Hideki Yamochi

Photoinduced phase transition (PIPT) involves substantial changes in both electronic and crystallographic structures caused by electron excitations. Because PIPT is ultrafast and exhibits inhomogeneous behavior, its fundamental processes, in particular, how the ultrafast dynamics of photoexcited electrons trigger lattice deformation and the timescales involved, remain poorly understood. In this study, phase transition and its spatial separation in organic charge-transfer complexes was investigated using photoemission electron microscopy with femtosecond pulsed laser excitation. First, the thermally induced phase transition was confirmed by imaging the morphology change and also by observing the change of spectral shape around the Fermi level. Second, the escape time of photoexcited electrons or the heat generated by the laser pulses is estimated by observing the morphology change depending on the laser repetition rate. Finally, the dynamics of the photoexcited electrons and the subsequent morphology change are directly visualized. The results show that an anisotropic morphology change is initiated following the accumulation of electrons in unoccupied states over approximately a picosecond.

Crystals of {Cryptand(K ⁺ )} ₂ {(CoI2Co ^II O) ²⁺ (Hhp˙ ⁴⁻ )} ²⁻ ·2C ₆ H ₄ Cl ₂ (1) were obtained from H ₃ Hhp and Co ^II (acac) ₂ under reduction. Salt 1 demonstrates a metal-to-macrocycle charge transfer and low-energy d–π optical transition.

Radical anion salts (astrophloxine⁺)₂(C₆₀^·-)₂×C₆H₄Cl₂×C₆H₁₄ (1) and (astrophloxine⁺)₂(C₇₀⁻)₂ (2) have been obtained with cationic dye. Singly bonded (C₆₀⁻)₂ dimers are partially formed in 1, whereas singly bonded (C₇₀⁻)₂ dimers are found in 2.

A series of complexes containing dianionic and radical-trianionic hexaazatriphenylene (HATNA) and hexazatrianthracene (HATA) ligands and high-spin Mn^II ions (S = 5/2) were obtained. It is shown that elongation of the Mn–N bonds results in a decrease of both Mn–ligand and Mn–Mn couplings.

Crystalline {Cryptand(Na⁺)}[(COD)Rh^ICl⋅Sn^II(Pc³⁻)]⁻⋅2C₆H₄Cl₂ (1) and {Cryptand(Cs⁺)}[(COD)Rh^I⋅Sn^II(Pc⁴⁻)]⁻⋅C₆H₅CH₃ (2) complexes were obtained via the interaction of [Sn^II(Pc³⁻)]⁻ and [Sn^II(Pc⁴⁻)]²⁻, respectively, with organometallic {(COD)RhCl}₂ dimer (COD is 1,5‐cyclooctadiene). Dissociation of {(COD)RhCl}₂ followed by the Rh−Sn binding is observed at the formation of 1. Elimination of the chlorine atom at the rhodium atom is observed in 2, and rhodium is additionally coordinated to the imine nitrogen atom of Pc⁴⁻. The complexes contain mono‐ Pc⋅³⁻ and doubly reduced Pc⁴⁻ species, respectively, that is supported by the data of XRD analysis as well as optical and magnetic properties of 1 and 2. There is an alternation of C‐N_imine bonds in the macrocycles, which gradually increases with increasing negative charge on the macrocycle. The difference between shorter and longer bonds increases from 0.051 Å in Pc³⁻ to 0.075 Å in Pc⁴⁻. The formation of 1 is accompanied by an essential blue shift of the Q‐band of starting SnPc and the appearance of a new intense band at 1031 nm. The even stronger shift of the Q‐band is observed in the spectrum of 2, but the band in the near‐IR range becomes weaker. The value of effective magnetic moment of 1 is 1.76 μ_B at 300 K corresponding the contribution of the Pc³⁻ radical trianions (S=1/2). Only weak magnetic coupling with the Weise temperature of −3 K is observed in 1 due to weak π–π interaction between the macrocycles in the chains. Paramagnetic Pc³⁻ species additionally monitored by EPR spectroscopy show a strong temperature dependence of g‐factor and linewidth of the EPR signal. Complex 2 is diamagnetic and EPR silent.

Heterobimetallic {[Co(CO)₄]⁻[In^III(Pc²⁻)]} (1) and (Cp*₂Cr⁺){[Mn(CO)₅]⁻[In^III(Pc˙³⁻)]}·2C₆H₄Cl₂ (2) complexes based on indium(III) phthalocyanine (Pc) were obtained as crystals. The complexes were synthesized by single (1) and double (2) reduction of indium(III) phthalocyanine chloride in the presence of transition metal carbonyls. Complex 1 contains dianionic Pc²⁻ macrocycles. Thus, the coordinated Co(CO)₄ carbonyl accepts an electron in the one-electron reduction forming a diamagnetic [Co(CO)₄]⁻ anion. Complex 2 contains a heterobimetallic {[Mn(CO)₅]⁻[In^III(Pc˙³⁻)]}⁻ anion and paramagnetic Cp*₂Cr⁺ counter cations. Therefore, in the double reduction, electrons are transferred to Mn(CO)₅ forming a diamagnetic [Mn(CO)₅]⁻ anion and to the Pc²⁻ macrocycle forming a paramagnetic radical Pc˙³⁻ trianion. Such assignments for 1 and 2 are in line with optical spectra, crystal structures and the data of magnetic measurements. The spectrum of 1 in the UV-visible range is similar to that of the starting In^IIIClPc. The formation of 2 is accompanied by an essential blue-shift of the Q-band of Pc as well as by the appearance of an intense NIR band at 1005 nm characteristic of Pc˙³⁻. Compound 1 is EPR silent and diamagnetic, whereas the value of the effective magnetic moment of 2 is 4.24μ_B at 300 K, which corresponds to the contribution of S = 1/2 (Pc˙³⁻) and S = 3/2 (Cp*₂Cr⁺) spins. Both weakly coupled paramagnetic centers (J = −0.41 cm⁻¹) are observed in the EPR spectra.

A series of trianion assemblies with hexaazatriphenylenehexacarbonitrile {HAT(CN) ₆ } and hexaazatrinaphthylenehexacarbonitrile {HATNA(CN) ₆ } with Fe( ii ) or Co( ii ) ions are obtained. Parameters affecting the magnetic properties of the assemblies are discussed.