Azobenzene-Linked Porphyrin−Fullerene Dyads

Abstract

A new group of porphyrin-fullerene dyads with an azobenzene linker was synthesized, and the photochemical and photophysical properties of these materials were investigated using steady-state and time-resolved spectroscopic methods. The electrochemical properties of these compounds were also studied in detail. The synthesis involved oxidative heterocoupling of free base tris-aryl-p-aminophenyl porphyrins with a p-aminophenylacetal, followed by deprotection to give the aldehyde, and finally Prato 1,3-dipolar azomethineylide cycloaddition to C60. The corresponding Zn(II)-porphyrin (ZnP) dyads were made by treating the free base dyads with zinc acetate. The final dyads were characterized by their 1H NMR, mass, and UV-vis spectra. 3He NMR was used to determine if the products are a mixture of cis and trans stereoisomers, or a single isomer. The data are most consistent with the isolation of only a single configurational isomer, assigned to the trans (E) configuration. The ground-state UV-vis spectra are virtually a superimposition of the spectral features of the individual components, indicating there is no interaction of the fullerene (F) and porphyrin (H2P/ZnP) moieties in the ground state. This conclusion is supported by the electrochemical data. The steady-state and time-resolved fluorescence spectra indicate that the porphyrin fluorescence in the dyads is very strongly quenched at room temperature in the three solvents studied: toluene, tetrahydrofuran (THF), and benzonitrile (BzCN). The fluorescence lifetimes of the dyads in all solvents are sharply reduced compared to those of H2P and ZnP standards. In toluene, the lifetimes of the free base dyads are 600-790 ps compared to 10.1 ns for the standard, while in THF and BzCN the dyad lifetimes are less than 100 ps. For the ZnP dyads, the fluorescence lifetimes were 10-170 ps vs 2.1-2.2 ns for the ZnP references. The mechanism of the fluorescence quenching was established using time-resolved transient absorption spectroscopy. In toluene, the quenching process is singlet-singlet energy transfer (k approximately 10(11) s-1) to give C60 singlet excited states which decay with a lifetime of 1.2 ns to give very long-lived C60 triplet states. In THF and BzCN, quenching of porphyrin singlet states occurs at a similar rate, but now by electron transfer, to give charge-separated radical pair (CSRP) states, which show transient absorption spectra very similar to those reported for other H2P-C60 and ZnP-C60 dyad systems. The lifetimes of the CSRP states are in the range 145-435 ns in THF, much shorter than for related systems with amide, alkyne, silyl, and hydrogen-bonded linkers. Thus, both forward and back electron transfer is facilitated by the azobenzene linker. Nonetheless, the charge recombination is 3-4 orders of magnitude slower than charge separation, demonstrating that for these types of donor-acceptor systems back electron transfer is occurring in the Marcus inverted region.

Synthesis of Novel Fullerene-Porphyrin Conjugates for the Formation of Langmuir Monolayers

“…[18][19][20][21][22][23][24] Thus to derive the fullerene-porphyrin conjugates with long chain alkyl substituents we employed the Prato reaction that is 1,3-dipolar cycloaddition to the double bond in a 6,6 ring position in a fullerene of azomethine ylide generated by the condensation of an a-amino acid and an aldehyde. [25][26] The reaction offers the following advantages:…”

Photochemical Transformations Involving Porphyrins and Phthalocyanines

“…For a related triad (C 60 + 2 Zn(II) porphyrins) it was shown that the main quenching pathway involved charge-separation from the excited singlet state Zn(II)TPP to the C 60 unit in noncoordinating solvents and from the triplet excited state of Zn(II)TPP in coordinating solvents (El-Khouly et al 2003). Porphyrin-fullerene dyads with an azobenzene linker were synthesized, and the photophysicochemical properties were investigated using steady-state and time-resolved spectroscopic methods (Schuster et al 2007). Photosynthetic models containing dendritic C 60 -H 2 P-(ZnP) 3 conjugates were analyzed in relation to the primary events in photosynthesis, i.e., light harvesting, unidirectional energy transfer, charge transfer, and charge-shift reactions.…”

New Donor-Acceptor Porphyrin-Fullerene Dyades

“…Однако он образует различные по природе диады с молекулами -донорами электронов, таким как тетратиафульвалены, металлоцены, N,N-диметиламинофенил-производные, рутений(II)-трис-бипиридин, порфирины, фталоцианины, являющиеся предметом многочисленных исследований. [2][3][4][5][6][7][8][9][10][11] Значительный интерес представляют порфирин-фуллереновые диады, образованные за счет донорно-акцепторного координационного взаимодействия между электронодонорным атомом заместителя в фуллерене и атомом металла в металлопорфирине. Пиридильные или имидазольные производные пирролидинил- [60] фуллерена имеют дополнительные реакционные центры и могут выступать донорами электронной плотности по отношению к координационно ненасыщенным металлопорфиринам (MP), образуя с ними координационно-связанные диады [12][13][14][15] и молекулярные комплексы более высокого порядка.…”