Temperature dependencies of transient photocharge and magnetophotoconductance effect of columnar self-assemblies of the hexabenzocoronene derivative (HBC-C14), which is a prospective one-dimensional photoconductor, presented different thermal activation processes for carrier generation and transportation, respectively. Thermal equilibrium between the low-lying short distance and high-lying long-distance geminate electron–hole (e–h) pairs is the origin for activation in carrier generation. The energy difference between these e–h pairs is estimated to be 7 meV, which was mainly due to the Coulomb interaction. On the other hand, the carrier transport with thermal activation was understood by the multiple trapping model. Carrier detrapping from localized states located in the band gap causes the thermal activation in the carrier transport. The shallow energy depth at the density peak of the localized state from the mobility edge (10 ± 3 meV) is a unique nature of HBC-C14 self-assemblies. A very narrow Gaussian distribution for density of the localized states was also clarified.Topics: Electric transport processes and properties; Self-assembly;

Dynamics of the photogenerated charge-separated (CS) state is studied for a newly synthesized molecular triad, in which the donor (D) dimethoxytriphenylamine, 1,3-bis(2-pyridylimino)isoindolate platinum (BPIPt), and the acceptor (A) naphthaldiimide are linked with a triethynylbenzene unit (BPIPt–DA). Photoexcitation of BPIPt gives rise to generation of a long-lived (∼4 μs) CS state BPIPt–D+A–, of which the lifetime is considerably increased by an applied magnetic field of 270 mT. The positive magnetic field effect (MFE) is in contrast to the negative MFE for the reference DA molecule, which indicates successful switching of the initial spin state of the CS state from singlet to triplet. Simulations of the MFE and time-resolved electron paramagnetic resonance show that spin-selective charge recombination and spin relaxation are unaffected by attachment of BPIPt. The minimum impact of heavy atom substitution on the electronic and magnetic properties has been realized by the small electronic coupling mediated by the rigid meta-triethynylbenzene.

The effect of the solvent viscosity dependence of time-resolved magnetoluminescence (ML) on the delayed fluorescence of 9,10-diphenylanthracene (DPA) sensitized by platinum octaethylporphyrin has clarified the structure and dynamics of the triplet–triplet pair (TT), i.e., the transition state of triplet fusion. Phase inversion of the ML effect with time provides evidence for the recycle dynamics of the excited triplet state for DPA in triplet fusion. The electron spin-relaxation by random molecular rotation causes intersystem crossing among the different spin states of the triplet–triplet pair and allows the 3,5TT to engage in triplet fusion. Therefore, slow-down of the molecular diffusion by an increase in the solvent viscosity can enhance the triplet fusion yield. However, the reduction of the ML effect observed in quite high viscosity solvents suggests that the substantially slow rotational motion decreases the triplet fusion yield due to steric factors in electron exchange from the triplet–triplet pair.

The spin sublevel dynamics of the excited triplet state in thermally activated delayed fluorescence (TADF) molecules have not been investigated for high-intensity organic light-emitting diode materials. Understanding the mechanism for intersystem crossing (ISC) is thus important for designing novel TADF materials. We report the first study on the ISC dynamics of the lowest excited triplet state from the lowest excited singlet state with charge-transfer (CT) character of TADF molecules with different external quantum efficiencies (EQEs) using time-resolved electron paramagnetic resonance methods. Analysis of the observed spin polarization indicates a strong correlation of the EQE with the population rate due to ISC induced by hyperfine coupling with the magnetic nuclei. It is concluded that molecules with high EQE have an extremely small energy gap between the 1CT and 3CT states, which allows an additional ISC channel due to the hyperfine interactions.

We demonstrate the magnetoconductance (MC) effect originated from depressing the spin mixing in encounter pairs under the external magnetic field provides quantitative information about the singlet fission, the charge recombination, and the trap-related dynamics with triplet exciton in a bilayer device of pentacene (Pen) and fullerene (C60). Three MC effects in low-, moderate-, and high-fields were detected in the bilayer device at room temperature. Kinetic analysis of the low-field MC effect showed that the charge recombination yield at the Pen|C60 interface is ∼1%. Quantum mechanical simulations for dynamics of spin-carrying pairs following the conservation rule of spin angular momentum in recombination showed that the moderate- and high-field MC effects are caused by, respectively, the trap-related dynamics with triplet exciton and the singlet fission with a maximum yield of 52% in the layers. The quantitative information obtained by investigating the MC effect will contribute to the development of high-efficiency organic solar cells devices.

Measurements of transient photoconductance under an external magnetic field were used to investigate photocarrier dynamics in low-dimensional hexabenzocoronene (HBC) self-assemblies, which are a promising material group for highly efficient solar cells achieved by bottom-up technology, and to clarify the effect of lamination with electron acceptor layer on the surfaces of HBC nanotubes. In an HBC column without an acceptor, the carrier generation yield from a geminate electron–hole (e-h) pair is dependent on the external electric and magnetic fields. The time dependence of the magnetic field effect on geminate e-h pair dynamics in the HBC column structure was analyzed to estimate the recombination rate constants of the singlet and triplet e-h pairs (krS and krT), which were 1.5 × 108 and 1.2 × 108 s–1 respectively. The same kinetic analysis with consideration of the electric field effect on the photocarrier generation yield provided an electric field dependent dissociation rate constant in the range of 107–108 s–1 in the HBC column structure. However, neither electric nor magnetic field effects on the carrier generation process were observed in acceptor-appended HBC nanotubes. The disappearance of the external field effects in acceptor-appended HBC indicates that the geminate recombination is reduced substantially by a well-organized donor/acceptor heterojunction with an interval of a few nanometers due to some σ-bonds. However, efficient nongeminate recombination with a ratio of krS:krT = 1.0:0.8 in the acceptor-appended HBC nanotubes was also elucidated by the incident photon density and magnetic field effects, which is an inherent nature in materials with high carrier density.

By means of the integral mode time-of-flight measurement of the drift motion of the photoinjected charge at room temperature, we observed magnetic field effect (MFE) on the photocharge in self-assembled nanotube of hexabenzocoronene (HBC) at the configurations where the magnetic field is parallel and perpendicular to the electric field. The detected MFEs are interpreted in terms of two mechanisms that are caused by the fast charge transportation (Hall effect) and the spin selective recombination (electron–hole (e–h) pair mechanism). A high mobility in the nanotubes of π-stacked HBC was implied from the Hall data. The time dependence of the low field MFE due to the e–h pair mechanism clarified the recombination from both the singlet and triplet e–h pairs with the initial rate of ∼108 s−1.

An amorphous molecular semiconductor, poly(N-vinylcarbazole) (PVCz), exhibits negative giant magnetoresistance (MR) under ambient conditions. The application of a weak magnetic field of 10 mT to PVCz films doped with lumichrome, in which light excitation immediately creates triplet electron-hole (e-h) pairs that are precursors of photocarriers, causes a steep decrease in resistivity by more than 20% at ambient temperature. Further, the resistivity of the doped film gradually reduces by approximately one-half under a field of 1 T, equivalent to an MR ratio of −55%. In addition, anomalous spikes are also detected at 0.07, 0.30, and 9.0 T, indicative of an exponential dependence with a decay distance of 0.1 nm in the exchange interaction of the e-h pair. A quantum mechanical calculation based on the density operator formalism clarifies that the observed MR effect can be comprehensively understood by the spin-selective charge dynamics and the coherent and incoherent spin dynamics of the e-h pair in a quasi-one-dimensional lattice for photocarrier generation. Model calculations also indicate the importance of the spin-lattice relaxation for the giant MR effect in organic molecular semiconductors.

By means of the electron spin resonance (ESR) technique, we have investigated the electronic structures of the tridentate imino nitroxyl diradical complex with copper(II) (Cu−bisimpy), which has a square planar structure and a ground quartet state with an extremely strong ferromagnetic exchange interaction, and its related compounds (bisimpy = 2,6-bis(1′-oxyl-4′,4′,5′,5′-tetramethyl-4′,5′-dihydro-1′H-imidazol-2′-yl)pyridine). It was clarified that Cu−bisimpy had unique magnetic orbitals, compared with the biradical ligand (bisimpy), a zinc(II) biradical complex (Zn−bisimpy) and a copper(II) terpyridine complex (Cu−tpy) (tpy = 2,2′;6′,2′′-terpyridine). Multifrequency ESR spectroscopy provided a reliable set of magnetic parameters of Cu−bisimpy, which has a small g anisotropy (gx = 2.02, gy = 2.01, gz = 2.08) and small hyperfine coupling with Cu (|Ax| = 42.0 MHz, |Ay| ≤ 14 MHz, |Az| = 153 MHz) but huge zero-field splitting (D = +17.4 GHz, E = −1.0 GHz). The maximum principal axis of zero-field interaction (zZF) is perpendicular to the z axis for the g and A tensors, which is normal to the molecular plane. These characteristics of the magnetic properties prove that the substantial spin transfer from the dx2−y2 orbital of copper to the n-orbitals of the ligand is caused by a σ-type covalent bonding effect between the central metal and the ligand nitrogens. The covalent bonding effect produces carbene configurations on the nitrogen atoms of the imino nitroxyl radicals. The carbene configuration was concluded to be the main reason for the strong ferromagnetic coupling in Cu−bisimpy. Multifrequency electron spin resonance spectroscopy clarified the unique electronic structure of a square planar copper(II) complex with an imino nitroxyl diradical, which undergoes a strong ferromagnetic interaction caused by a covalent bonding effect.