Kenkichi Sakai

Luminescent charge-transfer complexes of 4,4′-bipyridinium ion with tetrakis[3,5-bis(trifluoromethyl)phenyl]borate anion

Abstract 4,4′-bipyridinium ions form ion-pair charge-transfer (CT) complexes with tetrakis[3,5-bis(trifluoromethyl)phenyl] borate anion with a discrete absorption maximum at 420 nm in methanol and at 475 nm in 1,2-dimethoxyethane. These CT complexes in 1,2-dimethoxyethane at 20°C show broad structureless luminescence with a maximum at 553 nm, almost a mirror image of the CT absorption spectrum. The luminescence could not be observed for CT complexes in methanol. This is the first observation of luminescence from excited ion-pair CT complexes in solutions at room temperature.

Photoinduced electrochromism in monolayer assemblies of ion pair change transfer complexes of 4,4′-bipyridinium ions

Abstract A 1:4 mixture of N,N ′-dihexadecyl-4-4′-bipyridinium salt with tetrakis[3,5-bis(trifluoromethyl)phenyl]borate (TFPB − ) and arachidic acid formed a Y-type Langmuir-Blodgett (LB) film. On excitation of an ion pair change transfer (CT) band (λ ex > 365 nm) in vacuo or in argon, the colour of the mixed monolayer assemblies changed from pale yellow to blue. From UV-visible and electron spin resonance spectra, such a colour change was attributed to 4,4′-bipyridinium cation radicals formed by the photoinduced electron transfer from TFPB − . 4,4′-bipyridinium cation radicals in monolayer assemblies reversibly decayed in an inert atmosphere. These results demonstrated the photoinduced electrochromism by ion pair CT complexes in LB films.

Novel photochromism in 4,4′-bipyridinium monolayer assemblies via excitation of ion-pair charge-transfer complexes

N,N′-Dihexadecyl-4,4′-bipyridinium salts with tetrakis[3,5-bis(trifluoromethyl)phenyl]borate in mixed monolayer assemblies with arachidic acid showed photochromism due to the electron transfer reaction via excitation of an ion-pair charge-transfer band (λex. > 365 nm).

Reversible and steady photogeneration of 4,4′-bipyridinium radical cations via the excitation of ion-pair charge-transfer complexes between 4,4′-bipyridinium and tetrakis-[3,5-bis(trifluoromethyl)phenyl]borate in organic solutions

The excitation of an ion-pair charge-transfer (CT) band (λex > 365 nm) of 4,4′-bipyridinium salts with tetrakis[3,5-bis(trifluoromethyl)phenyl]borate (TFPB–), in degassed organic solutions, results in the accumulation of 4,4′-bipyridinium radical cations which disappear reversibly with a lifetime of ca. 1 h at 20 °C. The photogeneration and thermal decay of 4,4′-bipyridinium radical cations were shown to be highly reversible by both u.v.-visible and e.s.r. spectroscopies. The radical formation efficiency was almost constant over a wide timescale of the CT complex formation apart from high value in the early stages. These results suggest that the extraordinarily bulky and stable TFPB– anion works as a reversible electron donor in the reversible and steady photoreduction of 4,4′-bipyridinium ion via the excitation of an ion-pair CT band.

Reversible solid-state photoreduction of 4,4′-bipyridinium ion via excitation of its charge-transfer complex with tetrakis[3,5-bis(trifluoromethyl)phenyl]borate anion

Excitation of charge-transfer absorption in the solid state of 4,4′-bipyridinium salts of tetrakis[3,5-bis(trifluoromethyl)phenyl]borate in vacuo or under Ar gas resulted in the accumulation of 4,4′-bipyridinium cation radicals which disappeared reversilbly upon heating or dissolution.

Control of molecular orientation of 4,4′-bipyridinium cation radicals in novel photochromic monolayer assemblies

The orientation of 4,4′-bipyridinum cation radicals, reversibly formed in novel photochromic monolayer assemblies of tetrakis[3,5-bis(trifluoromethyl)phenyl]borate salts with N,N′-dihexadecyl-4,4′-bipyridinium and N-ethyl-N′-(2-ethylamide)-N″, N″-dihexadecyl-4,4′-bipyridinium ions via excitation of an ion-pair charge-transfer (CT) band (λex.>365 nm), was controlled by the substituents of the 4,4′-bipyridinium ion.

Molecular orientation of photogenerated radicals in novel photoelectrochromic Langmuir-Blodgett films

Abstract Y-Type Langmuir-Blodgett (LB) films were formed from several mixtures of tetrakis[3,5-bis(trifluoromethyl) phenyl] borate salts of N , N ′-dihexadecyl-4,4′-bipyridium (HV) and N -ethyl- N ′-(2-ethylamide)- N ″, N ″-dihexadecyl-4,4′-bipyridinium (AV) ions with arachidic acid (AA). 4,4′-Bipyridinium radical cations were generated in LB films upon irradiation of their charge-transfer (CT) bands (λ ex > 365 nm) in vacuo , which reversibly and very slowly decayed in the dark. Molecular orientation of photogenerated 4,4′-bipyridinium radical cations was determined by polarized absorption spectra measured in vacuo . It was shown that 4,4′-bipyridinium radical cations lay flat in AV/AA and inclined by about 46° in HV/AA LB films.

Reversible control of emission from ion-pair charge-transfer complex of 4,4′-bipyridinium with tetrakis[3,5-bis(trifluoromethyl)phenyl]borate anion by electron transfer reaction

Abstract 4,4′-Bipyridinium salts with tetrakis[3,5-bis(trifluoromethylphenyl)]borate anion (TFPB − ) showed a charge-transfer (CT) absorption above 350 nm and broad structureless CT fluorescence with a maximum at 525 nm in 1,2-dimethoxyethane (DME). The excitation of a CT band ( λ ex = 475 nm) in an oxygen-free atmosphere resulted in the quenching of CT fluorescence accompanied by a remarkable colour change from pale yellow to blue owing to the accumulation of 4,4′-bipyridinium radical cations. After disappearance of blue colour in about an hour at 20°C, the CT absorbance and fluorescence intensity in DME recovered very gradually to the equilibrium values. The CT fluorescence and colour changes were repeated reversibly. These results demonstrated that 4,4′-bipyridium TFPB − salts can be applied to a photon-mode-type optical memory which can be read with either absorption or emission.