Overview of Research
1. Photon-Gated Function of Complex Molecular Systems
2. Supramolecular Assembly Through Metal Complexation
3. Modulation of Nematic Liquid Crystalline Alignment By Photochromic Molecules
4. Photoredox Chemistry of Transition Metal Complexes
5. Coulombic Effect on Photoinduced Electron Transfer Reactions
6. Photochromoic Reaction of Thin Solid Films
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1. Photon-Gated Function of Complex Molecular Systems.
The chemistry of spiropyrans has been extensively studied with a special
regard to their photochemical C-O bond rupture and recombination at the
spiro position, providing remarkable photochromism between colorless, neutral
spiro-form (Sp) and colored, zwitter-ionic merocyanine form (Mc) as shown
in Scheme-1. It is amazing that such the intense change of molecular color
can be successively given by generating p-conjugation over the whole molecule
after rearrangement of the s-p hybridization of a single C atom. Introduction
of ionic atmosphere in the vicinity of the molecule or replacement of the
solvent can modify the state energy of Sp and that of Mc independently,
and the total photochromic reactivity as well. Strong coupling between
the photochromic reactions and the molecular environment indicates potential
usefulness of photochromism in enhancing, switching or controlling the
physicochemical functions of neighboring molecular systems, e.g. electronic
structures, catalytic reactivity, intermolecular interaction, etc, under
light stimuli. There have been explored fundamental structures of photochromic
molecules, and we are trying to develop a secondary effect by construction
of hybrid molecules.
Get 'Chime plug-in' to see the structure of calixsp2 in MDL format.
We use calixarenes as substructure to hold several molecules together.
Calixarene family has been widely used as a building block for the design
and construction of supramolecular structures, especially in terms of molecular
recognition functionality. Multiple functional molecules can be brought
together onto an identical calixarene rim in a spatially controlled manner,
within short distances, photochromism-promoted chemical function should
be enhanced in such molecular architecture. We are investigating a novel
photo-ionic switching behavior of a metal-inclusion complex of a p-tert-butylcalix[4]arene
derivative bearing two photochromic nitrospirobenzopyran moieties (Calix-Sp2
, Figure-1). In this system, complexation between ions and Calix-Sp2 was
well-controlled by the photochromic reaction of appended spiropyrans. UV
irradiation to the metal free acetonitrile solution gave a rise of colored
Mc form with its maximum absorption at 567nm, the lifetime of which was
ca.140s in the dark. Addition of LiI (2.0 mM) resulted remarkable absorption
shift (537nm) and reduced the apparent decay rate down to one-tenth of
the metal-free condition. ESI-MS measurement clearly indicates formation
of the metal-inclusion complex Calix-Sp2/Li+. By careful examination of
decoloration process, complexation and its photoionic process were clarified.
Stability constant (Kc) of Calix/Li+ strongly depends on the number of
photogenerated Mc and on the structure of spacer. ESI-MS signal of the
Li+ complex drastically dropped off in Calix-Mc from that of Calix-Sp.
Diminution of ESI-MS signal corresponds to photo-switching of the ligating
principle from cavity-based one to MC-based ionic complexation (Scheme-2).
As the former, amid based bonding would dominate complexation, since even
under high Li+ concentration, Calix-Mc analogue without amid unit shows
little change in visible spectra and decoloration rate compare to the metal
free condition.
Scheme-2
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2. Supramolecular Assembly Through Metal Complexation
Assembly of functional pigments toward a well-designed structure with suitable
sequence, alignment, distance and molecular interaction has been a matter
of increasing attention in view of a precisely controlled photoreaction
system. Such strategic assembly would be achieved by incorporating the
pigments into a relatively immobile polymer backbone by a particular manner.
Metal complex system would be a good backbone, since variety of functional
groups can be introduced as a ligand structure so that it keeps specific
orientation to a metal center. On the other, many efforts have been made
to develop photocatalytic abilities of the metal complexes consisting of
platinum group elements. Those complexes often exhibit intense absorption
in visible region, high redox potential and a long-lived, emissive excited
state based on the metal-to-ligand charge transfer (MLCT). Connecting hetero
metal elements through a-diimine type bridging ligands (BL), we can anticipate
a vectorial and sequential electron or energy transfer over the complex.
As for such systems have been demonstrated trimetallic complexes consisting
of Ru, Os, Ir, and Rh as metal elements, and 2,2'-bipyrimidine (bpm), 2,3-bis(2'-pyridyl)pyrazine
(dpp), 2,3- bis(2'-pyridyl)quinoxaline and 2,3-bis(2'-pyridyl)benzoquinoxaline
as BL. Although such the d6 octahedral transition metal center has been
preferably used as a principal constitutional unit, consecutive connection
of octahedral centers will restrict structural development of the polymetallic
system. My efforts are thus focusing on incorporation of a square planar
d8 center, Pt(II), to bring a variety of functioning structure and reactivity
into polymetallic architecture.
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3. Modulation of Nematic Liquid Crystalline Alignment By Photochromic Molecules
Transformation of photochromic molecules induces a reversible change in
the organized structure of nematic liquid crystalline (LC) phase. When
azobenzene (Az) moieties are introduced into the inner wall of a nematic
LC cell, the alignment axis of the nematic phase can be switched between
perpendicular and parallel to the wall as responding to photo-transformation
of the surface Az between trans and cis forms, respectively. The factors
dominating this "command-surface" phenomenon were studied on
the commanding ability of various Az structures including side-chain Az
polymers. One of the most important factors is compatibility or affinity
at the interface between the trans-Az and the LC molecules to establish
a photo-controllable, strong interaction. Thus. photochemical induction
and rotation of the nematic in-plane axis also become possible under irradiation
of linearly polarized UV light or slantwise irradiation of nonpolarized
UV light. These events are explicit by concerning photoselection of the
trans-Az chromophore in a restricted configuration. Conoscopic observation
for the latter indicates that the nematic tilt develops toward the reaction
light, namely phototropic. Relaxation time of the nematic alignment varies
from 1000 to 12 ms depending on the incident energy of reaction Nd-YAG
THG pulses, which suggests a considerable change of the surface anchoring
force by transforming trans-Az into cis-Az.
Scheme 3-1 Reversible alignment change of Nematic LC phase
indueced by the photochromic reaction of surface Az molecules.
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4. Photoredox Chemistry of Transition Metal Complexes
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5. Coulombic Effect on Photoinduced Electron Transfer Reactions
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6. Photochromic Reaction of Thin Solid Films
Switching capability based on photochromic transformation has been attracting attention. Photochromism brings about not only reversible changes in molecular and electronic structures of a single chromophore but also those of assembled molecules applicable to optic/photonic molecular arrays . There are quite beneficial aspects in application of photochromism such as quick response as fast as photochemical rearrangement, high resolution close to the light wavelength, reversibility, use of light characteristics (polarization, phase, wavelength), etc., and to which we can adapt materials by introducing molecular architecture in variety of organic and polymeric compounds. We have been studying orientation of a nematic liquid crystalline phase on thin films made up of a series of photochromic molecules(command surface). On a way, it has been found that the nematic orientation can be conducted precisely by the photochemical transformation of interfacial chromophores, e.g. azobenzene (Az), and the nematic tilt angle is tunable as responding to the photo-stationary ratio between trans-cis isomers given by the steady irradiation at various wavelengthen We have also demonstrated stepwise increment of the nematic tilt and acceleration of the response time along with excitation by YAG laser-pulses. Evaluation of the photochromic process in a thin solid film becomes essential in the sense of controlling molecular arrays. We have attempted simple formulation of the laser-pulse promoted photochromic process in a thin solid film and its application to the photoisomerization of an Az film.
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