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The " precursor approach " has proved particularly valuable for the preparation of insoluble and unstable π-conjugated polycyclic compounds (π-CPCs), which cannot be synthesized via in-solution organic chemistry, for their improved processing, as well as for their electronic investigation both at the material and single-molecule scales. This method relies on the synthesis and processing of soluble and stable direct precursors of the target π-CPCs, followed by their final conversion in situ, triggered by thermal activation, photoirradiation or redox control. Beside well-established reactions involving the elimination of carbon-based small molecules, i.e., retro-Diels-Alder and decarbonylation processes, the late-stage extrusion of chalcogen fragments has emerged as a highly promising synthetic tool to access a wider variety of π-conjugated polycyclic structures and thus to expand the potentialities of the " precursor approach " for further improvements of molecular materials' performances. This review gives an overview of synthetic strategies towards π-CPCs involving the ultimate elimination of chalcogen fragments upon thermal activation, photoirradiation and electron exchange., (Copyright © 2024, Okba et al.)

Novel hybrid porphyrin(2.1.2.1)s and their boron and copper complexes were synthesized using the "toy bricks" synthetic method. Crystal data, frontier molecular orbital calculations, and electrostatic potential surface maps reveal that hybridization in the porphyrin(2.1.2.1) donor-acceptor unit controls the selective coordination of BF 2 .

To investigate the host ability of a simple macrocycle, 1,3-phenylene-bridged naphthalene hexamer N6, we evaluated the complexation of N6 with fullerenes in toluene and in the crystals. The complexes in the solid-state demonstrate the one-dimensional alignment of fullerenes. The single-crystals of the C 60 @N6 composite have semiconductive properties revealed by photoconductivity measurements., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)

Anthriporphyrinoid and its dimeric homologues were synthesized by Suzuki-Miyaura coupling and subsequent oxidation. Both porphyrinoids were smoothly converted to their Pd II complexes and were further decorated by Suzuki-Miyaura coupling with thiophene derivatives and subsequent oxidative fusion reaction to provide multiply fused compounds. Most Pd II anthriporphyrinoids have been structurally well characterized to be planar for monomeric and helically twisted for dimeric species. The dimeric anthriporphyrinoids show paratropic ring currents due to their global antiaromatic networks, the extent of which increases with an increase of conjugated network. Multiply fused dimeric anthriporphyrinoids show helical structures, fully reversible six redox potentials, small HOMO-LUMO gaps, and absorption tails reaching in the near-infrared region, suggesting the high potential of this approach to explore molecular graphene. Optical separations of the dimeric helical species were accomplished, and racemization barrier heights were determined., Competing Interests: The authors declare no competing financial interest., (© 2022 The Authors. Published by American Chemical Society.)

We demonstrate on-surface deprotection of methylenedioxy groups which yielded graphene nanoribbons (GNRs) with edges functionalized by hydroxy groups. While anthracene trimer precursors functionalized with hydroxy groups did not yield GNRs, it was found that hydroxy groups are first protected as methylenedioxy groups and then deprotected during the cyclo-dehydrogenation process to form GNRs with hydroxy groups. The X-ray photoemission spectroscopy and non-contact atomic force microscopy studies revealed that ∼20% of the methylenedioxy turned into hydroxy groups, while the others were hydrogen-terminated. The first-principles density functional theory (DFT) study on the cyclo-dehydrogenation process was performed to investigate the deprotection mechanism, which indicates that hydrogen atoms emerging during the cyclo-dehydrogenation process trigger the deprotection of methylenedioxy groups. The scanning tunneling spectroscopy study and DFT revealed a significant charge transfer from hydroxy to the Au substrate, causing an interface dipole and the HOMO being closer to the Fermi level when compared with hydrogen-terminated GNR/Au(111). This result demonstrates on-surface deprotection and indicates a possible new route to obtain GNRs with desired edge functionalization, which can be a critical component for high-performance devices., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)

Heteroatom substitution in acenes allows tailoring of their remarkable electronic properties, expected to include spin-polarization and magnetism for larger members of the acene family. Here, we present a strategy for the on-surface synthesis of three undecacene analogs substituted with four nitrogen atoms on an Au(111) substrate, by employing specifically designed diethano-bridged precursors. A similarly designed precursor is used to synthesize the pristine undecacene molecule. By comparing experimental features of scanning probe microscopy with ab initio simulations, we demonstrate that the ground state of the synthesized tetraazaundecacene has considerable open-shell character on Au(111). Additionally, we demonstrate that the electronegative nitrogen atoms induce a considerable shift in energy level alignment compared to the pristine undecacene, and that the introduction of hydro-aza groups causes local anti-aromaticity in the synthesized compounds. Our work provides access to the precise fabrication of nitrogen-substituted acenes and their analogs, potential building-blocks of organic electronics and spintronics, and a rich playground to explore π-electron correlation., (© 2022. The Author(s).)

Optically active fullerenes, including C 60 and C 70 derivatives carrying organic substituents, are used in a range of applications because of their unique spectroscopic, catalytic, and chiral recognition properties. However, their inherent photoexcited chirality is yet to be elucidated because of their very poor fluorescence quantum yield (Φ f ). We synthesised a new chiral C 70 derivative, X70A, with 20% yield, by reacting bis-borylated xanthene with C 70 in a one-step double addition reaction, followed by a successful optical resolution. The isolation of two separate X70A enantiomers was confirmed by mirror-image circular dichroism spectroscopy in the range of 300-750 nm. In toluene, the enantiomeric pair of X70A clearly revealed mirror-image circularly polarised luminescence (CPL) spectra with a high |g lum | value of 7.0 ×  10 -3 at 690 nm. The first fullerene-based deep-red CPL of X70A should provide a new guideline for the design of chiral nanocarbon materials.

Vinylene-bridged cyclic boron-difluoride complex of dipyrrin (BODIPY) trimers were successfully prepared from expanded dimethyl-vinylene bridged hexaphyrin(2.1.2.1.2.1) Me-Hex that has the structure of alternate dipyrrins and vinylene bridges. The hexaphyrin(2.1.2.1.2.1) Me-Hex can coordinate with boron ions to afford five kinds of cyclic BODIPYs given by step-by-step boron complexations. Crystal structures of all cyclic BODIPYs except for 3BF 2 -Me-Hex(b) formed non-planar structures. The theoretical calculation predicted that mono-/bis-boron cyclic BODIPYs show the intramolecular charge transfer (ICT) characteristics, whereas tri-boron cyclic BODIPYs have no ICT characteristics. Reflecting these electronic properties, tri-boron cyclic BODIPYs exhibit weak fluorescence in the red region, but mono-/bis-boron cyclic BODIPYs exhibit no emission. Vinylene bridged cyclic dipyrrin trimer Me-Hex is the novel porphyrinoid ligand allowed to control the boron coordination under different reaction conditions to form various boron complexes.

Preparation of high-performance organic semiconductor devices requires precise control over the active-layer structure. To this end, we are working on the controlled deposition of small-molecule semiconductors through a photoprecursor approach wherein a soluble precursor compound is processed into a thin-film form and then converted to a target semiconductor by light irradiation. This approach can be applied to layer-by-layer solution deposition, enabling the preparation of p-i-n-type photovoltaic active layers by wet processing. However, molecular design principles are yet to be established toward obtaining desirable thin-film morphology via this unconventional method. Herein, we evaluate a new windmill-shaped molecule with anthryl blades, 1,3,5-tris(5-(anthracen-2-yl)thiophen-2-yl)benzene, which is designed to deposit via the photoprecursor approach for use as the p-sublayer in p-i-n-type organic photovoltaic devices (OPVs). The new compound is superior to the corresponding precedent p-sublayer materials in terms of forming smooth and homogeneous films, thereby leading to improved performance of p-i-n OPVs. Overall, this work demonstrates the effectiveness of the windmill-type architecture in preparing high-quality semiconducting thin films through the photoprecursor approach.

The preparation of large rylenes often needs the use of solubilizing groups along the rylene backbone, and all the substituents of the terrylenes and quaterrylenes were introduced before creating the rylene skeleton. In this work, we successfully synthesized 2,5,10,13-tetrakis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)terrylene ( TB4 ) by using an iridium-catalyzed direct borylation of C-H bonds in terrylene in 56% yield. The product is soluble in common organic solvents and could be purified without column chromatography. Single crystal X-ray diffraction analysis revealed that the terrylene core is not disturbed by the substituents and is perfectly flat. The photophysical properties of TB4 are also unchanged by the substituents because the carbon atoms at 2,5,10,13-positions have less coefficients on its HOMO and LUMO, estimated by theoretical calculations. Finally, the same borylation reaction was applied for quaterrylene, resulting in the formation of soluble tetra-borylated quaterrylene despite a low yield. The post modification of rylenes enables us to prepare their borylated products as versatile units after creating the rylene skeletons., (Copyright © 2020, Kano et al.; licensee Beilstein-Institut.)

The recent surge in the efficiency of organic photovoltaic devices (OPVs) largely hinges on the reduction of energy loss ( E loss ) that leads to improvements in open-circuit voltage ( V OC ). However, there are still many unclarified factors regarding the relationship between the molecular structure and V OC , hampering the establishment of widely applicable, effective principles for the design of active-layer materials. In this contribution, we examine the origin of the large V OC shifts induced by minor structural differences in end-alkyl substituents on a series of anthracene-based p-type compounds. The examined p-type compounds are all highly crystalline, thereby enabling detailed investigation of the molecular packing with X-ray diffraction analysis. At the same time, they are strongly aggregating and hardly soluble; therefore, they are deposited with the aid of a photoprecursor approach which we have been employing for controlled deposition of insoluble acene-based organic semiconductors. The resultant OPVs afford the highest V OC of 0.966 V when the end-alkyl groups are 2-ethylbutyl, and the lowest of 0.419 V when n -butyl is used in replacement of 2-ethylbutyl. X-ray diffraction analyses and density-functional-theory calculations indicate a critical impact of the non-slipped herringbone arrangement on the observed large loss in V OC . This type of molecular arrangement is prohibited when branched alkyl chains are introduced at the ends of linear π-systems, which we consider an important factor contributing to the relatively high V OC obtained with the 2-ethylbutyl derivative. These results may serve as a basis of useful molecular-design rules to avoid unnecessary losses in V OC ., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)

Tetrabenzoporphyrin (BP) holds attractive characteristics for optoelectronic applications, such as the large π-conjugated framework and high photoabsorption capability. However, its use in organic solar cells (OSCs) has been limited because of the extremely low solubility that hampers direct solution processing and also the high frontier-orbital energies that lead to low open-circuit voltage ( V OC ). Herein, we examine BP derivatives equipped with multiple strongly electron-withdrawing groups for photovoltaic applications. The derivatives are generated in thin films through a thermal precursor approach, wherein the corresponding bicyclo[2.2.2]octadiene-fused porphyrin derivatives are solution-cast, and then annealed to carry out the in situ retro-Diels-Alder reaction. The frontier-orbital energies of the resulting derivatives are effectively stabilized as compared to pristine BP to such a degree that they afford high V OC of up to 0.94 V when used as a donor or can even work as a new class of nonfullerene acceptor in OSCs. Single-crystal X-ray diffraction analyses demonstrate that the conformation of the BP framework largely varies from being near planar to highly curved depending on its substituents. The morphology of polymer:BP-derivative bulk-heterojunction films prepared by the thermal precursor approach also varies between the BP derivatives. These results can greatly extend the scope of both molecular design and morphology control for utilization of the BP chromophore toward achieving viable organic optoelectronic devices.

The study assessed care for high-risk pregnancy, including access, functioning, and use of health services, ranging from primary healthcare (PHC) to specialized care. This was an evaluative study anchored in an analytical model that used triangulation of different information sources and comparative analysis of care for high-risk pregnancy in four Brazilian metropolises. The selected analytical categories were: access to consultations and tests, linkage between components of the healthcare networks and between users and healthcare professionals, and the care provided. Contexts of care for high-risk pregnancy were characterized by health system information indicators and a qualitative descriptive study produced by key informants, in addition to a questionnaire applied to 1,886 pregnant women seen at specialized services, having been referred by PHC. Triangulation of data obtained from secondary sources, contextualization of healthcare networks, and a survey with high-risk pregnant women allowed encompassing the complexity of the organizational arrangements in the healthcare network among the municipalities. Significant differences were identified in access, linkage, and care provided for high-risk pregnancy in the four metropolises. Campinas (São Paulo State, Brazil) was the municipality with the best performance in the indicators related to linkage and access by pregnant women to PHC. São Paulo and Porto Alegre (Rio Grande do Sul State) showed fair performance on the set of indicators as a whole, while Fortaleza (Ceará State) revealed the need for improvement in the three categories analyzed.

This study assessed prenatal care in high-risk pregnancies and associated factors in the city of São Paulo, Brazil. This was an evaluative study of 689 pregnant women referred from primary care (PHC) to specialized care in 2016. The type of PHC model was used as the main independent variable: family health units (USF), basic health units (UBS), and UBS/mixed. Multiple logistic regression models with forward selection were used for the outcomes "early initiation of prenatal care" and "shared follow-up between specialized care and PHC". Referral by a USF was associated with higher odds of "early initiation of prenatal care" and "shared follow-up between specialized care and PHC". Independently of the type of PHC, there were associations between "early initiation of prenatal care" and white skin color, and between "shared follow-up between specialized care and PHC" and having received a home visit. The results demonstrate the importance of services organized according to the USF model, which conducts home visits and is attuned to the implicit vulnerabilities in pregnant women's individual characteristics.

A remarkably strained cyclopyrenylene trimer CP3 was synthesized and it underwent the first biaryl C-C σ-bond cleavage by direct oxygen insertion without the aid of any metal agents. A priori highly strained CP3 exhibits the longest wavelength emission among all pyrene-based fluorophores due to the intensive electronic interactions between pyrenes. The color of the emission drastically changes from orange to light blue upon oxidation. Theoretical studies revealed that the release of ring strain reasonably drives the reaction between two CP3 molecules and O 2 . This strain-induced transformation could be also applied for sulfur atom insertion into a biaryl σ-bond.

The additive effect on small-molecule-based p-i-n-type devices has been little investigated so far. We focus on the improvement of the miscibility of tetrabenzoporphyrin (BP) and [6,6]-phenyl-C 61 -butyric acid methyl ester (PC 61 BM) blend film by addition of fullerene-linked tetrabenzoporphyrin (BP-C 60 ) as an additive to the interlayer (i-layer). BP is one of the most promising p-type organic semiconductors, and BP films can be prepared readily by heating as-cast films of the precursor (a bicyclo[2.2.2]octadiene-fused porphyrin; CP), that results in changes from amorphous CP films to polycrystalline BP films. Because of the high crystallinity of BP, large BP grains on the scale of tens to hundreds of nanometers are generated in blend films of BP and PC 61 BM during film fabrication. We found that the addition of BP-C 60 as an additive (3, 5, 7, and 10 wt%) to the i-layer composed of BP and PC 61 BM improves the miscibility of BP and PC 61 BM. The power conversion efficiency of p-i-n-type organic solar cells consisting of a blend film of BP and PC 61 BM (i-layer) sandwiched by BP (p-layer) and PC 61 BM (n-layer) improved by up to 50% as compared to that of a control device after the addition of BP-C 60 to the i-layer. The film morphology was investigated using atomic force microscopy, fluorescence microspectroscopy, two-dimensional grazing-incident wide-angle X-ray diffraction measurements, and scanning electron microscopy. Interacting with both BP and PC 61 BM, the addition of BP-C 60 led to changes in the grain size as well as an increase in the size of the BP/PC 61 BM interface and hence effective charge separation in the p-i-n device. This morphological improvement is attributable to the ability of BP-C 60 , which exhibits the characteristics of both BP and C 60 , to promote the compatibility of BP and PC 61 BM. This study is a significant step towards the development of high-performance p-i-n-type solar cells and should pave the way for the fabrication of high-performance bulk-heterojunction layers in solution-processed organic photovoltaic devices., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)

A large 1,3-phenylene-bridged hexameric naphthalene wheel N6 and a heptameric wheel N7 were synthesized simply by Suzuki-Miyaura coupling via one-pot reaction from monomers. We could control the distribution of N6 and N7 via the reaction conditions. The hexameric wheel structure was revealed by X-ray diffraction analysis. The wheel N6 exhibited C 60 encapsulation ability in the solid state, which was also confirmed by single crystal X-ray analysis., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)

We have investigated conformational structures of meso- meso linked porphyrin arrays (Z n) by single molecule fluorescence spectroscopy. Modulation depths ( M values) were measured by excitation polarization fluorescence spectroscopy. The M value decreases from 0.85 to 0.46 as the number of porphyrin units increases from 3 to 128, indicating that longer arrays exhibit coiled structures. Such conformational changes depending on the length have been confirmed by coarse-grained simulation. The histograms of M values and traces of centroid position of emitting sites by localization microscopy showed that the structures of longer arrays changed to more stretched after solvent vapor annealing with tetrahydrofuran.

9,10-Quinoxaline-fused porphycenes 1a-H₂ and 1b-H₂ were synthesized by intramolecular McMurry coupling. As a result of the annulation of the quinoxaline moiety on the porphycene skeleton, 1a-H₂ and 1b-H₂ display absorption and fluorescence in the near infra-red (NIR) region. Additionally, the quinoxaline moieties of 1a-H₂ and 1b-H₂ act as electron-withdrawing groups, introducing lower reduction potentials than for pristine porphycene. The protonation occurred at the nitrogen atoms in the cavity of freebase porphycenes and at the quinoxaline moieties for their nickel complexes to give diprotonic species., Competing Interests: The authors declare no conflicts interest.

Benzo[4,5]cyclohepta[1,2- b ]fluorene ( 5a ), a new π-conjugated polycyclic hydrocarbon containing linearly fused six-, five-, six-, seven- and six-membered rings (C 6 -C 5 -C 6 -C 7 -C 6 ), was designed and its stable derivatives 5b and 5c were synthesized. With 22 π electrons, 5a is an isomer of pentacene with quinoidal, dipolar ionic and diradical resonance forms. Molecules 5b and 5c were experimentally investigated with cyclic voltammetry, electronic absorption spectroscopy and X-ray crystallographic analysis, and theoretically studied by calculating the NICS value, diradical character and dipole moment. A comparison of 5a-c with pentacene and other pentacene analogues containing linearly fused five- or seven-membered rings was also conducted and discussed. It was found that 5b behaved as a p-type organic semiconductor in solution-processed thin film transistors with a field effect mobility of up to 0.025 cm 2 V -1 s -1 .

Borrowing an idea from the silicon industry, where the charge-carrier's characteristics can be changed through heteroatom implantation, we believe that the charge transport nature of organic semiconductors can be switched through molecular "doping" (co-crystallization). Here, we report a novel molecule 2,7-di- tert -butyl-10,14-di(thiophen-2-yl)phenanthro[4,5- abc ][1,2,5]thiadiazolo[3,4- i ]phenazine (DTPTP), which originally is a p-type (0.3 cm 2 V -1 s -1 ) compound, and can be switched to an n-type semiconductor (DTPTP 2 -TCNQ, 3 × 10 -3 cm 2 V -1 s -1 under air conditions) through tetracyanoquinodimethane (TCNQ) doping (co-crystallization). Single crystal X-ray studies revealed that TCNQ-doped DTPTP complexes (DTPTP 2 -TCNQ) adopt a dense one-dimensional (1D) mixed π-π stacking mode with a ratio of DTPTP and TCNQ of 2 : 1, while pure DTPTP molecules utilize a herringbone-packing pattern. Interestingly, theoretical analysis suggested that there is a quasi-2D electron transport network in this host-guest system. Our research results might provide a new strategy, to switch the charge transport characteristics of an original system by appropriate molecular "doping" (co-crystal engineering).

An unexpected "kinked" N-heteroacene with a slipped two-dimensional ladder-like packing feature is produced from the conventional condensation reaction. The as-obtained compound [2,2']bi(5,12-bis(TIPS)piperazin-3-one[2,3- b ]phenazine) ( 2BPP ) consists of two identical backbones (5,12-bis(TIPS)piperazin-3-one[2,3- b ]phenazine), which are fused together through a C[double bond, length as m-dash]C double bond and two intramolecular H-bonds. A study on the charge carrier transport indicates that a 2BPP single crystal has a hole mobility up to 0.3 cm 2 V -1 s -1 , while theoretical calculations suggest that this compound might possess potential well-balanced ambipolar charge-transport characteristics., (This journal is © The Royal Society of Chemistry 2016.)

meso-β doubly linked Zn(II) porphyrin arrays and their indene-fused analogues were prepared by Suzuki-Miyaura coupling and subsequent oxidation with DDQ-Sc(OTf)(3). The excited state properties of these conjugated porphyrin arrays have been investigated by steady-state and time-resolved spectroscopy along with quantum mechanical calculations to reveal their molecular shape dependencies with respect to syn- versus anti-connectivity as well as nonfused versus fused structures. Nonfused syn-trimer exhibits nearly the same S(1)-state lifetime and two-photon absorption (TPA) value as the nonfused dimer, while the nonfused anti-trimer exhibits different values. The TD-DFT calculations also demonstrate that the nonfused anti-trimer has a different electronic structure from the corresponding syn-trimer. Indene-fused dimers and trimers exhibit the enhanced TPA values and shortened S(1)-state lifetime because of the elongated π-conjugation and the perturbation of electronic structures. Our research provides further insight into the molecular-shape-dependent electronic properties of meso-β doubly linked Zn(II) porphyrin arrays.

Crystalline solids with extended non-interpenetrating three-dimensional crystal structures were synthesized that support well-defined pores with internal diameters of up to 48 angstroms. The Zn4O(CO2)6 unit was joined with either one or two kinds of organic link, 4,4',4''-[benzene-1,3,5-triyl-tris(ethyne-2,1-diyl)]tribenzoate (BTE), 4,4',44''-[benzene-1,3,5-triyl-tris(benzene-4,1-diyl)]tribenzoate (BBC), 4,4',44''-benzene-1,3,5-triyl-tribenzoate (BTB)/2,6-naphthalenedicarboxylate (NDC), and BTE/biphenyl-4,4'-dicarboxylate (BPDC), to give four metal-organic frameworks (MOFs), MOF-180, -200, -205, and -210, respectively. Members of this series of MOFs show exceptional porosities and gas (hydrogen, methane, and carbon dioxide) uptake capacities. For example, MOF-210 has Brunauer-Emmett-Teller and Langmuir surface areas of 6240 and 10,400 square meters per gram, respectively, and a total carbon dioxide storage capacity of 2870 milligrams per gram. The volume-specific internal surface area of MOF-210 (2060 square meters per cubic centimeter) is equivalent to the outer surface of nanoparticles (3-nanometer cubes) and near the ultimate adsorption limit for solid materials.

A series of covalently linked cyclic porphyrin arrays CNZ that consist of N/2 of meso-meso directly linked zinc(II) porphyrin dimer subunits Z2 bridged by 1,3-phenylene spacers have been prepared by Ag(I)-promoted oxidative coupling reaction. We have investigated the excitation energy migration processes of CNZ in toluene by using femtosecond transient absorption anisotropy decay measurements by taking 2Z2 composed of two Z2 units linked by 1,3-phenylene as a reference molecule. On the basis of the excitation energy transfer rate determined for 2Z2, we have revealed the excitation energy hopping rates in the cyclic arrays CNZ by using a regular polygon model. The number of excitation energy hopping sites N(flat) calculated by using a regular polygon model is close to the observed N(expt) value obtained from the transient absorption anisotropy decays for C12Z-C18Z with circular and well-ordered structures. On the other hand, a large discrepancy between N(flat) and N(expt) was found for smaller or larger arrays (C10Z, C24Z, and C32Z). In the case of C10Z, m-phenylene linked 2Z2 motif with the interchromophoric angle of 120 degrees is not well suited to make a cyclic pentagonal array C10Z based on planar pentagonal structure. This geometrical factor inevitably causes a structural distortion in C10Z, leading to a discrepancy between N(expt) and N(flat) values. On the contrary, the presence of highly distorted conformers such as figure-eight structures reduces the number of effective hopping sites N(expt) in large cyclic arrays C24Z and C32Z. Thus, our study demonstrates that not only the large number of porphyrin chromophores in the cyclic arrays CNZ but the overall rigidity and three-dimensional orientation in molecular architectures is a key factor to be considered in the preparation of artificial light harvesting porphyrin arrays.

We have investigated the relationship between the photophysical properties and structures of a series of directly linked zinc(II) porphyrin dimers (mmZ2, mbZ2, and bbZ2) using time-resolved spectroscopic measurements and theoretical calculations. Their unique characters such as CT nature and torsional motion are caused by interporphyrin interactions and steric effects, respectively, which can be fully understood in terms of three structural factors: linking position, dihedral angle, and linkage length. The orthogonal geometry and heterolinking of mmZ2 and mbZ2 induce the localized MOs and electron unbalance in the constituent porphyrin units, respectively, and consequently lead to distinct CT characters in spite of their different origin. On the other hand, a small interporphyrin steric hindrance in bbZ2 makes a torsional motion possible around the direct beta-beta' linkage in the excited state, which is correlated with the solvent dependence of the fast S(1) fluorescence decay component. On the basis of this work, we can gain further insight into the effect of individual structural factors on the photophysical properties, which provides a firm basis for further understanding of the photophysical properties mainly determined by the structural factors in multiporphyrin systems.

A belt-shaped hexagonal cyclic porphyrin array 2 that comprises of six meso-meso, beta-beta, beta-beta triply linked diporphyrins 3 bridged by 1,3-phenylene spacers is prepared by oxidation from cyclic dodecameric array 1 consisting of six meso-meso directly linked diporphyrins 4 with DDQ and Sc(OTf)3. The absorption spectrum of 2 is similar to that of the constituent subunit 3 but shows a slight red-shift for the Q-bands in near-infrared (NIR) region, indicating the exciton coupling between the neighboring diporphyrin chromophores. Observed total exciton coupling energies in the absorption spectra were largely matched with the calculated values based on point-dipole exciton coupling approximation. It was found that the experimental exciton coupling strength (292 cm(-1)) of the Q-band in 2 is slightly larger than the calculated one (99 cm(-1)), indicating that the electronic communications are enhanced through 1,3-phenylene linkers in hexameric macromolecule. A rate of the excitation energy hopping (EEH) that occurs in 2 at the lowest excited singlet state in the near-infrared region has been determined to be (1.8 ps)(-1) on the basis of the pump-power dependent femtosecond transient absorption (TA) and the transient absorption anisotropy (TAA) decay measurements. The 2 times faster EEH rate of 2 than that of 1 (4.0 ps)(-1) mainly comes from involving through-bond energy transfer among diporphyrin subunits via 1,3-phenylene bridges as well as Förster-type through-space EEH processes. STM measurement of 2 in the Cu(100) surface revealed that it takes several discrete conformations with respect to the relative orientation of neighboring diporphyrins. Collectively, an effective EEH in the NIR region is realized in 2 due largely to the intensified oscillator strength in the S(1) state (Q-band) and the close proximity held by 1,3-phenylene spacers.

We have investigated the overall excitation energy relaxation dynamics in linear porphyrin arrays as well as the energy transport phenomena by attaching an energy acceptor to one end of a linear porphyrin array by using steady state and time-resolved spectroscopic measurements. We have revealed that the solvation dynamics as well as the conformational dynamics contributes significantly to the energy relaxation processes of linear porphyrin arrays. Consequently, long porphyrin arrays no longer serve as good energy transmission elements in donor-acceptor linked systems due to conformational heterogeneities which provide the non-radiative deactivation channels as energy quenchers.

Intramolecular excitation energy hopping (EEH) time within a dodecameric porphyrin wheel C6ZA, in which six meso-meso linked zinc(II) diporphyrin (Z2) subunits are bridged by 1,3-phenylene spacers, is deduced by a Förster energy hopping model based on S(1)-S(1) exciton-exciton annihilation and anisotropy depolarization. Under the assumption that the energy hopping sites are six Z2 subunits, two different observables (e.g., exciton-exciton annihilation and anisotropy depolarization times) consistently give the EEH time of 4.0 +/- 0.4 ps via 1,3-phenylene spacer of C6ZA, which is faster than 9.4 ps of linear 2Z2 (1,3-phenylene-linked zinc(II) tetraporphyrin). As a consequence, C6ZA serves as a well-defined two-dimensional model for a light-harvesting complex.

The excited state dynamics and two-photon absorption properties of four novel triply linked porphyrin dimers in toluene have been investigated. The fused porphyrin dimers exhibit extremely increased two-photon absorption cross-section values (12,000-15,000 GM) compared with porphyrin monomers owing to much enhanced pi-electron delocalization throughout the porphyrin dimer ring.