Significance of the connecting position between Zn(ii) porphyrin and Re(i) bipyridine tricarbonyl complex units in dyads for room-temperature phosphorescence and photocatalytic CO2 reduction: unexpected enhancement by triethanolamine in catalytic activity.

We synthesized three new dyads composed of a Zn porphyrin and fac-Re(bpy)(CO)3Br (bpy = 2,2'-bipyridine) units, ZnP-nBpy[double bond, length as m-dash]ReBr (n = 4, 5, and 6), in which the porphyrin is directly connected at the meso-position through the 4-, 5-, or 6-position of the bpy. We investigated the relationships between the connecting positions and the photophysical properties as well as catalytic activity in the CO2 reduction reaction. The dyad connected through the 6-position, ZnP-6Bpy[double bond, length as m-dash]ReBr, showed obvious phosphorescence with a lifetime of 280 µs at room temperature, in N,N-dimethylacetamide (DMA), whereas the other two dyads showed almost no phosphorescence under the same conditions. The photocatalytic CO2 reduction reactions in DMA using 1,3-dimethyl-2-phenyl-2,3-dihydro-1H-benzo[d]imidazole as the electron donor and the three dyads ZnP-nBpy[double bond, length as m-dash]ReBr selectively produced CO with similar initial rates, but the durabilities were low. The addition of triethanolamine (TEOA) suppressed the decomposition of dyads, improving their durabilities and reaction efficiencies. In particular, ZnP-5Bpy[double bond, length as m-dash]ReBr was remarkably improved-it gave the highest durability and reaction efficiency among the three dyads; the reaction quantum yield reached 24%. The reason for this significant activity is no accumulation of electrons on the Zn porphyrin in ZnP-5Bpy[double bond, length as m-dash]ReBr, which would be caused by dual interactions of TEOA with the Re and Zn ions in the dyad. As the highest catalytic activity was observed in ZnP-5Bpy[double bond, length as m-dash]ReBr among the three dyads, which had no room-temperature phosphorescence (RTP), the catalytic activities and RTP properties are considered independent, but they are greatly influenced by the connecting positions on the bpy ligand in ZnP-nBpy[double bond, length as m-dash]ReBr.

Star-Shaped ROMP Polymers Coated with Oligothiophenes That Exhibit Unique Emission.

A series of oligo(thiophene)-modified "soluble" star-shaped ring-opening metathesis polymerization (ROMP) polymers were prepared by sequential living ROMP of norbornene and a cross-linking agent using a molybdenum-alkylidene catalyst, followed by Wittig-type coupling for termination with oligo(thiophene) carboxaldehydes. The resultant star-shaped ROMP polymers displayed unique emission properties affected by the core size and arm repeat units as well as the kind of oligothiophene coated. The effects of the thiophene groups on photophysical properties of star-shaped/linear polymers were studied via time-resolved fluorescence spectroscopy. Fluorescence lifetimes were determined in THF as 400, 640, 730, and 820 ps for Star 3TPh, Linear 3TPh, Star 4T, and Linear 4T, respectively. A significant enhancement of the nonradiative rate constants k nr in the star-shaped polymers results in relatively lower fluorescence quantum yields and shorter fluorescence lifetimes compared to the corresponding linear polymers.

S,C,C- and O,C,C-Bridged Triarylamines and Their Persistent Radical Cations.

This work reports the synthesis, crystal structures, and electronic properties of structurally constrained S,C,C- and O,C,C-bridged triarylamine derivatives and their persistent radical cations. O,C,C-Bridged triphenylamines and a dinaphthylphenylamine were obtained through a straightforward synthetic protocol. Similar to a previously reported S,C,C-bridged triphenylamine, the O,C,C-bridged triarylamines were easily oxidized to afford the corresponding radical cations, which were obtained as hexachloroantimonate salts. X-ray crystallographic analyses showed almost planar structures for these O,C,C-bridged triarylamine radical cations, which represent new members of the family of planar triarylamine radical cations without substituents on the aryl rings. Detailed investigations of the electronic properties of the S,C,C- and O,C,C-bridged triarylamine radical cations demonstrated that the spin and positive charge are sufficiently delocalized over the planar triarylamine scaffolds. The results provide the following insights into the effects of the bridging unit (sulfur vs oxygen) and the dibenzo-annulation on the spin delocalization in the bridged triarylamine radical cations: (1) An effective decrease of the spin density on the nitrogen atom is observed for the sulfur bridge relative to the oxygen bridge; and (2) a moderate decrease of the spin density on the oxygen atom rather than the nitrogen atom is induced by the dibenzo-annulation.

Observation of Intramolecular Interaction in Fluorescent Star-Shaped Polymers: Evidence for Energy Hopping between Branch Chains.

Time-resolved fluorescence anisotropy measurements were performed on three-branched star-shaped polymers, based on precisely synthesized poly(9,9-di-n-octyl-fluorene vinylene)s containing C6F5 end groups. The star-shaped polymers showed identical fluorescence spectra, fluorescence lifetimes, and quantum yields to those of the reference single-chain oligomer. However, a rapid fluorescence anisotropy decay was observed in two kinds of star-shaped polymers, while such decay was not seen in the corresponding single-chain oligomer. On the basis of the analysis using an incoherent hopping model, the observed rapid anisotropy decay is attributable to energy hopping processes between branches within a single polymer species, and its rate was deduced to be ca.100 ps depending upon the core part.

Open-shell singlet diradicaloid difluoreno[4,3-b:3',4'-d]furan and its radical cation and dianion.

Difluoreno[4,3-b:3',4'-d]furan (DFFu) was prepared via a short, scalable, multi-step synthesis, and characterized by crystallography, spectroscopic measurements, and theoretical calculations. Even though the results revealed an open-shell singlet diradical character, DFFu is stable under ambient conditions. The radical cationic and dianionic species of DFFu were also synthesized and characterized.

Synthesis, Structures, and Properties of Neutral and Radical Cationic S,C,C-Bridged Triphenylamines.

A structurally constrained S,C,C-bridged triphenylamine was synthesized, and the corresponding radical cation was obtained as a hexachloroantimonate by chemical oxidation. An X-ray crystallographic analysis revealed an almost planar structure for this radical cation, which thus represents the first example of a planar, para-unsubstituted triphenylamine radical cation analogue with a sulfur bridge. The electronic properties of the radical cation were examined by UV-vis-NIR and ESR spectroscopy as well as DFT calculations.

Visualization of nonemissive triplet species of Zn(ii) porphyrins through Cu(ii) porphyrin emission via the reservoir mechanism in a porphyrin macroring.

A macroring composed of three Cu(ii) porphyrins (CuP) and three slipped-cofacial Zn(ii) porphyrin (ZnP2) dimers exhibited near-IR emission from the CuP part. The emission lifetime of the macroring (15 µs) was 500 times longer compared to that of a Cu porphyrin monomer (e.g., Cu(ii)TPP; TPP = meso-tetraphenylporphyrin). The observed emission is ascribed to emission via the reservoir mechanism from the trip-doublet (2T1) state in CuP thermally activated from the T1 state of ZnP2, which is located ca. 1030 cm-1 below the 2T1 state of CuP. The near-IR emission of the macroring was significantly quenched by O2, whereas that of the Cu porphyrin monomer was hardly quenched, indicating that the quenching event mainly occurred on the T1 state of the ZnP2 parts. The nonemissive triplet state of a Zn porphyrin at room temperature was visualized through emission from a neighboring Cu porphyrin.

Terthiophene Functionalized Conjugated Triarm Polymers Containing Poly(fluorene-2,7-vinylene) Arms Having Different Cores-Synthesis and Their Unique Optical Properties.

Optical properties of three types of terthiophene (3T) functionalized conjugated triarm (star-shaped) polymers consisting of poly(9,9-di-n-octyl-fluorene-2,7-vinylene) (PFV) arms and different [2,4,6-tri(biphenyl)benzene (TBP), 1,3,5-tri(benzyl)benzene (TBB), and triphenylamine (TPA)] cores, prepared by combined olefin metathesis with Wittig coupling, have been studied. Relative intensities [increases in the higher vibronic bands, (0, 1) fluorescence (FL)] of the fully conjugated TPA-core polymers, TPA(PFV-3T)3, in the fluorescence (FL) spectra in tetrahydrofuran (toluene) solution were higher than those in the other triarm polymers, TBP(PFV-3T)3, TBB(PFV-3T)3, whereas no significant differences were observed in their UV-vis spectra; notable temperature dependences were not observed in the UV-vis and FL spectra (at -5, 25, and 55 °C). Remarkable differences were not observed in the spectra in these polymer thin films, whereas λmax values red-shifted due to the formation of J-type aggregates. The observation for the time-resolved study well corresponds to results for the steady-state fluorescence measurements. The observed unique emission by the star-shaped (triarm) polymer containing the TPA core would be assumed to be due to a difference in nature of the core (higher coplanarity) compared to that of the others.

Pyrrole-fused azacoronene family: the influence of replacement with dialkoxybenzenes on the optical and electronic properties in neutral and oxidized states.

A novel pyrrole-fused azacoronene family was synthesized via oxidative cyclodehydrogenation of the corresponding hexaarylbenzenes as the key step, and the crystal structures of tetraazacoronene 3b and triazacoronene 4a were elucidated. The photophysical properties for neutral compounds 1-4 were investigated using steady-state UV-vis absorption/emission spectroscopy and time-resolved spectroscopy (emission spectra and lifetime measurements) at both room temperature and 77 K. The observation of both fluorescence and phosphorescence allowed us to estimate the small S1-T1 energy gap (ΔES-T) to be 0.35 eV (1a), 0.26 eV (2a), and 0.36 eV (4a). Similar to the case of previously reported hexapyrrolohexaazacoronene 1 (HPHAC), electrochemical oxidation revealed up to four reversible oxidation processes for all of the new compounds. The charge and spin delocalization properties of the series of azacoronene π-systems were examined using UV-vis-NIR absorption, ESR, and NMR spectroscopies for the chemically generated radical cations and dications. Combined with the theoretical calculations, the experimental results clearly demonstrated that the replacement of pyrrole rings with dialkoxybenzene plays a critical role in the electronic communication, where resonance structures significantly contribute to the thermodynamic stability of the cationic charges/spins and determine the spin multiplicities. For HPHAC 1 and pentaazacoronene 2, the overall aromaticity predicted for closed-shell dications 1(2+) and 2(2+) was primarily based on the theoretical calculations, and the open-shell singlet biradical or triplet character was anticipated for tetraazacoronene 3(2+) and triazacoronene 4(2+) with the aid of theoretical calculations. These polycyclic aromatic hydrocarbons (PAHs) represent the first series of nitrogen-containing PAHs that can be multiply oxidized.

Catalyst-free aromatic nucleophilic substitution of meso-bromoporphyrins with azide anion: efficient synthesis and structural analyses of meso-azidoporphyrins.

meso-Mono- or diazidoporphyrins were readily obtained in high yields by the catalyst-free aromatic nucleophilic reaction of the corresponding bromoporphyrins with azide anions under mild conditions. The molecular structures of the obtained azides were unambiguously determined by X-ray crystallographic analysis.

Superexchange mediated energy transfer in zinc(II) porphyrin-free base porphyrin dimers: comparison of m- and p-bis(phenylethynyl) phenylene linked dimers.

The singlet-singlet energy transfer rate in a new zinc(II) porphyrin-free base porphyrin dimer, having a m-bis(phenylethynyl)phenylene bridge, was found to be slower than that in the corresponding p-bis(phenylethynyl)phenylene-bridged dimer, despite the shorter donor-acceptor distance and pathway. The slower rate is interpreted as evidence for a major contribution of the superexchange mechanism.

Light-induced electron spin polarization in vanadyl octaethylporphyrin: I. Characterization of the excited quartet state.

Laser flash induced spin-polarized transient electron paramagnetic resonance (TREPR) spectra for vanadyl octaethylporphyrin in isotropic and partially ordered frozen solutions are presented and compared with corresponding luminescence data. The TREPR spectra show well-resolved hyperfine couplings to the vanadium nucleus and a multiplet polarization pattern with features typical of zero-field splitting (ZFS). The principal values of the vanadium hyperfine coupling tensor evaluated from the spectra are 1/3 of the corresponding values found from steady-state EPR spectra of the ground state. On the basis of these characteristics and numerical simulations, the polarization patterns are assigned to the excited quartet state. The values of the ZFS parameters of the trip-quartet obtained from simulation of the spectra (D = 17.5 mT and E = 1.5 mT) are comparable to those of the triplet state of the zinc and free base octaethyl porphyrin. The lifetime of the spin polarization is found to be temperature dependent and is essentially the same as that of the optical emission. The temperature dependence is rationalized using a model in which the decay to the ground state occurs from both the trip-quartet and trip-doublet, which are in thermal equilibrium even at 15 K. A fit of the model to the observed spin polarization lifetimes yields an energy gap of 47 cm(-1) between the trip-quartet and trip-doublet. It is shown that the spin polarization evolves from a multiplet pattern at early times to a net absorptive pattern at late times following the laser flash. It is proposed that the establishment of thermal equilibrium leads to the evolution of the spin from multiplet to net polarization.

Light-induced electron spin polarization in vanadyl octaethylporphyrin: II. Dynamics of the excited states.

The dynamics of the low-lying excited states of vanadyl octaethylporphyrin (OEPVO) in frozen solution is investigated by transient electron paramagnetic resonance (TREPR). The observation of spin-polarized TREPR spectra from the lowest excited trip-quartet state of OEPVO, reported in the preceding paper, opens a new avenue for investigation of the excited states of such molecules. Here, a model based on the back-and-forth transitions between the trip-quartet and trip-doublet states is developed and used to explain the time dependence of the low-temperature laser flash-induced electron spin polarization of OEPVO. At early times, the TREPR spectra show predominantly multiplet polarization, whereas strong net polarization develops at later times. An analysis of the time dependence reveals two well-separated processes: (i) fast evolution of the polarization from the multiplet pattern to the net absorptive pattern and (ii) very slow decay of the net polarization. Both processes are temperature dependent and are faster at higher temperature. All of these observed features can be reproduced, and the experimental data can be simulated within the framework of the model. For simplicity, only the two nearly degenerate orbital states resulting from the a(1) --> e triplet excitation of the porphyrin are considered. Each of these is split into a trip-doublet and trip-quartet giving a total of four low-lying excited states. Transitions between the trip-doublet and trip-quartet states are assumed to be governed by spin-orbit coupling, which mixes the four low-lying states. It is known that following light excitation, the molecule initially decays to the lowest trip-doublet state and then to the trip-quartet state. In agreement with the observed TREPR spectra, the model predicts that this decay results in predominantly multiplet polarization of the trip-quartet. However, a small amount of net polarization is also predicted due to the spin selectivity associated with the Zeeman interaction. Because the energy gap between the trip-doublet and trip-quartet states is small, back-and-forth electronic transitions between the trip-doublet and trip-quartet are expected to occur as thermal equilibrium is established. The model predicts that it is these transitions that lead to the observed evolution of the initial multiplet polarization to net absorptive polarization.

Observation of a photoexcited state of a paramagnetic transition metal complex by time-resolved electron paramagnetic resonance spectroscopy.

The first observation of a spin polarized excited state of a paramagnetic metal-complex using time-resolved electron paramagnetic resonance (TREPR) spectroscopy is reported for octaethylporphinatooxovanadium(iv). The TREPR spectra show well resolved orientation dependent hyperfine splitting to the I = 7/2 vanadium nucleus. The reduction of the hyperfine splitting by a factor of 3 compared to the ground state and the observation of a multiplet pattern of spin polarization allow the TREPR spectra to be assigned to the excited quartet state of the complex. The spin polarization patterns evolve with time and it is postulated that this is a result of the equilibration between the lowest excited quartet and doublet states.