Linear π-conjugated oligomers have been widely investigated, but the behavior of the corresponding cyclic oligomers is poorly understood, despite the recent synthesis of π-conjugated macrocycles such as [n]cycloparaphenylenes and cyclo[n]thiophenes. Here we present an efficient template-directed synthesis of a π-conjugated butadiyne-linked cyclic porphyrin hexamer directly from the monomer. Small-angle X-ray scattering data show that this nanoring is shape-persistent in solution, even without its template, whereas the linear porphyrin hexamer is relatively flexible. The crystal structure of the nanoring-template complex shows that most of the strain is localized in the acetylenes; the porphyrin units are slightly curved, but the zinc coordination sphere is undistorted. The electrochemistry, absorption, and fluorescence spectra indicate that the HOMO-LUMO gap of the nanoring is less than that of the linear hexamer and less than that of the corresponding polymer. The nanoring exhibits six one-electron reductions and six one-electron oxidations, most of which are well resolved. Ultrafast fluorescence anisotropy measurements show that absorption of light generates an excited state that is delocalized over the whole π-system within a time of less than 0.5 ps. The fluorescence spectrum is amazingly structured and red-shifted. A similar, but less dramatic, red-shift has been reported in the fluorescence spectra of cycloparaphenylenes and was attributed to a high exciton binding energy; however the exciton binding energy of the porphyrin nanoring is similar to those of linear oligomers. Quantum-chemical excited state calculations show that the fluorescence spectrum of the nanoring can be fully explained in terms of vibronic Herzberg-Teller (HT) intensity borrowing.

The photophysics of a butadiyne-linked porphyrin dimer has been investigated by spectroscopy and quantum mechanical calculations. Primarily, the influence of conformation on the ground and first singlet excited states was studied, and two spectroscopically distinct limiting cases were identified. Experiments show that the twisted and planar conformers are separate spectroscopic species that can be selectively excited and have unique absorption and emission spectra. Calculated ground-state spectra compare well with experimental spectra of the two species. A spectrum of the planar conformer was obtained by the addition of a dipyridyl pyrrole ligand, which forms a 1:1 complex with the dimer and thus forces it to stay planar. The absorption spectrum of the twisted conformer could be deduced from the excitation spectrum of its emission. The interpretation of the ground-state spectrum of the free noncomplexed dimer is that it represents an average of a broad distribution of conformations. Calculations support this conclusion by indicating that the barrier for rotation is relatively small in the ground state (0.7 kcal/mol). Studies of the temperature dependence of the fluorescence spectrum of the dimer indicate a mother-daughter relationship between the twisted and planar conformations in the excited state, where the former has approximately 3.9 kcal/mol higher energy. Furthermore, time-correlated single-photon counting experiments also suggest that the twisted population adopts a planar configuration in the first singlet excited state with a rate constant of k rot ) 8.8 × 10 9 s -1 in 2-MTHF at room temperature. The temperature dependence of the fluorescence lifetimes indicated that an activation energy barrier of approximately 2 kcal/mol, in part related to solvent viscosity, is associated with this rate constant.

Dedicated to Professor Jeremy Sanders on the occasion of his 60th birthdayBelt-shaped chromophores provide fascinating insights into electronic p delocalization over curved surfaces with radially oriented p orbitals.[1] Examples include the cyclic para-phenylacetylenes [2] and the [4 6 ]paracyclophanedodecayne of Tsuji and coworkers, [3] as well as fullerenes and carbon nanotubes. A variety of belt-shaped porphyrin arrays have been synthesized; [4] however, the vast majority of them lacks a complete pconjugation pathway around the whole macrocycle. Recently we reported the synthesis of a belt-shaped D 8h symmetric porphyrin[8] nanoring on an octadentate template.[5] Herein we present an efficient synthesis of an even more strained p conjugated D 6h porphyrin [6] nanoring 1, by template-directed trimerization of a porphyrin dimer 2 on a hexapyridyl template 3 (Scheme 1). This route is more direct than the synthesis of the cyclic octamer since both starting materials, 2 and 3, are readily accessible. The cyclic hexamer complex 1·3 is phenomenally stable (K f = 7 10 38 m À1 ; EM = 340 m) but the free macrocycle can be liberated from the 1·3 complex with amines such as quinuclidine. The UV/Vis/NIR absorption and emission show that there is efficient p conjugation around the porphyrin[6] nanoring 1, and that its S 0 -S 1 gap is even smaller than that of the corresponding linear porphyrin hexamer 4; this conclusion is supported by time-dependent density functional (TD-DFT) calculations.The key to the synthesis of the hexamer nanoring is the design of a complementary template (Scheme 2). The hexadentate template 3 has a calculated nitrogennitrogen distance of 20.1 , which is a good fit for the cavity of cyclic hexamer 1 with a zinc-zinc ring diameter of 24.2 (assuming a Zn À N bond length of 2.2 ). The template was synthesized by a six-fold Suzuki coupling of 4-pyridineboronic acid with hexakis(4-bromophenyl)benzene [6] in 50 % yield (Supporting Information). Two versions of cyclic hexamer 1 were synthesized-1 a·3 with tert-butyl side chains in 44 % and 1 b·3 with octyloxy side chains in 33 % yield-by oxidative

Electron-transfer reactions are fundamental to many practical devices, but because of their complexity, it is often very difficult to interpret measurements done on the complete device. Therefore, studies of model systems are crucial. Here the rates of charge separation and recombination in donor–acceptor systems consisting of a series of butadiyne-linked porphyrin oligomers (n = 1–4, 6) appended to C60 were investigated. At room temperature, excitation of the porphyrin oligomer led to fast (5–25 ps) electron transfer to C60 followed by slower (200–650 ps) recombination. The temperature dependence of the charge-separation reaction revealed a complex process for the longer oligomers, in which a combination of (i) direct charge separation and (ii) migration of excitation energy along the oligomer followed by charge separation explained the observed fluorescence decay kinetics. The energy migration is controlled by the temperature-dependent conformational dynamics of the longer oligomers and thereby limits the quantum yield for charge separation. Charge recombination was also studied as a function of temperature through measurements of femtosecond transient absorption. The temperature dependence of the electron-transfer reactions could be successfully modeled using the Marcus equation through optimization of the electronic coupling (V) and the reorganization energy (λ). For the charge-separation rate, all of the donor–acceptor systems could be successfully described by a common electronic coupling, supporting a model in which energy migration is followed by charge separation. In this respect, the C60-appended porphyrin oligomers are suitable model systems for practical charge-separation devices such as bulk-heterojunction solar cells, where conformational disorder strongly influences the electron-transfer reactions and performance of the device.