Mechanisms, Pathways, and Dynamics of Excited-State Energy Flow in Self-Assembled Wheel-and-Spoke Light-Harvesting Architectures

Design, Synthesis, and Utility of Defined Molecular Scaffolds

A growing theme in chemistry is the joining of multiple organic molecular building blocks to create functional molecules. Diverse derivatizable structures—here termed “scaffolds” comprised of “hubs”—provide the foundation for systematic covalent organization of a rich variety of building blocks. This review encompasses 30 tri- or tetra-armed molecular hubs (e.g., triazine, lysine, arenes, dyes) that are used directly or in combination to give linear, cyclic, or branched scaffolds. Each scaffold is categorized by graph theory into one of 31 trees to express the molecular connectivity and overall architecture. Rational chemistry with exacting numbers of derivatizable sites is emphasized. The incorporation of water-solubilization motifs, robust or self-immolative linkers, enzymatically cleavable groups and functional appendages affords immense (and often late-stage) diversification of the scaffolds. Altogether, 107 target molecules are reviewed along with 19 syntheses to illustrate the distinctive chemistries for creating and derivatizing scaffolds. The review covers the history of the field up through 2020, briefly touching on statistically derivatized carriers employed in immunology as counterpoints to the rationally assembled and derivatized scaffolds here, although most citations are from the past two decades. The scaffolds are used widely in fields ranging from pure chemistry to artificial photosynthesis and biomedical sciences.

Synthesis and Photophysical Properties of Porphyrin Macrorings Composed of Free-Base Porphyrins and Slipped-Cofacial Zinc Porphyrin Dimers

The self-assembled macroring N-(Zn-Fb-Zn)₃ has been constructed by intermolecular complementary coordination among three trisporphyrin Zn-Fb-Zn molecules, each of which consists of a central free-base porphyrin and two imidazolyl-zinc-porphyrin ends. Thus, N-(Zn-Fb-Zn)₃ has three slipped-cofacial zinc porphyrin dimers ("special pair model") and three free-base porphyrins, alternately. The zinc porphyrin dimers in N-(Zn-Fb-Zn)₃ are covalently connected by a ring-closing olefin metathesis reaction between the allyl ether groups substituted on the zinc porphyrin dimers, giving a covalently linked macroring C-(Zn-Fb-Zn)₃. The fluorescence spectra of C-(Zn-Fb-Zn)₃ in several solvents show that the photoinduced energy transfer from one of the zinc porphyrin dimers to a free-base porphyrin occurs intramolecularly in toluene, whereas the photoinduced electron transfer predominantly occurs intramolecularly in N,N-dimethylformamide. Treatment of C-(Zn-Fb-Zn)₃ with copper(II) acetate gives a Cu-containing heteromultinuclear porphyrin macroring C-(Zn-Cu-Zn)₃, demonstrating that C-(Zn-Fb-Zn)₃ could be a good precursor to construct various heteromultinuclear porphyrin macrorings.

Energy transfer from an individual silica nanoparticle to graphene quantum dots and resulting enhancement of photodetector responsivity

Förster resonance energy transfer (FRET), referred to as the transfer of the photon energy absorbed in donor to acceptor, has received much attention as an important physical phenomenon for its potential applications in optoelectronic devices as well as for the understanding of some biological systems. If one-atom-thick graphene is used for donor or acceptor, it can minimize the separation between donor and acceptor, thereby maximizing the FRET efficiency (E_FRET). Here, we report first fabrication of a FRET system composed of silica nanoparticles (SNPs) and graphene quantum dots (GQDs) as donors and acceptors, respectively. The FRET from SNPs to GQDs with an E_FRET of ∼78% is demonstrated from excitation-dependent photoluminescence spectra and decay curves. The photodetector (PD) responsivity (R) of the FRET system at 532 nm is enhanced by 10⁰∼10¹/10²∼10³ times under forward/reverse biases, respectively, compared to the PD containing solely GQDs. This remarkable enhancement is understood by network-like current paths formed by the GQDs on the SNPs and easy transfer of the carriers generated from the SNPs into the GQDs due to their close attachment. The R is 2∼3 times further enhanced at 325 nm by the FRET effect.

Recent advances in the template-directed synthesis of porphyrin nanorings

This Feature Article reviews recent advances in the template-directed synthesis of porphyrin nanorings, including new templating methods, novel structures, and their applications in host–guest chemistry and artificial light-harvesting.

A bifurcated molecular pentad capable of sequential electronic energy transfer and intramolecular charge transfer

The title compound absorbs strongly over much of the solar range and undergoes a variety of photophysical events under illumination.

Energy transfer in non-covalent porphyrin assemblies: through-space or through-bond?

Photosynthetic antenna arrays found in nature funnel photoexcited energy into the reaction center. Attempts have been made to mimic the antenna function by using artificial chromophores, porphyrins in particular, not only to better understand the energy-transfer processes but also to create light-harvesting devices. This review covers non-covalent porphyrin assemblies, for which intra-ensemble energy-transfer processes were characterized. The essence of the mechanisms of energy transfer is summarized and specific examples are reviewed with an emphasis put on the rate and mechanism of singlet-singlet energy transfer. As these examples demonstrate, non-covalent intra-ensemble energy-transfer processes have been ascribed to the Förster-type through-space mechanism in almost all cases. The exception is porphyrin dyad and pentad from our group based on amidinium-carboxylate salt bridges. Through-bond superexchange mechanism is proposed to account for the fast excited energy-transfer processes for these unique assemblies. The importance of intermolecular interactions not only in terms of the structural aspects but also in terms of the electronic aspects is highlighted for the design of supramolecular systems in which efficient energy transfer is desired.

Solution-state conformational ensemble of a hexameric porphyrin array characterized using molecular dynamics and X-ray scattering

Solution-phase X-ray scattering measurements in combination with coordinate-based modeling have been used to characterize the conformational ensemble of a hexameric, diphenylethyne-linked porphyrin array in solution. Configurationally broadened X-ray scattering patterns measured at room temperature for dilute toluene solutions of the porphyrin array were compared to scattering patterns calculated from structural ensembles in constant pressure and temperature molecular dynamics simulations. Thermal fluctuations sampled at picosecond intervals within nanosecond time scale dynamic simulations show large-amplitude motions that include porphyrin ring "tipping" around the porphyrin linkage axes and extended hexameric porphyrin array "breathing" motions involving torsional distortions collectively distributed along porphyrin and diphenylethyne groups. Each type of group motion produced characteristic, angle-dependent dampening of scattering features that are needed to reproduce dampening features in the experimental X-ray scattering. However, mismatches in the magnitudes of experimental and simulated dampening of high-angle X-ray scattering patterns show that large-amplitude hexamer array breathing-type motions are significantly under-represented in the simulated ensembles. This comparison between experiment and simulation provides a means not only to interpret scattering data in terms of an explicit atomic model but more generally demonstrates the use of solution X-ray scattering as an experimental benchmark for the development of simulation methods that more accurately predict configurational dynamics of supramolecular assemblies.

Energy- and hole-transfer dynamics in oxidized porphyrin dyads

The mechanisms and dynamics of quenching of a photoexcited free base porphyrin (Fb*) covalently linked to a nearby oxidized zinc porphyrin (Zn(+)) have been investigated in a set of five dyads using time-resolved absorption spectroscopy. The dyads include porphyrins joined at the meso-positions by a diphenylethyne linker or a diarylethyne linker with 2,6-dimethyl substitution on either one or both of the aryl rings. Another dyad is linked at the beta-pyrrole positions of the porphyrins via a diphenylethyne linker. The type of linker and attachment site modulate the interporphyrin through-bond electronic coupling via steric hindrance (porphyrin-linker orbital overlap) and attachment motif (porphyrin electron density at the connection site). For each ZnFb dyad, the zinc porphyrin is selectively electrochemically oxidized (to produce Zn(+)Fb), the free base porphyrin is selectively excited with a 130 fs flash (to produce Zn(+)Fb*), and the subsequent dynamics monitored. The Zn(+)Fb* excited state has a lifetime of approximately 3 to approximately 30 ps (depending on the linker steric hindrance and attachment site) and decays by parallel excited-state energy- and hole-transfer pathways. The relative yields of the two channels depend on a number of factors including the linker-mediated through-bond electronic coupling and a modest (< or =20%) Forster through-space contribution for the energy-transfer route. One product of Zn(+)Fb* decay is the metastable ground-state ZnFb(+), which decays to the Zn(+)Fb preflash state by ground-state hole transfer with a linker-dependent rate constant of (20 ps)(-1) to (150 ps)(-1). Collectively, these results provide a detailed understanding of the mechanism and dynamics of quenching of excited porphyrins by nearby oxidized sites, as well as the dynamics of ground-state hole transfer between nonequivalent porphyrins (Zn and Fb). The findings also lay the foundation for the study of ground-state hole transfer between identical porphyrins (e.g., Zn/Zn, Fb/Fb) in larger multiporphyrin arrays wherein a hole is selectively placed via electrochemical oxidation.

Photophysics of Soret-excited tetrapyrroles in solution. I. Metalloporphyrins: MgTPP, ZnTPP, and CdTPP

The photophysical behavior of three Soret-excited diamagnetic meso-substituted tetraphenylmetalloporphyrins, MgTPP, ZnTPP, and CdTPP, have been examined in a wide variety of solvents using both steady-state and femtosecond fluorescence upconversion methods. The S 2 population of MgTPP decays to S 1 on the time scale of a few picoseconds with unit S 2-S 1 internal conversion efficiency, and the decay rates conform to the weak coupling case of radiationless transition theory. The energy gap law parameters characterizing the coupling of the S 2 and S 1 states of MgTPP have been obtained. The most important accepting vibrational modes in the S 1 state are multiple in-plane C-C and C-N stretches in the 1200-1500 cm (-1) range. Net S 2-S 1 decay is the dominant decay path for ZnTPP and CdTPP as well, but the process occurs at rates that exceed (in the case of CdTPP, they vastly exceed) those predicted by weak interstate coupling. Alternate mechanisms for the radiationless decay of the S 2 states of ZnTPP and CdTPP have been explored. Large spin-orbit coupling constants and the presence of multiple, near-equiergic triplet states suggest that S 2-T n intersystem crossing might occur at rates competitive with internal conversion. However, the measured efficiencies of S 2-S 1 internal conversion show that, at most, only a few percent of the S 2 population of ZnTPP and no more than about 30% of the S 2 population of CdTPP can decay by a "dark" path such as intersystem crossing.

Langmuir-Schaefer films of a set of achiral amphiphilic porphyrins: aggregation and supramolecular chirality

A series of new achiral porphyrins with hydrophobic dodecyl chains and hydrophilic substituents were designed in order to clarify the relationship between molecular geometry and aggregation as well as the supramolecular chirality in Langmuir-Schaefer films. These achiral porphyrins have zero (TPPA0), one (TPPA1), two (TPPA2a, TPPA2b), three (TPPA3) and four (TPPA4) long hydrophobic chains, respectively. Most of the compounds showed good spreading behaviour on a water surface except TPPA4 and could be fabricated into LS films easily. Depending on the number of alkyl chains, the porphyrin derivatives showed different aggregation behaviour in the LS films. The transferred films were characterized by UV-vis, circular dichroism (CD) and FTIR spectroscopy, and atomic force microscopy (AFM). Interestingly, some of the porphyrin assemblies were found to be optically active in the LS films although the compounds themselves are achiral. TPPA3 showed a strong Cotton effect in the LS film and fiber-like morphology on mica surface. Weak supramolecular chirality was detected from the LS films of TPPA0, TPPA1 and TPPA4, but no Cotton effect was observed for the LS films of TPPA2a or TPPA2b. On the other hand, when the LS film of TPPA3 was dipped into hexane or exposed to HCl gas, which altered the hydrophobic effect among the dodecyl chains and destroyed the π-π interactions between the porphyrin macrocyclic rings, the CD signal decreased or disappeared. The different aggregation behaviour and supramolecular chirality in the films was suggested to be related to the molecular structure and subsequent packing in their organized molecular films.