Nanographene-Fused Expanded Carbaporphyrin Tweezers.

A nanographene-fused expanded carbaporphyrin (5) and its BF2 complex (6) were synthesized. Single-crystal X-ray structures revealed that 5 and 6 are connected by two hexa-peri-hexabenzocoronene (HBC) units and two dipyrromethene or BODIPY units, respectively. As prepared, 5 and 6 both show nonaromatic character with figure-of-eight carbaoctaphyrin (1.1.1.0.1.1.1.0) cores and adopt tweezers-like conformations characterized by a partially confined space between the two constituent HBC units. The distance between the HBC centers is >10 Å, while the dihedral angles between the two HBC planes are 30.5 and 35.2° for 5 and 6, respectively. The interactions between 5 and 6 and fullerene C60 were studied both in organic media and in the solid state. Proton NMR spectral titrations of 5 and 6 with C60 revealed a 1:1 binding mode for both macrocycles. In toluene-d8, the corresponding binding constants were determined to be 1141 ± 17 and 994 ± 10 M-1 for 5 and 6, respectively. Single-crystal X-ray diffraction structural analyses confirmed the formation of 1:1 fullerene inclusion complexes in the solid state. The C60 guests in both complexes are found within triangular pockets composed of two HBC units from the tweezers-like receptor most closely associated with the bound fullerene, as well as an HBC unit from an adjacent host. Femtosecond transient absorption measurements revealed subpicosecond ultrafast charge separation between 5 (and 6) and C60 in the complexes. To the best of our knowledge, the present report provides the first example wherein a nanographene building block is incorporated into the core of a porphyrinic framework.

Cyclic Azobenzene-BODIPY Hybrids.

Cyclic azobenzene-BODIPY hybrids were synthesized via cyclization by 1) acid-catalysed condensation of azobenzene-bridged dipyrroles with 3,5-di-tert-butylbenzaldehyde, 2) oxidation with DDQ, and 3) metalation with BF3 ⋅ Et2 O. The structures of many cyclic hybrids have been confirmed by single crystal X-ray analysis. The absorption spectra of the hybrids reveal the effective cyclic conjugation. The ultrafast measurements reveal that the photoexcited decays of these cyclic hybrids depend upon the ring size and connectivity.

Ultrafast Excited State Aromatization in Dihydroazulene.

Excited-state aromatization dynamics in the photochemical ring opening of dihydroazulene (DHA) is investigated by nonadiabatic molecular dynamics simulations in connection with the mixed-reference spin-flip (MRSF)-TDDFT method. It is found that, in the main reaction channel, the ring opening occurs in the excited state in a sequence of steps with increasing aromaticity. The first stage lasting ca. 200 fs produces an 8π semiaromatic S1 minimum (S1, min) through an ultrafast damped bond length alternation (BLA) movement synchronized with a partial planarization of the cycloheptatriene ring. An additional ca. 200 fs are required to gain the vibrational energy needed to overcome a ring-opening transition state characterized by an enhanced Baird aromaticity. Unlike other BLA motions of ππ* state, it was shown that their damping is a characteristic feature of aromatic bond-equalization process. In addition, some minor channels of the reaction have also been discovered, where noticeably higher barriers of the S1 non/antiaromatic transition structures must be surmounted. These anti-Baird channels led to reformation of DHA or other closed-ring products. The observed competition between the Baird and anti-Baird channels suggests that the quantum yield of photochemical products can be controllable by tipping their balance. Hence, here we suggest including the concept of anti-Baird, which would expand the applicability of Baird rule to much broader situations.

Leveraging Charge-Transfer Interactions in Through-Space-Coupled Pentacene Dendritic Oligomer for Singlet Exciton Fission.

Singlet exciton fission in organic chromophores has received much attention during the past decade. Inspired by numerous spectroscopic studies in the solid state, there have been vigorous efforts to study singlet exciton fission dynamics in covalently bonded oligomers, which aims to investigate underlying mechanisms of this intriguing process in simplified model systems. In terms of through-space orbital interactions, however, most of covalently bonded pentacene oligomers studied so far fall into weakly interacting systems since they manifest chain-like structures based on various (non)conjugated linkers. Therefore, it remains as a compelling question to answer how through-space interactions in the solid state intervene this photophysical process since it is hypersensitive to displacements and orientations between neighboring chromophores. Herein, as one of experimental studies to answer this question, we introduced a tight-packing dendritic structure whose mesityl-pentacene constituents are coupled via moderate through-space orbital interactions. Based on the comparison with a suitably controlled dendritic structure, which is in a weak coupling regime, important mechanistic viewpoints are tackled such as configurational mixings between singlet, charge-transfer, and triplet pair states and the role of chromophore multiplication. We underscore that our through-space-coupled dendritic oligomer in a quasi-intermediate coupling regime provides a hint on the interplay of multiconfigurational excited-states, which might have drawn complexity in singlet exciton fission kinetics throughout numerous solid-state morphologies.

V-Shaped Tröger Oligothiophenes Boost Triplet Formation by CT Mediation and Symmetry Breaking.

A new family of molecules obtained by coupling Tröger's base unit with dicyanovinylene-terminated oligothiophenes of different lengths has been synthesized and characterized by steady-state stationary and transient time-resolved spectroscopies. Quantum chemical calculations allow us to interpret and recognize the properties of the stationary excited states as well as the time-dependent mechanisms of singlet-to-triplet coupling. The presence of the diazocine unit in Tröger's base derivatives is key to efficiently producing singlet-to-triplet intersystem crossing mediated by the role of the nitrogen atoms and of the almost orthogonal disposition of the two thiophene arms. Spin-orbit coupling-mediated interstate intersystem crossing (ISC) is activated by a symmetry-breaking process in the first singlet excited state with partial charge transfer character. This mechanism is a characteristic of these molecular triads since the independent dicyanovinylene-oligothiophene branches do not display appreciable ISC. These results show how Tröger's base coupling of organic chromophores can be used to improve the ISC efficiency and tune their photophysics.

Near-Infrared-Responsive Hydrocarbons Designed by π-Extension of Indeno[1,2,3,4-pgra]perylene at the 1,2,12-Positions.

The relationship between the overall electronic structure of π-conjugated molecules and the arrangement of their constituent elements is of fundamental importance. Establishing rational design guidelines for conjugated hydrocarbons with narrow HOMO-LUMO gaps is useful to develop near-infrared (NIR) responsive dyes and redox-active materials. This study describes the synthesis and properties of three conjugated hydrocarbons, i. e., an indenonaphthoperylene, an indenoterrylene, and a diindenoterrylene. These molecules exhibit NIR absorption despite the absence of significant antiaromaticity and diradical character. Notably, the indenonaphthoperylene exhibits red-to-NIR emission in the 620-850 nm region. The indenoterrylene and the diindenoterrylene exhibit NIR absorption tailing to 870 and 940 nm, respectively. Moreover, the effect of the π-extension of indenoperylene is disclosed in order to propose guidelines for achieving a narrow HOMO-LUMO gap with negligible antiaromaticity and diradical character.

Porphyrinoids, a unique platform for exploring excited-state aromaticity.

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.

Modulations of a Metal-Ligand Interaction and Photophysical Behaviors by Hückel-Möbius Aromatic Switching.

In organometallic complexes containing π-conjugated macrocyclic chelate ligands, conformational change significantly affects metal-ligand electronic interactions, hence tuning properties of the complexes. In this regard, we investigated the metal-ligand interactions in hexaphyrin mono-Pd(II) complexes Pd[28]M and Pd[26]H, which exhibit a redox-induced switching of Hückel-Möbius aromaticity and subsequent molecular conformation, and their effect on the electronic structure and photophysical behaviors. In Möbius aromatic Pd[28]M, the weak metal-ligand interaction leads to the π electronic structure of the hexaphyrin ligand remaining almost intact, which undergoes efficient intersystem crossing (ISC) assisted by the heavy-atom effect of the Pd metal. In Hückel aromatic Pd[26]H, the significant metal-ligand interaction results in ligand-to-metal charge-transfer (LMCT) in the excited-state dynamics. These contrasting metal-ligand electronic interactions have been revealed by time-resolved electronic and vibrational spectroscopies and time-dependent DFT calculations. This work indicates that the conspicuous modulation of metal-ligand interaction by Hückel-Möbius aromaticity switching is an appealing approach to manipulate molecular properties of metal complexes, further enabling the fine-tuning of metal-ligand interactions and the novel design of functional organometallic materials.

Synthesis of Subporphyrin Free Bases.

BIII subporphyrins are the legitimate ring-contracted porphyrins consisting of three pyrroles and three meso-carbons and their chemistry has been extensively developed since the first synthesis in 2006. However, subporphyrin free bases have never been synthesized, despite tremendous attempts to remove the BIII ion. Here we report that Suzuki-Miyaura coupling between α,α'-diborylated tripyrrane 1 and tetrabromide 5 gave subporphyrin free bases 6, 6 A, and free base dimer 7 in 6 %, 4 %, and 2 % yields as the first examples. Subporphyrin free bases exhibit curved bowl-like structures and distinct 14π-aromaticity. Steady-state and time-resolved spectroscopy revealed that the excited-state behaviors of the subporphyrin free bases are comparable with those of the corresponding BIII subporphyrins. Rotational relaxation processes in the excited states have been revealed, which enhance the electronic interactions with the meso-aryl substituents and between the two subporphyrins.

meso-Oxoisocorroles: Tunable Antiaromaticity by Metalation and Coordination of Lewis Acids as Well as Aromaticity Reversal in the Triplet Excited State.

The corrole derivative meso-oxoisocorrole has been theoretically predicted to be antiaromatic, despite its formally cross conjugated electronic system. In this study, this prediction has been experimentally proven by the facile preparation of meso-oxoisocorrole via the oxidation of a meso free corrole with MnO2 and its comprehensive characterization using NMR, UV/vis absorption, FT-IR, and transient-absorption spectroscopy, cyclic voltammetry, and X-ray diffraction analysis. Furthermore, the free base meso-oxoisocorrole was metalated by treatment with Ni(acac)2, PdCl2(PhCN)2, and Zn(OAc)2 to give the corresponding metal complexes. These complexes are more strongly antiaromatic, and their degree of paratropicity depends on their planarity. Thus, fine tuning of their antiaromaticity was achieved with concomitant modulation of their HOMO-LUMO gaps. In the presence of tris(pentafluorophenyl)borane, their antiaromaticity is significantly enhanced due to the elongation of the C═O bond, which promotes the polarized C+-O- resonance state. Furthermore, a distinct frequency shift of the C═O vibrational mode in the triplet state was observed in the time-resolved IR spectra in accordance with the Baird rule, which indicates aromaticity reversal in the excited state.

Magnetic-Field-Induced Modulation of Charge-Recombination Dynamics in a Rosarin-Fullerene Complex.

Charge-recombination processes are critical for photovoltaic applications and should be suppressed for efficient charge transport. Here, we report that an applied magnetic field (0-1 T) can be used control the charge-recombination dynamics in an expanded rosarin-C60 complex. In the low magnetic field regime (<100 mT), the charge-recombination rate slows down due to hyperfine coupling, as inferred from transient absorption spectroscopic analyses. In contrast, in the high field regime, i.e., over 500 mT, the charge-recombination rate recovers and increases because the Δg mechanism facilitates spin conversion to a triplet charge-separated state (S to T0 ) that undergoes rapid charge-recombination to a localized rosarin triplet state. Therefore, we highlight the charge-recombination rate and the localized triplet state population can be modulated by the magnetic field in charge donor/acceptor non-covalent complexes.

Site-Selective N-Methylation of 5,15-Diazaporphyrins: Reactive Cationic Porphyrinoids that Provide Isoporphyrin Analogues.

N-Alkylation significantly changes the electronic and optical properties, as well as the reactivity of nitrogen-containing π-conjugated molecules. In this study, it is found that treating 5,15-diazaporphyrins with methyl triflate selectively affords the corresponding N-methyl-5,15-diazaporphyrinium cations in good yield. N-Methylation substantially alters the electronic properties and reactivity of diazaporphyrins. The electron-accepting properties of the N-methyl-5,15-diazaporphyrinium cations are enhanced due to their lowered LUMO level. Stabilization of the LUMO energy enables regio- and stereoselective Diels-Alder reactions of the cationic diazaporphyrin with cyclopentadiene. N-Methylation also enhances the acidity of the inner NH protons, and thus, allows facile deprotonation to provide nitrogen-substituted isoporphyrin analogues with only one NH group in the central cavity.

Noncovalent Intermolecular Interaction in Cofacially Stacked 24π Antiaromatic Hexaphyrin Dimer.

π-π Stacking is omnipresent not only in nature but in a wide variety of practical fields applied to our lives. Because of its importance in a performance of natural and artificial systems, such as light harvesting system and working layer in device, many researchers have put intensive effort into identifying its underlying nature. However, for the case of π-π stacked systems composed of antiaromatic units, the understanding of the fundamental mechanisms is still unclear. Herein, we synthesized a new type of planar ß,ß'-phenylene-bridged hexaphyrin (1.0.1.0.1.0), referred as naphthorosarin which possesses the 24π-electron conjugated pathway. Especially, the corresponding antiaromatic porphyrinoid shows the unique property to form dimeric species adopting the face-to-face geometry which is unprecedented in cases of known annulated naphthorosarins. In order to elucidate the intriguing properties derived from the stacked dimer, the current study focuses on the experimental support to rationalize the observed π-π interactions between the two subunits.

Synthesis of Ag/Mn Co-Doped CdS/ZnS (Core/Shell) Nanocrystals with Controlled Dopant Concentration and Spatial Distribution and the Dynamics of Excitons and Energy Transfer between Co-Dopants.

We report lightly Ag/Mn co-doped CdS/ZnS (core/shell) nanocrystals (NCs) as a model system for studying interactions between co-dopants and between NCs and dopants. The co-doped NCs were prepared with a varying average number of Ag dopant atoms per CdS core of the NC from zero to eight; at the same time, the depth profile of the Mn dopants in the ZnS shells was controlled to be either close to or far from the Ag dopants. The incorporation of an average of one to two Ag dopant atoms per NC increased the band-edge photoluminescence (PL); however, it was quenched at higher doping concentration. This alternation is attributed to change of the Ag ion occupancy from PL-enhancing interstitial sites to PL-quenching substitutional sites. Mn PL increased as the number of Ag atoms per NC increased up to approximately seven and then decreased. For NCs doped only with Ag ions, the Ag dopants in substitutional sites acted as PL-quenching hole traps. In Ag/Mn co-doped NCs, the Ag dopants acted as Dexter-type relay sites that enhanced the energy transfer from NC to Mn ions; this effect increased as the distance between Ag and Mn dopants decreased. This model study demonstrates that the simultaneous control of dopant concentrations and spatial distributions in co-doped semiconductor NCs enables sophisticated control of their optical properties.

Multiexcitonic Triplet Pair Generation in Oligoacene Dendrimers as Amorphous Solid-State Miniatures.

Singlet fission in organic semiconducting materials has attracted great attention for the potential application in photovoltaic devices. Research interests have been concentrated on identifying working mechanisms of coherent SF processes in crystalline solids as ultrafast SF is hailed for efficient multiexciton generation. However, as long lifetime of multiexcitonic triplet pair in amorphous solids facilitates the decorrelation process for triplet exciton extractions, a precise examination of incoherent SF processes is demanded in delicate model systems to represent heterogeneous structures. Heterogeneous coupling and energetics for SF were developed in our oligoacene dendrimers, which mimic complicated SF dynamics in amorphous solids. SF dynamics in dendritic structures was thoroughly investigated by time-resolved spectroscopic techniques and quantum chemical calculations in respect of the relative orientation/distance between chromophores and though-bond/-space interactions.

Excited-State Aromaticity of Gold(III) Hexaphyrins and Metalation Effect Investigated by Time-Resolved Electronic and Vibrational Spectroscopy.

Expanded porphyrins with appropriate metalation provide an excellent opportunity to study excited-state aromaticity. The coordinated metal allows the excited-state aromaticity in the triplet state to be detected through the heavy-atom effect, but other metalation effects on the excited-state aromaticity were ambiguous. Herein, the excited-state aromaticity of gold(III) hexaphyrins through the relaxation dynamics was revealed via electronic and vibrational spectroscopy. The SQ states of gold [26]- and [28]-hexaphyrins showed interconvertible absorption and IR spectra with those of counterparts in the ground-state, indicating aromaticity reversal. Furthermore, while the T1 states of gold [28]-hexaphyrins also exhibited reversed aromaticity according to Baird's rule, the ligand-to-metal charge-transfer state of gold [26]-hexaphyrins contributed by the gold metal showed non-aromatic features arising from the odd-number of π-electrons.

Spectroscopic Diagnosis of Excited-State Aromaticity: Capturing Electronic Structures and Conformations upon Aromaticity Reversal.

Aromaticity, the special energetic stability derived from cyclic [4 n + 2]π-conjugated electronic structures, has been the topic of intense interest in chemistry because it plays a critical role in rationalizing molecular stability, reactivity, and physical/chemical properties. Recently, the pioneering work by Colin Baird on aromaticity reversal, postulating that aromatic (antiaromatic) character in the ground state reverses to antiaromatic (aromatic) character in the lowest excited triplet state, has attracted much scientific attention. The completely reversed aromaticity in the excited state provides direct insight into understanding the photophysical/chemical properties of photoactive materials. In turn, the application of aromatic molecules to photoactive materials has led to numerous studies revealing this aromaticity reversal. However, most studies of excited-state aromaticity have been based on the theoretical point of view. The experimental evaluation of aromaticity in the excited state is still challenging and strenuous because the assessment of (anti)aromaticity with conventional magnetic, energetic, and geometric indices is difficult in the excited state, which practically restricts the extension and application of the concept of excited-state aromaticity. Time-resolved optical spectroscopies can provide a new and alternative avenue to evaluate excited-state aromaticity experimentally while observing changes in the molecular features in the excited states. Time-resolved optical spectroscopies take advantage of ultrafast laser pulses to achieve high time resolution, making them suitable for monitoring ultrafast changes in the excited states of molecular systems. This can provide valuable information for understanding the aromaticity reversal. This Account presents recent breakthroughs in the experimental assessment of excited-state aromaticity and the verification of aromaticity reversal with time-resolved optical spectroscopic measurements. To scrutinize this intriguing and challenging scientific issue, expanded porphyrins have been utilized as the ideal testing platform for investigating aromaticity because they show distinct aromatic and antiaromatic characters with aromaticity-specific spectroscopic features. Expanded porphyrins exhibit perfect aromatic and antiaromatic congener pairs having the same molecular framework but different numbers of π electrons, which facilitates the study of the pure effect of aromaticity by comparative analyses. On the basis of the characteristics of expanded porphyrins, time-resolved electronic and vibrational absorption spectroscopies capture the changes in electronic structure and molecular conformations driven by the change in aromaticity and provide clear evidence for aromaticity reversal in the excited states. The approaches described in this Account pave the way for the development of new and alternative experimental indices for the evaluation of excited-state aromaticity, which will enable overarching and fundamental comprehension of the role of (anti)aromaticity in the stability, dynamics, and reactivity in the excited states with possible implications for practical applications.

Control and Switching of Aromaticity in Various All-Aza-Expanded Porphyrins: Spectroscopic and Theoretical Analyses.

Modification of aromaticity is regarded as one of the most interesting and important research topics in the field of physical organic chemistry. Particularly, porphyrins and their analogues (porphyrinoids) are attractive molecules for exploring various types of aromaticity because most porphyrinoids exhibit circular conjugation pathways in their macrocyclic rings with various molecular structures. Aromaticity in porphyrinoids is significantly affected by structural modification, redox chemistry, NH tautomerization, and electronic states (singlet and triplet excited states). Conversely, aromaticity significantly affects the spectroscopic properties and chemical reactivities of porphyrinoids. In this context, considerable efforts have been devoted to understanding and controlling the aromaticity and antiaromaticity of porphyrinoids. Thus, a series of porphyrinoids are in the limelight, being expected to shed light on this field because they have some advantages to demonstrate the switching of aromaticity; it is possible to control the aromaticity by lowering the temperature, adding and removing the protons of expanded porphyrins, changing the chemical environment, and switching the electronic states (triplet and singlet excited states) by photoexcitation. In this regard, this Review describes the control of aromaticity in various expanded porphyrins from the spectroscopic point of view with assistance from theoretical calculations.

Photoinduced Intermolecular Electron Transfer Mediated by the Colloidal Tyrosyl Bolaamphiphile Assembly.

The self-assembly of tyrosyl bolaamphiphiles is exploited to create a colloidal protein-like host matrix, upon which sacrificial electron-donor molecules associate to create a photosystem II (PSII) mimetic electron-relay system. This system harnesses the tyrosine phenol groups abundant on the surface of the assemblies to mediate photoinduced intermolecular electron transfer. Compared with the l-tyrosine molecules, the tyrosyl bolaamphiphile assembly facilitates electron transfer from the sacrificial electron donor to the oxidized photosensitizer. The enhanced electron relay is likely to be driven by the host function of the assembly associated with the sacrificial electron donor and by the suppression of the oxidative cross-linking of phenoxyl radicals. The tyrosyl bolaamphiphile assembly is advantageous in the construction of a PSII mimetic system with a protein-like nature and displaying biochemical functions.

Guest-Induced Modulation of the Energy Transfer Process in Porphyrin-Based Artificial Light Harvesting Dendrimers.

A series of dendritic multiporphyrin arrays (PZnTz-nPFB; n = 2, 4, 8) comprising a triazole-bearing focal zinc porphyrin (PZn) with a different number of freebase porphyrin (PFB) wings has been synthesized, and their photoinduced energy transfer process has been evaluated. UV/vis absorption, emission, and time-resolved fluorescence measurements indicated that efficient excitation energy transfer takes place from the focal PZn to PFB wings in PZnTz-nPFB's. The triazole-bearing PZn effectively formed host-guest complexes with anionic species by means of axial coordination with the aid of multiple C-H hydrogen bonds. By addition of various anionic guests to PZnTz and PZnTz-nPFB's, strong bathochromic shifts of PZn absorption were observed, indicating the HOMO-LUMO gap (ΔEHOMO-LUMO) of PZn decreased by anion binding. Time-resolved fluorescence measurements revealed that the fluorescence emission predominantly takes place from PZn in PZnTz-nPFB's after the addition of CN-. This change was reversible because a treatment with a silver strip to remove CN- fully recovered the original energy transfer process from the focal PZn to PFB wings.