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, meta-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 p-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 (ΦT) 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.

Pentacene dimers bridged by a phenylene at ortho and meta positions [denoted as o-(Pc) and m-(Pc)] 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 o-(Pc) relative to m-(Pc). 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 m-(Pc), which is in sharp contrast to a major recombination process in o-(Pc). Time-resolved electron spin resonance (TR-ESR) measurements demonstrated that the recombination and dissociation proceed from the quintet state of (TT) in m-(Pc). The rate constant of the SF was 2 orders of magnitude greater in o-(Pc) than that in m-(Pc) and was rationalized by enhanced electronic coupling between adjacent HOMOs of the Pc units.

Singlet fission (SF) theoretically enables the performance of the sequential photoenergy conversion process starting from the singlet state and leading to electron transfer (ET) with the radical ion pair quantum yield approaching 200%. Additionally, the long lifetime of the triplet state opens the possibility for an intermolecular ET process in a diffusion-limited reaction. However, the quantitative two-electron transfer process through SF has yet to be reported. Herein we demonstrate the quantitative sequential process involving SF and leading to intermolecular two-electron transfers using 2,2′-biphenyl-bridged tetracene dimer (Tet-BP-Tet: SF and electron donor) and chloranil (Ch: electron acceptor). The high-yield and long-lived individual triplet excited states of Tet-BP-Tet by SF (ΦT = 175 ± 5% and τT = 0.29 ms) resulted in the quantitative two-electron transfer process (ΦET = 173 ± 5%) with Ch in benzonitrile.

We present a novel design concept of molecular dimers for quantitative individual triplet yield (Φ Τ ) through intramolecular singlet fission. We synthesized a series of tetracene (Tc) dimers bridged by different phenylene-based linkers, focusing on the conformational flexibility in addition to the electronic coupling. In transient absorption measurements, a 4,4′-biphenyl-bridged Tc dimer with weaker electronic coupling and larger conformational flexibility exhibited the quantitative Φ Τ : 196 ± 12% at high excitation energy. Moreover, decoupled spins associated with conformational change were directly characterized by time-resolved electron paramagnetic resonance. Strong interplay of the geometry change and electronic localization was revealed on the triplet pair dissociation through magnitudes of spin−spin exchange couplings. Thus, a synergetic role of conformational flexibility and electronic coupling in quantitative Φ Τ was clarified.

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 (ΦFL) of [7]­Hl-NAIQx was 0.25, which is more than 10 times larger than that of the reference [7]­Hl (ΦFL = 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 g 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.