a view to enhancing the light-harvesting efficiency at approximately 550 nm with a diketopyrrolopyrrole (DPP) unit. The strength of the donor side of the push-pull porphyrin was tuned by affixing the electron-rich 4,4'-dimethoxydiphenylamine group at the meso position of the macrocycle, and the influence of the distance between the semiconductor surface and the porphyrin chromophore was assessed by introducing different π-conjugated spacers. Charge-transfer transitions over great distances were characterised by electronic absorption spectroscopy and DFT calculations. The absorption and photoactivity spectra of the new bichromophoric dyes spans the whole visible spectrum to the red, implying a better light-harvesting efficiency than regular porphyrin as the absorption spectra of DPP and porphyrin complement one another. Photovoltaic conversion efficiencies accordingly increase from 2.40 to 5.19 %. Interestingly, the best overall efficiency was reached with dye 3, which lacks the powerful donating group in the meso position of the porphyrin core. Optical and electrochemical measurements coupled to time dependent (TD)-DFT calculations give insight into the deleterious effect of the 4,4'-dimethoxydiphenylamine unit on the photovoltaic performances, paving the way towards the design of efficient push-pull porphyrin-based sensitizers.

The Heck reaction for porphyrin functionalisation: synthesis of meso-alkenyl monoporphyrins and palladium-catalysed formation of unprecedented meso–β ethene-linked diporphyrins

Palladium-catalysed coupling of the vinyl derivatives methyl acrylate, styrene and acrylonitrile with 5-bromo-10,15,20-triphenylporphyrin (MTriPPBr; M = 2H, Ni, Zn) and 5,15-dibromo-10,20-bis(3,5-di-tert-butylphenyl)porphyrin (MDAPBr(2)) produced a series of mono- and disubstituted alkenylporphyrins, thus demonstrating the applicability of meso-haloporphyrins in Heck-type reactions. The same technique was also applied to meso-ethenylporphyrins and simple aryl halides, with mixed results. Only meso-vinyl nickel(ii) porphyrins showed any reactivity under our conditions. A mixture of 1,1- and 1,2-disubstitution across the alkene was observed for 5-vinyl-10,15,20-triphenylporphyrinatonickel(ii) (meso-vinylNiTriPP), whereas 5-vinyl-10,20-bis(3,5-di-t-butylphenyl)porphyrinatonickel(ii) (meso-vinylNiDAP) produced a mixture of meso-1,1-, meso-1,2- and, surprisingly, beta-1,2-disubstituted Heck products. Coupling meso-vinylNiDAP with MTriPPBr under similar Heck conditions led unexpectedly to transbeta-meso-NiDAP-ethene-MTriPP dyads, affording the first members of a new class of alkenyl-linked diporphyrins. A mechanism for the unusual meso to beta rearrangement is discussed. The electronic absorption spectra of the dyads have a red-shifted shoulder on the Soret (B) band, which is evidence of a moderate degree of electronic interaction between the porphyrins via the ethenyl bridge.

Photoinduced electron transfer in platinum(II) terpyridinyl acetylide complexes connected to a porphyrin unit

A series of six new dyads consisting of a zinc or magnesium porphyrin appended to a platinum terpyridine acetylide complex via a para-phenylene bisacetylene spacer are described. Different substituents on the 4' position of the terpyridinyl ligand were explored (OC7H15, PO3Et2, and H). The ground-state electronic properties of the dyads are studied by electronic absorption spectroscopy and electrochemistry, and they indicate some electronic interactions between the porphyrin subunit and the platinum complex. The photophysical properties of these dyads were investigated by steady-state, time-resolved, and femtosecond transient absorption spectroscopy in N,N-dimethylformamide solution. Excitation of the porphyrin unit leads to a very rapid electron transfer (2-20 ps) to the nearby platinum complex followed by an ultrafast charge recombination, thus preventing any observation of the charge separated state. The variation in the rate of the photoinduced electron transfer in the series of dyads is consistent with Marcus theory. The results underscore the potential of the para-phenylene bisacetylene bridge to mediate a rapid electron transfer over a long donor-acceptor distance.