Inter- and Intramolecular Electron-Transfer Reduction Properties of Coronenediimide Derivatives via Photoinduced Processes

Coronenediimide derivatives with five- or six-membered maleimide groups [denoted as Cor­(5Im)₂ and Cor­(6Im)₂] were employed as electron acceptors to examine the electron-transfer reduction properties through photochemical and electrochemical measurements. In steady-state absorption and fluorescence measurements, the spectra of Cor­(5Im)₂ and Cor­(6Im)₂ became remarkably broadened and red-shifted as compared to pristine coronene (Cor). These results are supported by electrochemical measurements and DFT calculations. The rate constants of photoinduced intermolecular electron transfer from various donor molecules to ³Cor­(5Im)₂* or ³Cor­(6Im)₂* are determined by nanosecond transient absorption measurements. Although the back-electron-transfer reactions examined in this study proceed with the diffusion-limited rate constant in benzonitrile (PhCN), the rate constants of forward electron-transfer reactions (<i>k</i>_et) increase with an increase in the driving force of electron transfer (−Δ<i>G</i>_et) to approach the diffusion-limited rate constant. When the driving force dependence of <i>k</i>_et was fit on the basis of the Marcus theory of electron transfer, the reorganization energy (λ) of the electron-transfer reduction of Cor­(5Im)₂ and Cor­(6Im)₂ are determined to be 0.77 and 1.15 eV, respectively. A new covalently perylene-linked donor–acceptor dyad was also synthesized to investigate the dynamics of ultrafast photoinduced intramolecular electron transfer

Synthesis, Structural and Photophysical Properties of Pentacene Alkanethiolate Monolayer-Protected Gold Nanoclusters and Nanorods: Supramolecular Intercalation and Photoinduced Electron Transfer with C₆₀

6,13-Bis­(triisopropylsilylethynyl)-pentacene (TIPS-pentacene: TP) alkane­thiolate monolayer-protected gold nanoclusters (TP-C<i>n</i>-<i>X</i>-MPCs: <i>X</i> stands for small (<i>S</i>) and large (<i>L</i>) nanocluter sizes) and nanorods (TP-C<i>n</i>-MPRs) with different alkyl chain lengths (<i>n</i> = 7, 11) were synthesized to examine the structural and photophysical properties as well as intercalation trends with C₆₀. The syntheses of TP-C<i>n</i>-<i>X</i>-MPCs and TP-C<i>n</i>-MPRs were successfully performed using two different precursors: TP disulfides and TP alkanethiols. The detailed structural properties were confirmed by ¹H NMR, elemental analyses, and transmission electron micrograph (TEM). In the spectroscopic absorption and fluorescence excitation measurements, spectral shapes of TP units on the gold surface were clearly observed, whereas fluorescence intensities of TP units were strongly quenched as compared to the corresponding reference monomer (TP-Ref). Then, fluorescence quenching titration experiments to determine the association constants (<i>K</i>_app) between C₆₀ and TP assemblies (TP-C<i>n</i>-<i>X</i>-MPCs and TP-C<i>n</i>-MPRs) were performed by adding C₆₀ in toluene. The <i>K</i>_app values were largely dependent on the sizes of nanoclusters and alkyl chain lengths in TP-C<i>n</i>-<i>X</i>-MPC. For example, the <i>K</i>_app value of TP-C7-<i>S</i>-MPC (73 800 M^–1) was much larger than those of TP-C11-<i>S</i>-MPC (37 800 M^–1) and TP-C7-<i>L</i>-MPC (5350 M^–1). This trend is in sharp contrast with the similar <i>K</i>_app values (∼66 000 M^–1) in TP-C<i>n</i>-MPR (<i>n</i> = 7, 11). These results suggest that the intercalation behaviors are dependent on the surface structures (nanocluster vs nanorod). Such fluorescence quenching processes by photoinduced electron transfer (PET) in the complex between TP-C7-<i>S</i>-MPC and C₆₀ were directly observed by femtosecond transient absorption measurements, monitoring the TP radical cation and C₆₀ radical anion

Multiexciton Dynamics Depending on Intramolecular Orientations in Pentacene Dimers: Recombination and Dissociation of Correlated Triplet Pairs

Pentacene dimers bridged by a phenylene at ortho and meta positions [denoted as <i>o</i>-(Pc)₂ and <i>m</i>-(Pc)₂] were synthesized to examine intramolecular orientation-dependent multiexciton dynamics, especially focusing on singlet fission (SF) and recombination from correlated triplet pairs [(TT)]. Absorption and electrochemical measurements indicated strong intramolecular couplings of <i>o</i>-(Pc)₂ relative to <i>m</i>-(Pc)₂. Femtosecond and nanosecond TA measurements successfully demonstrated efficient SF in both dimers. In contrast, the dissociation process from the (TT) to the individual triplets [(2 × T)] was clearly observed in <i>m</i>-(Pc)₂, which is in sharp contrast to a major recombination process in <i>o</i>-(Pc)₂. Time-resolved electron spin resonance (TR-ESR) measurements demonstrated that the recombination and dissociation proceed from the quintet state of ⁵(TT) in <i>m</i>-(Pc)₂. The rate constant of the SF was 2 orders of magnitude greater in <i>o</i>-(Pc)₂ than that in <i>m</i>-(Pc)₂ and was rationalized by enhanced electronic coupling between adjacent HOMOs of the Pc units

Long-Lived Triplet Excited States of Bent-Shaped Pentacene Dimers by Intramolecular Singlet Fission

Intramolecular singlet fission (ISF) is a promising photophysical process to construct more efficient light energy conversion systems as one excited singlet state converts into two excited triplet states. Herein we synthesized and evaluated bent-shaped pentacene dimers as a prototype of ISF to reveal intrinsic characters of triplet states (e.g., lifetimes of triplet excited states). In this study, <i>meta-</i>phenylene-bridged TIPS-pentacene dimer (PcD-3Ph) and 2,2′-bipheynyl bridged TIPS-pentacene dimer (PcD-Biph) were newly synthesized as bent-shaped dimers. In the steady-state spectroscopy, absorption and emission bands of these dimers were fully characterized, suggesting the appropriate degree of electronic coupling between pentacene moieties in these dimers. In addition, the electrochemical measurements were also performed to check the electronic interaction between two pentacene moieties. Whereas the successive two oxidation peaks owing to the delocalization were observed in a directly linked-pentacene dimer (PcD) by a single bond, the cyclic voltammograms in PcD-Biph and PcD-3Ph implied the weaker interaction compared to that of <i>p-</i>phenylene-bridged TIPS-pentacene dimer (PcD-4Ph) and PcD. The femtosecond and nanosecond transient absorption spectra clearly revealed the slower ISF process in bent-shaped pentacene dimers (PcD-Biph and PcD-3Ph), more notably, the slower relaxation of the excited triplet states in PcD-Biph and PcD-3Ph. Namely, the quantum yields of triplet states (Φ_T) by ISF approximately remain constant (ca. 180–200%) in all dimer systems, whereas the lifetimes of the triplet excited states became much longer (up to 360 ns) in PcD-Biph as compared to PcD-4Ph (15 ns). Additionally, the lifetimes of the corresponding triplet states in PcD-Biph and PcD-3Ph were sufficiently affected by solvent viscosity. In particular, the lifetimes of PcD-Biph triplet state in THF/paraffin (1.0 μs) increased up to approximately three times as compared to that in THF (360 ns), whereas those of PcD-4Ph were quite similar in both solvent

Siloxy Group-Induced Highly Efficient Room Temperature Phosphorescence with Long Lifetime

The design and development of organic phosphors that exhibit efficient emission at room temperature but do not contain precious metals such as iridium and platinum have attracted increasing attention. We report herein highly efficient green phosphorescence-emitting 1,4-dibenzoyl-2,5-bis­(siloxy)­benzene crystals in air at room temperature. Remarkable luminescence quantum yields of 0.46 to 0.64 and long lifetimes ranging from 76.0 to 98.3 ms were observed. X-ray diffraction analysis of the single crystals revealed that there were several intermolecular interactions causing suppression of intramolecular motion, thereby minimizing nonradiative decay of the triplet excited state. Comparison with the corresponding 2,5-bis­(dimethylphenylsilylmethyl) and 2,5-bis­(trimethylsilyl) derivatives revealed that the siloxy groups are essential for efficient room temperature phosphorescence. Density functional calculations suggested that σ–n conjugation was operative in the siloxy moieties. Electron spin resonance measurement indicated that the radiative process included generation of the triplet diradical species, whose electron distribution was very similar to that of naphthalene. The present study largely expands the possibilities for the molecular design of precious metal- and halogen-free organic phosphors exhibiting efficient room temperature phosphorescence

Highly Fluorescent [7]Carbohelicene Fused by Asymmetric 1,2-Dialkyl-Substituted Quinoxaline for Circularly Polarized Luminescence and Electroluminescence

A new 1,2-dialkylquinoxaline-fused [7]­carbohelicene ([7]­Hl-NAIQx) was designed and synthesized by asymmetrically introducing two alkyl chains onto the quinoxaline unit. Direct alkylation of the quinoxaline ring of quinoxaline-fused helicene leads to discontinuity in the conjugated structure. In the single-crystal analysis, the parent quinoxaline-fused [7]­carbohelicene ([7]­Hl-Qx) was found to have a helical structure formed by two phenanthrene units and a nonplanar twisted angle between the phenanthrene and quinoxaline units. In contrast, [7]­Hl-NAIQx possesses an almost planar aromatic structure between the alkyl-quinoxaline and phenanthrene units (torsion angle: 179°), in addition to the similar helical structure between the two phenanthrene units. The steady-state absorption, fluorescence, and circular dichroism (CD) spectra of [7]­Hl-NAIQx were significantly red-shifted compared to those of [7]­Hl-Qx and [7]­carbohelicene ([7]­Hl). These spectral changes were mainly explained by electrochemical measurements and density functional theory calculations. Moreover, the absolute fluorescence quantum yield (Φ_FL) of [7]­Hl-NAIQx was 0.25, which is more than 10 times larger than that of the reference [7]­Hl (Φ_FL = 0.02). Such a large enhancement of the fluorescence of [7]­Hl-NAIQx has provided excellent circularly polarized luminescence (CPL). The value of the anisotropy factor <i>g</i>_lum (normalized difference in emission of right-handed and left-handed circularly polarized light) was estimated to be 4.0 × 10^–3. The electroluminescence of an organic light-emitting diode utilizing [7]­Hl-NAIQx was successfully observed

Synthetic Control of Photophysical Process and Circularly Polarized Luminescence of [5]Carbohelicene Derivatives Substituted by Maleimide Units

A series of [5]­carbohelicene derivatives substituted by electron-withdrawing maleimide and electron-donating methoxy, such as maleimide-substituted [5]­carbohelicene (HeliIm) and methoxy-substituted HeliIm (MeO-HeliIm), were newly designed and synthesized to examine the electrochemical properties, excited-state dynamic and circularly polarized luminescence (CPL). First, electrochemical measurements and DFT calculations of [5]­carbohelicene derivatives were performed by comparing with the structural isomers: picene derivatives. Introduction of an electron-withdrawing maleimide group onto a [5]­carbohelicene core contributes to the stabilized LUMO state in HeliIm as compared to that of [5]­carbohelicene (Heli), whereas the energy level of HOMO state in MeO-HeliIm increases by introducing electron-donating methoxy (MeO) groups onto a HeliIm skeleton. The HOMO–LUMO gap of MeO-HeliIm is smaller than those of HeliIm and Heli, which is similar to the steady-state spectroscopic measurements. The absolute fluorescence quantum yield (Φ_FL) of HeliIm (0.37) largely increased as compared to [5]­carbohelicene, Heli (0.04), whereas Φ_FL of MeO-HeliIm (0.22) was slightly smaller than that of HeliIm. Theses photophysical processes including intersystem crossing are successfully explained by the kinetic discussions. Since [5]­carbohelicene derivatives show the chirality, measurements of circular dichroism (CD) and circularly polarized luminescence (CPL) were successfully performed. In particular, HeliIm and MeO-HeliIm have provide excellent circularly polarized luminescence (CPL) and the values of the anisotropy factor <i>g</i>_lum were estimated to be ∼2.4 × 10^–3 and ∼2.3 × 10^–3, relatively. This is the first observation of CPL in [5]­carbohelicene derivatives

Controlled Excited-State Dynamics and Enhanced Fluorescence Property of Tetrasulfone[9]helicene by a Simple Synthetic Process

Tetrasulfone­[9]­helicene (PTSH) was newly synthesized to improve and evaluate its fluorescence and excited-state dynamics through a single-step oxidation reaction of tetrathia[9]­helicene (PTTH). In electrochemical measurements, the reduction potential of PTSH was shifted in a positive direction by approximately 1.0 V when compared to that of PTTH because of its electron-accepting sulfone units. The results of the electrochemical measurements agree with the energy levels calculated by density functional theory (DFT) methods and steady-state spectroscopic measurements. Furthermore, a significant enhancement of the absolute fluorescence quantum yield (Φ_FL) was achieved. The absolute fluorescence quantum yield of PTSH attained 0.27, which is approximately 10 times larger than that of PTTH (Φ_FL = 0.03). Such an enhancement of Φ_FL can be successfully explained by the corresponding kinetic comparison. The reason is mainly the increased energy gap Δ<i><i>E</i></i>_ST between the lowest singlet (S₁) and triplet (T₁) excited states. Finally, excellent circularly polarized luminescence of PTSH was also observed. The value of the anisotropy factor <i>g</i>_CPL was estimated to be 8.3 × 10^–4 in PTSH