Temperature dependence of conformation, chemical state, and metal-directed assembly of tetrapyridylporphyrin on Cu (111) Two-dimensional localization of adsorbate/substrate charge-transfer excited states of molecules adsorbed on metal surfaces

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.

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A precise, one-pot synthesis of end-functionalized block copolymers consisting of poly(9,9-di-n-octylfluorene-2,7-vinylene)s (PFVs) and oligo(2,5-dialkoxy-1,4-phenylenevinylene) or terthiophene units as the middle segment have been prepared by olefin metathesis of the vinyl group in the PFV chain ends followed by subsequent Wittig-type coupling. Formation of the block copolymers and the quantitative efficiency in the end-functionalization can be confirmed by synthesis of amphiphilic block copolymers containing poly-(ethylene glycol) at the both polymer chain ends. Effect of PFV conjugation length, middle segment, and the end groups toward the emission properties have been studied: the polymers containing ferrocene moiety at the chain ends displayed unique emission/quenching properties.

Free-base meso-propanoyloxylated (–OCOEt) porphyrins and dioxoporphodimethenes (quinoid forms of dihydroxyporphyrins) were efficiently synthesized by the treatment of the corresponding meso-diarylporphyrins with PhI(OAc)2 in propionic acid. Basic hydrolysis of the obtained meso-mono- and -dipropanoyloxyporphyrins provided meso-hydroxyporphyrins (oxophlorins) and dioxoporphodimethenes, respectively. Reduction of the dioxoporphodimethenes generated unstable meso-dihydroxyporphyrins, and one-pot etherification of the meso-dihydroxyporphyrins yielded stable meso-dialkoxyporphyrins.

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.

Time-resolved fluorescence study has been explored for end-functionalized conjugated triblock copolymers consisting of poly(9,9-di-n-octylfluorene-2,7-vinylene)s (PFVs, FV repeat units ca. 10 or 20) and oligo(2,5-dialkoxy-1,4-phenylenevinylene)s [OPV, alkoxy = O(CH 2 ) 2 OSi i Pr 3 , 3 or 7 PV repeat unit] as the middle segment. Unit (FV and PV) length dependence of each block on fluorescence was examined. The polymer sample having three PV repeat units as the middle segment with C 6 F 5 end-groups, [10PFV-3PV]F 2 , showed time-dependent fluorescence spectra, in which relative intensities of the vibronic bands increase in later delay time, whereas similar observations were not seen in the other polymer samples containing 7PV as the middle segment even by varying the PFV chain lengths (ca. 20) or the end-group (C 6 F 5 vs C 6 H 5 ). Timeresolved of fluorescence signals for [10PFV-3PV]F 2 can be analyzed by assuming two components: one is a faster decay component and the other is a slower red component, which rises with the time constant of the former one. On the other hand, the other three polymer samples showed single-exponential decays without significant wavelength dependence. Dynamical structural relaxation in the excited state is proposed as the origin of time dependence of the fluorescence spectra in the polymer having 3PV repeat unit on the basis of analysis of time-resolved fluorescence signals as well as viscosity dependence. ■ INTRODUCTIONConjugated polymers are of continuing interest as organic electronics due to their promising applications in optoelectronic and electrochemical devices, 1−5 such as photovoltaic cells, 3b light-emitting diodes, 5c,6 sensors, 5d,7 electrochromics, 8 and field-effect transistors. 5a,b,9 The synthesis of structurally regular, chemically pure polymers by development of new synthetic methods attracts notable attention 1 because their device performances are governed by the structural regularity, chemical purity, and supramolecular order. 2,3 Emerging applications of conjugated polymers require the patterning of materials on the nanometers length scale, and block copolymers made of covalently linked polymers represent an ideal route to control the self-assembly of the nanosized morphologies. Formation of regular one-dimensional conjugated structures on the nanoscale should be thus expected by exploiting the specific assembling properties of rod−coil block copolymers. 10−13 End-group modification of conjugated polymers has also been considered as an attractive topic of investigation because the important properties that have been demonstrated for end-functional conjugated polymers and the significant effect end-groups can have on electronic properties. For example, simple end-capping of conjugated polymers via postpolymerization reactions showed improvement in the performance of bulk heterojunction photovoltaic devices by eliminating traps for charge carriers. 14 Many reports concerning poly(fluorene)s (PFs) containing discrete end-groups (such as triarylamines, perylene mon...