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Anion bound and palladium complexes of 2,6-di-m-phenylpyridine embedded isosmaragdyrin [1.1.1.0.0] with N3C2 core are successfully synthesized. The anion bound complex is achieved by the addition of trifluoroacetic acid and the core is also utilized to stabilize the Pd(II) ion in the (N^C)N bonding mode. Single-crystal X-ray analysis of both complexes ratifies the structures unequivocally. Structural and spectral studies reveal the nonaromatic character of the complexes and the results were substantiated by theoretical studies. In addition to trifluoroacetate anion and palladium cation stabilization, the potential triplet excited state features of both complexes are comprehensively investigated, which indicates that there is an emergence of the triplet excited state only for the Pd(II) complex due to the heavy atom effect-enticed intersystem crossing process. [ABSTRACT FROM AUTHOR]

We synthesised thionium-ion embedded aromatic porphyrinoids: a free-base 5-thiaporphyrinium cation and its zinc complex. The sulphur atom effectively participates in the macrocyclic π-conjugation. Fluorescence quantum yields of thiaporphyrinium cations were lower than 1% unlike oxaporphyrinium cations. Detailed photophysical analysis and DFT calculations clarified the vibrational mode regarding the out-of-plane motion of the sulphur atom induced ultrafast quenching of the excited state in comparison to the corresponding oxaporphyrinium cations. [ABSTRACT FROM AUTHOR]

This communication describes the photophysical behavior of three analogs of cyclophane bearing the dipyrrolonaphthyridinedione (DPND) core. In these molecules, intersystem crossing (ISC) can be successfully induced by distinct changes in the deviation from planarity within the DPND core, allowing at the same time the emission maximum to shift from the green to red region of the visible spectrum without any synthetic modifications of the chromophore structure. This finding may build the foundation for a new paradigm for inducing ISC-type transitions within other centrosymmetric and planar cross-conjugated chromophores. [ABSTRACT FROM AUTHOR]

Recently, Baird (anti)aromaticity has been referred to as a description of excited-state (anti)aromaticity. With the term of Baird's rule, recent studies have intensively verified that the Hückel aromatic [4n + 2]π (or antiaromatic [4n]π) molecules in the ground state are reversed to give Baird aromatic [4n]π (or Baird antiaromatic [4n + 2]π) molecules in the excited states. Since the Hückel (anti)aromaticity has great influence on the molecular properties and reaction mechanisms, the Baird (anti)aromaticity has been expected to act as a dominant factor in governing excited-state properties and processes, which has attracted intensive scientific investigations for the verification of the concept of reversed aromaticity in the excited states. In this scientific endeavor, porphyrinoids have recently played leading roles in the demonstration of the aromaticity reversal in the excited states and its conceptual development. The distinct structural and electronic nature of porphyhrinoids depending on their (anti)aromaticity allow the direct observation of excited-state aromaticity reversal, Baird's rule. The explicit experimental demonstration with porphyrinoids has contributed greatly to its conceptual development and application in novel functional organic materials. Based on the significant role of porphyrinoids in the field of excited-state aromaticity, this review provides an overview of the experimental verification of the reversal concept of excited-state aromaticity by porphyrinoids and the recent progress on its conceptual application in novel functional molecules. [ABSTRACT FROM AUTHOR]

A novel carbazole-containing porphyrinoid 4H and its oligomers 6H and 7H were synthesized for the first time via the Suzuki–Miyaura cross-coupling reaction. The structures of 4H, 4Pd, 6H, and 6Zn were finally confirmed by X-ray analysis. The exciton coupling strength (628 cm−1) in 4Zn, 6Zn, and 7Zn was found to be larger than that of the directly-linked porphyrin arrays (570 cm−1), extending their absorption spectra to the NIR region as well as enhancing the extinction coefficients. [ABSTRACT FROM AUTHOR]

As the first example of expanded heteroporphyrins with heteroatoms at meso-positions, 5,20-dithiahexaphyrins and 5,20-diazahexaphyrins were synthesized via nucleophilic substitution reactions of α,α′-dibromotripyrrin as the key step. While 5,20-dithiahexaphyrins are practically nonaromatic, 5,20-[28]diazahexaphyrins show weakly antiaromatic characters and can be cleanly converted into aromatic 5,20-[26]diazahexaphyrins upon oxidation with PbO2, demonstrating distinct aromaticity switching with maintenance of dumbbell-like conformations. [ABSTRACT FROM AUTHOR]

Significant changes of macrocyclic aromaticity in expanded porphyrins through C60 complexation were studied by 1H NMR spectroscopy and nucleus-independent chemical shift calculations. This work is a detailed research study of how the formation of a complex of dual aromatic expanded porphyrin with fullerene affects the electron densities in the main conjugation pathways and meso-substituents. Furthermore, we found that the formation of the photoinduced charge-separated state and the triplet excited-state populations of the bowl-shaped and rigid expanded porphyrin can be controlled by a simple complexation with C60. [ABSTRACT FROM AUTHOR]

Diazachlorin and diazabacteriochlorin have been prepared through reduction of diazaporphyrin and their in vitro and in vivo activity in photodynamic therapy has been investigated. [ABSTRACT FROM AUTHOR]

π-Conjugated oligothiophene is considered a chain segment of its polymeric counterpart, whose size and shape can be precisely controlled. Because of its simplified structure, it is possible to understand complex excited-state dynamics of the π-conjugated polymers by employing a bottom-up approach. We review theoretical and experimental aspects of π-conjugated oligothiophenes by summarizing recent works employing time-resolved spectroscopy. The extent of exciton delocalization, which is a prerequisite to efficient charge generation at organic heterojunctions, is described sequentially in model linear and cyclic oligothiophenes, and their analogues. The heterogeneous nature of these systems is highlighted by illustrating the results at both ensemble and single-molecule levels. Exciton dynamics that arise in the polymers are also covered and the signifcance of exciton and charge delocalization in photovoltaic materials is highlighted. [ABSTRACT FROM AUTHOR]

Excitation transfer from 1BODIPY* to bisstyrylBODIPY followed by electron transfer to C60 leading to a charge separated state of appreciable lifetime in a supramolecularly assembled triad is demonstrated, as a mimic of the photosynthetic ‘antenna-reaction centre’. [ABSTRACT FROM AUTHOR]

We have investigated the fundamental photophysical properties of surface-bound perylene bisimide (PBI) molecules in a solution-phase at the single-molecule level. By efficient immobilization of single PBIs on glass, we were able to simultaneously monitor fluorescence intensity trajectories, fluorescence lifetimes, and emission spectra of individual PBIs in organic and aqueous media using confocal microscopy. We showed that the fluorescence dynamics of single PBIs in the solution phase is highly dependent on their local and chemical environments. Furthermore, we visualized different spatial-fluctuations of surface-bound PBIs using defocused wide-field imaging. While PBIs show more steric flexibility in organic media, the flexible motion of PBI molecules in aqueous solution is relatively prohibited due to a cage effect by a hydrogen bonding network, which is previously unobserved. Our method opens up a new possibility to investigate the photophysical properties of multi-chromophoric systems in various solvents at the single-molecule level for developing optimal molecular devices such as water-proof devices. [ABSTRACT FROM AUTHOR]

meso-to-meso 2,5-Pyrrolylene bridged zig-zag porphyrin arrays have been synthesized via a stepwise Suzuki–Miyaura cross coupling reaction. Both the dimeric and trimeric structures of ZnII porphyrin were confirmed by X-ray diffraction analysis. DFT calculations indicate that the porphyrin oligomers are all in zig-zag conformations. The UV/Vis absorption and emission spectra of these porphyrin oligomers were red-shifted as the number of their porphyrin units increased. [ABSTRACT FROM AUTHOR]

Herein, we revealed a symmetry-breaking charge transfer (SBCT) process in the excited state of a directly linked push–pull porphyrin dyad (AD) and triad (ADA) via spectroscopic measurements including steady-state absorption and fluorescence, time-resolved fluorescence (TRF), femtosecond transient absorption (fs-TA), and time-resolved infrared (TRIR) measurements. Unprecedented broad fluorescence spectra were observed for porphyrin arrays in polar solvents; these were attributed to the existence of a charge transfer state as evidenced by the TRF measurements. TA measurements also revealed emerging features of a CT state for AD and ADA in polar solvents. These dynamics were also confirmed via TRIR measurements, which provided further information on the solvation and structural relaxation processes of the SBCT process. This is the first observation of an SBCT process in porphyrin arrays, providing fundamental understanding of the strongly coupled porphyrin arrays. Thus, the results of this study reveal the potential of the porphyrin arrays in relevant applications requiring SBCT. [ABSTRACT FROM AUTHOR]

The photophysical properties of molecular arrays are strongly dependent on a variety of structural factors: the constituent chromophores, dihedral angle, linkage length, linkage position, the center-to-center distance between chromophores, and the linker itself. Here, we investigated the exciton coupling dynamics of syn- and anti-type β–β directly linked Zn(ii) porphyrin linear arrays. Focusing on the relationship between the origin of the lowest excited electronic state and the linkage position, we evaluated the exciton coupling strength and carried out time-dependent density functional theory (TDDFT) calculations on model compounds as well as femtosecond transient absorption anisotropy (fs-TAA) measurements. Based on our experiments and calculations, we propose that a different origin of the lowest excited state leads to linkage-position-dependent exciton coupling. In short, compared with syn-type porphyrin arrays, anti-type arrays induce distinct and stronger exciton coupling in the lowest excited state. [ABSTRACT FROM AUTHOR]

Herein, we present a combined spectroscopic and computational study on 5,5′-linked and 10,10′-linked corrole dimers, 5CD and 10CD, respectively, to reveal their strikingly different excited-state dynamics. The excited-state dynamics of 10CD indicate relatively small electronic interactions between the two corrole units, which is similar to the case of the meso–meso directly linked Zn(ii) porphyrin dimer. On the other hand, 5CD exhibits characteristic excitation-wavelength-dependent dual fluorescence. Transient absorption spectra of 10CD on the picosecond timescale showed torsional relaxation with a time constant of 25 ps, whereas the torsional relaxation of 5CD was faster, exhibiting a time constant of 10 ps. Quantum calculations have indicated that the eccentric dual fluorescence and the faster torsional relaxation process of 5CD are consequences of a large π-orbital coefficient at the connecting 5-position, which enhances the conjugative stabilization in the excited-state. In contrast, a small π-orbital coefficient at the 10-position and a larger torsional barrier of 10CD cause monomer-like characters in the excited-state. [ABSTRACT FROM AUTHOR]

Porphyrin and subphthalocyaninatoboron(iii) chromophores have been fused through a quinoxaline moiety, resulting in the first porphyrin–subphthalocyaninatoboron(iii)-fused hybrid with intramolecular charge transfer from tetrapyrrole/tripyrrole chromophores to the quinoxaline moiety. The unique plane-bowl molecular structure of this hybrid was revealed based on single crystal X-ray diffraction analysis for the first time. [ABSTRACT FROM AUTHOR]

The photophysical properties of a series of highly π-conjugated benzoporphyrin molecules (BPNs) with different shapes were investigated in the condensed phase using single-molecule fluorescence spectroscopy. The fluorescence properties of single BPNs were found to be affected by the number of porphyrin units and their molecular shapes. Notably, the single-molecule fluorescence dynamics of the BPNs revealed an increase in the fluorescence lifetimes and blue shifts of the fluorescence spectra indicative of decreasing π-conjugation pathways in the molecules. The distributions of the spectroscopic parameters and the photostability for the molecules also suggest conformational complexities and heterogeneities. Specifically, as the number of constituent porphyrin units increased, the one-step photobleaching behavior ratio and photostability decreased, and the spectroscopic parameter distributions broadened. The structural properties of the BPNs were also directly determined using defocused wide-field imaging and linear dichroism analyses. In particular, molecules with the same number of constituent porphyrins but different molecular shapes exhibited distinct photophysical properties. In summary, these observations provide guidance for the design of molecular systems that can enhance the performance of molecular electronic devices. [ABSTRACT FROM AUTHOR]

By utilizing single-molecule defocused wide-field fluorescence microscopy, we have investigated the molecular structural properties such as transition dipole moment orientations and the angular relationship among chromophores, as well as structural distortions and flexibilities depending on the ring size, in a series of cyclic porphyrin arrays bearing close likeness in overall architectures to the LH2 complexes in purple bacterial photosynthetic systems. Furthermore, comparing the experimental results with molecular dynamics simulations, we ascertained site selection for fluorescent trapping sites. Collectively, these experimental and computational results provide the basis for structure–property relationships and energy hopping/emitting processes in an important class of artificial light-harvesting molecular systems widely used in molecular electronics technology. [ABSTRACT FROM AUTHOR]

The excited-state energy relaxation processes of a Zn(ii)porphyrin–[26]hexaphyrin–Zn(ii)porphyrin triply linked hybrid tape, FZn, have been investigated by femtosecond transient absorption spectroscopy (TA), using a directly meso–meso linked hybrid trimer, HZn, as a reference compound. FZn has a very small S1–S0 energy gap through the expansion of π-conjugation and the absorption band at 1897 nm corresponds to its lowest singlet excited-state as a consequence of enhanced transition dipole moment that lies parallel to the long molecular axis. In TA measurements, we observe an energy transfer process (0.4 ps) from the Zn(ii)porphyrin moiety to the [26]hexaphyrin core in HZn. In contrast to HZn, a biexponential decay with the time constants of 0.25 and 6.5 ps was observed in the decay profile of FZn. The detailed analysis of excitation wavelength, temperature and solvent dependent TA in FZn revealed that the electronic relaxation process (0.25 ps) from S1 to S0 is faster than the vibrational relaxation processes (5.9 ps) in the excited and ground states due to a very small S1–S0 energy gap through the expansion of π-conjugation. Accordingly, we demonstrate that electronic deactivation overtakes vibrational relaxation processes in a highly conjugated FZn. [ABSTRACT FROM AUTHOR]

A2B-type meso-(4-nitrophenyl)-substituted subporphyrins have been synthesized and shown to undergo very fast photoinduced intramolecular charge separation (CS) and charge recombination (CR) between the subporphyrin core and the meso-4-nitrophenyl group in CH2Cl2 as probed by femtosecond time-resolved transient absorption spectroscopy. Red-shifted emissions were detected from charge-separated states as a rare case for porphyrinoids. [ABSTRACT FROM AUTHOR]

Foldamer systems comprised of two perylene bisimide (PBI) dyes attached to the conjugated backbones of 1,2-bis(phenylethynyl)benzene and phenylethynyl-bis(phenylene)indane, respectively, were synthesized and investigated with regard to their solvent-dependent properties. UV/Vis absorption and steady-state fluorescence spectra show that both foldamers exist predominantly in a folded H-aggregated state consisting of π–π-stacked PBIs in THF and in more random conformations with weaker excitonic coupling between the PBIs in chloroform. Time-resolved fluorescence spectroscopy and transient absorption spectroscopy reveal entirely different relaxation pathways for the photoexcited molecules in the given solvents, i.e. photoinduced electron transfer leading to charge separated states for the open conformations (in chloroform) and relaxation into excimer states with red-shifted emission for the stacked conformations (in THF). Supported by redox data from cyclic voltammetry and Rehm–Weller analysis we could relate the processes occurring in these solution-phase model systems to the elementary processes in organic solar cells. Accordingly, only if relaxation pathways such as excimer formation are strictly avoided in molecular semiconductor materials, excitons may diffuse over larger distances to the heterojunction interface and produce photocurrent via the formation of electron/hole pairs by photoinduced electron transfer. [ABSTRACT FROM AUTHOR]

A novel electron deficient β-octakis(methylthio)porphycene, along with its Zn(ii) and Ni(ii) derivatives, was synthesized for the first time. The macrocyclic structure exhibits core ruffling with a largely red shifted absorption band (∼750 nm) and also a large enhancement in the third order nonlinear optical response. [ABSTRACT FROM AUTHOR]

Electronic couplings in a 1,3-butadiyne-bridged Zn(ii) porphyrin dimer D and trimer T have been probed by measuring their excited-state properties at ensemble and single molecular levels. While single chromophore-like, strongly interacting behaviors are revealed for D, the coupling in T is indicated to be not so strong. [ABSTRACT FROM AUTHOR]

In this study, we have investigated the shape-dependence of TPA cross-section values of two-dimensionally extended benzoporphyrin arrays to determine the relationship between the directionality of π-conjugation pathways and TPA properties. [ABSTRACT FROM AUTHOR]

A series of meso–meso, β–β, β–β triply linked linear, radial and square-type zinc(ii) porphyrin arrays consist of the constituent porphyrin units and naphthalene junctions. To understand the unique nature of triply linked porphyrin arrays, numerous research activities have been focused on the electronic structures of the constituent porphyrin units. In this study, however, we have paid attention to the role of the naphthalene junction in the electronic delocalization of various triply linked porphyrin arrays. On the basis of our study, we have unveiled that unique π-conjugation behaviors in triply linked porphyrin arrays are induced by their intrinsic molecular orbital interactions and subsequently by antiaromatic junctions. Furthermore, the structural deformation by triple linkages gives rise to a deteriorative effect on the electronic delocalization between inner and outer porphyrin units. Finally, we propose a different type of electron delocalization in linear multichromophoric systems by alternating aromatic and antiaromatic units. [ABSTRACT FROM AUTHOR]

A covalently linked trimer of 4,4-dimethyl-4H-cyclopenta[def]phenanthrene (MCPP), referred to as tri-MCPP, undergoes spontaneous self-assembly to form highly ordered colloidosomes in the absence of a template when subjected to reprecipitation from THF–water. The colloidosomes, and the oligomers in neat tetrahydrofuran have also been analyzed using optical methods. While the monomer, MCPP, was found to display a low fluorescence quantum yield (ΦF∼ 0.03) and low molar absorptivity ( ∼ 900 M−1cm−1) for the S1← S0transition due to symmetry restrictions, short oligomers formed viaconnection at the paraphenylene positions displayed corresponding optical features that were found to increase gradually as a function of oligomer length. In the case of the largest species studied, the pentamer (penta-MCPP), the molar absorptivity was ca.140 times larger than that of MCPP(= 126 500) for the S1← S0transition. The ΦFand radiative rates for the MCPPoligomers were found to be enhanced by a factor of ∼30 and ∼180, respectively, while going from monomer to pentamer, presumably because of the larger radiative size of the oligomers. Further, the colloidosomes derived from tri-MCPPdisplayed a fluorescence quantum yield (ca.0.35 to ∼0.20) that was reduced by approximately 1.8 times relative to tri-MCPP. The fluorescence lifetime was likewise decreased by ∼10 times compared to tri-MCPP; presumably, this is the result of intermolecular interactions between the very closely organized oligomers. The fact that highly ordered structures are obtained in the case of tri-MCPP, but not the other species included in this study (monomer; other oligomers), is thought to reflect the mutually orthogonal orientation between the constituent monomeric subunits in the trimer, as well as an ability to stabilize favorable hydrogen bonding interactions with residual solvent entrapped under the conditions of THF–water induced precipitation. Specifically, it is proposed that, in the case of tri-MCPP, these interactions aid in the formation and subsequent stabilization of precise colloidosome superstructures. An analogous stabilization of a colloidosome superstructure was not found for the corresponding tetramer (tetra-MCPP) or pentamer (penta-MCPP), in spite of the fact that these species contain a greater number of monomeric MCPPsubunits and would be expected to give rise to ostensibly similar structures. This leads us to suggest that specific structural requirements are required for MCPP-derived precursors to form self-assembled colloidosomes. [ABSTRACT FROM AUTHOR]