Abstract:
A photochromic device having a photochromic layer including a photochromic material which exhibits absorbance in a visible region upon being sensitized by a light having a wavelength of not less than 700 nm, and an ultraviolet light blocking device for blocking an ultraviolet light from sensitizing the photochromic material in the photochromic layer. The ultraviolet light blocking device includes at least one of an ultraviolet shielding layer positioned to shield the photochromic layer from the ultraviolet light and an ultraviolet light absorber included in the photochromic layer.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to a photochromic compound, a photochromic composition, and a photochromic functional element using the same. More particularly, the present invention relates to a photochromic functional element which can be used in an anti-glare mirror for automobile or such.  
         [0003]     2. Description of the Related Arts  
         [0004]     In the conventional automobile mirror, a technique has been developed which imparts mirror to an anti-glare property in order to improve visibility. The term “anti-glare property” used herein is a property that sunshine in the daytime, a headlight of next car in the nighttime and the like are reflected by an automobile mirror to decrease dazzling felt by a driver or a fellow passenger. As a mirror possessing such an anti-glare property, there is a mirror which utilizes a color mirror (so-called blue mirror) having a reflectance peak at a blue side (short wavelength side) and having a blue color on the surface thereof.  
         [0005]     The human visibility has a peak at a wavelength of approximately 555 nm under a light condition, and yellowish green color becomes clearly visible. As the surroundings become dark, the peak of the visibility is gradually shifted toward a blue side, and the peak wavelength is to be at approximately 505 nm. In the blue mirror, the peak of reflectance characteristic exists in a range of from 400 to 510 nm. Consequently, the image reflected by the blue mirror has a peak deviating from the peak of the human visibility in the daytime and, thus it looks slightly dark, preventing dazzling of the sunshine. In the nighttime, the blue mirror has a peak accorded with the peak of the human visibility and, thus, the reflected image looks bright, excelling in visibility.  
         [0006]     Typical examples of the conventional techniques which impart anti-glare property to an automobile mirror include those which apply a liquid crystal material, electrochromic material or such. Amongst them, a reflectance-variable, automobile mirror utilizing an electrochromic material will be described (See Japanese Patent Laid-Open No. 09-120,088).  
         [0007]     As shown in  FIG. 8 , in an automobile mirror  300  having a reversibly variable transmittance, a prism-shaped mirror  180  is laminated on a surface  131  of a device having a reversibly variable transmittance by means of a transparent laminating material (layer  19 ). The prism-shaped mirror  180  comprises a prism-shaped piece  18  composed of an essentially transparent solid material (such as glass or transparent plastic), and a layer  18 A of a highly reflective material (such as silver) adhered onto the surface of the solid material by a technique common to the mirror processing field, so that a light passing through the solid material and arriving at the reflective material layer is reflected toward the original direction via the solid material layer in a high ratio (preferably at least approximately 80%). The highly reflective layer  18 A makes up reflective means for mirror.  
         [0008]     The mirror comprises devices (electrochromic devices) having a reversibly variable transmittance through which a light before and after reflected from the reflecting means is passed. A wall  100 , electrochromic elements  10  and  10 A; a wall  130 , electrochromic elements  13  and  13 A; a spacer  11 ; a solution space  12 ; and a line or a strip  16  shown in  FIG. 8  correspond to elements for the device (not shown), and the line or strip shown in  FIG. 8  extends to lead lines of the device (not shown). The lead lines of the device (not shown) are connected to an element for controlling power supply (for example, switching means, means for controlling electric potential between electrodes).  
         [0009]     The automobile mirror having a reversibly variable transmittance configured as described above is usually in a bleached state, where a light entering from the wall  100  (which is a solid material comprising glass or transparent plastic) is passed through the interiors of the electrochromic elements  13  and  13 A, and is reflected at the reflective means  18 A to be in a highly reflective state, exhibiting a function as a mirror. At the time of coloration, the light transmittance thereof is decreased so that the device is to be in a low reflective state where the reflected light is decreased, exhibiting an anti-glare function. In the case where such electrochromic elements are utilized in antiglare mirror, responding to the application of voltage and to making a short circuit, mutual switching between the coloration at the time of exhibiting anti-glare function and the bleaching at a usual mode is carried out.  
         [0010]     However, in the automobile mirror having a reversibly variable transmittance utilizing such electrochromic elements, a sensor, which can sensitize a relatively strong light such as sunshine and a light from a head light of next car, a control circuit which controls the actuation of the electrochromic elements based on a signal produced from the sensor and other device should be separately required, leading to a problem associated with complicated configuration.  
         [0011]     So, glazing has been suggested in Japanese Patent Laid-Open No. 10-114007 in which a photochromic material, whose absorbance at a visible light region is reversibly varied upon irradiation of a light with a specific wavelength is applied to glazing so that the glazing can become dark upon receiving a strong light such as sunshine whereby the light reflectance thereof is decreased to prevent a driver and others who get into a car from being dazzled (Japanese Patent Laid-Open Publication No. 10-114007).  
         [0012]     Examples of the photochromic materials applied to this glazing include a material comprising a silver salt, particularly a silver halide, as an active component dispersed in a glassy matrix so that the active component such as the silver halide just mentioned is reversibly transformed into a state of a metal bonded body, and a material comprising an organic dyestuff (pigment) as an active component dispersed in a polymeric matrix.  
         [0013]     In the glazing utilizing the later material, the photochromic material absorbs a light at a ultraviolet region to thereby be reversibly isomerized, causing coloration or bleaching in a reversible manner. Typical examples of the organic pigments include compounds derived from spirooxazine and spiropyran.  
         [0014]     However, in the glazing utilizing the photochromic material described above, irradiation of ultraviolet contained in sunshine is required for causing coloration and bleaching of the photochromic material in a reversible manner. For this reason, even if a light from a head light of an oncoming car enters into the glazing, it has been difficult to colors the mirror enough for preventing a driver or such from dazing due to its weak strength of ultraviolet possessed by the automobile head light. Consequently, in such a case, a light source, which can irradiate with the photochromic materials a ultraviolet light at a strength sufficient for reversibly changing coloration and bleaching, is required to be separately placed. This poses a problem in terms of insufficient practical use.  
         [0015]     The present invention has been made in light of the above problems, and a first object of the present invention is to provide a photochromic material, which does not exhibit any photochromic property against a light having a wavelength within a visible region and which absorbs a given wavelength within a wavelength region of not less than 700 nm (particularly infrared region) into a visible region, and to provide a functional element using such a phenomenon. Specifically, in the photochromic material and the photochromic phenomenon according to the present invention, since it sensitizes a specific wavelength within a wavelength region of not less than 700 nm (particularly infrared region) to exhibit a photochromic property, a light having a ultraviolet wavelength region is not required for the exhibition of a photochromic property.  
         [0016]     A second object of the present invention is to provide a functional element such as a photochromic display element applicable to an automobile mirror having an anti-glare property utilizing such a photochromic material and such a photochromic phenomenon as described above.  
       SUMMARY OF THE INVENTION  
       [0017]     We have examined various compounds each having an electron donor and an electron acceptor within the molecule thereof, and exhibiting a photochromic phenomenon upon being excited with a specific light for the characteristics of their excitation wavelength.  
         [0018]     As a result, it has been clarified that when a light (such as a xenon light source) having a prescribed energy at a specific wavelength (for example, 830 nm) within an infrared region of not less than 750 nm is irradiated to 4,4′-bipyridine derivative represented by the following formula (1):  
                         
 
 the absorbance of the 4,4′-bipyridine derivative at approximately 610 nm as a center is increased and the 4,4′-bipyridine derivative is blue-colored, while it becomes gradually a transparent color, when the irradiation of the above-mentioned light is stopped or (the derivative is placed in a dark portion to decrease the light absorbency thereof. 
 
         [0019]     It is noted that the conventional photochromic compound is required to be irradiated with a light having a ultraviolet light region of from 380 nm to 400 nm to exhibit its photochromic phenomenon as a rule.  
         [0020]     With regard to compound analogue to the 4,4′-bipyridine derivative represented by the formula (1) described above (derivatives in which a benzene ring at the terminal of the dendrimer construction adducted to the 4,4′-bipyridine is substituted with other substituent (e.g., naphthalene)), Ghaddar et al., of North Carolinian University have been reported the change in the absorbance of various 4,4′-bipyridine derivatives at a wavelength of 600 nm, which is within a visible light region, excited by a ultraviolet light having a wavelength of 266 nm (Journal of American Chemical Society, 2002, 124, P8285-8289). However, this report shows the photochromic property within a ultraviolet light region, and does not show any photochromic property against irradiation of a light having a wavelength region of not less than 700 nm (especially infrared region). Also, this report does not disclose the 4,4′-bipyridine derivative represented by the formula (1) described above, (derivatives whose the terminal of the dendrimer construction adducted to the 4,4′-bipyridine is a benzene ring).  
         [0021]     In contrast, we have clarified for the first time that the 4,4′-bipyridine derivative represented by the formula (1) sensitizes a specific wavelength of an infrared region to exhibit a photochromic phenomenon. Also, by the use of the 4,4′-bipyridine derivative having such a characteristic, a photochromic display element has newly realized, which sensitizes a specific wavelength of an infrared region contained in a head light utilizing a white and bright xenon light source, which has recently appeared and which has been used in a part of an automobile, to exhibit a blue color. It has been found that a functional element utilizing a photochromic phenomenon that sensitizes a wavelength of not less than 700 nm or specific wavelength within a infrared region and exhibits absorption in a visible wavelength region can be realized, resulting in the present invention.  
         [0022]     Such a phenomenon that a colored state and bleached state are reversibly exhibited by irradiation of a specific wavelength within an infrared region and non-irradiation is considered to bring about a photochemical reaction of the 4,4′-bipyridine derivative represented by the formula (1) where at the time of coloration, it is reduced to be colored and at the time of bleaching, it is in a stable construction due to the reversible reaction.  
         [0023]     It has been clarified that the photochromic material, which attain these and other objects of the present invention may be made up of any of substances having a construction or properties analogies to that of the 4,4′-bipyridine derivative (biologen derivative) represented by the formula (1). It has also been clarified that depending upon the construction or such, these substances each sensitizes a light having a prescribed energy at a specific wavelength of not less than 700 nm, being not restricted to an infrared region of not less than 750 nm, to exhibit a photochromic phenomenon.  
         [0024]     For example, a 4,4′-bipyridine derivative represented by the following formula (2) in which the terminal benzene ring of the dendrimer construction adducted to 4,4′-bipyridine is substituted with other condensed ring (such as naphthalene) falls under the photochromic material according to the present invention.  
                         
 
 wherein R 1 , R 2 , R 3 , and R 4  may be the same or different from each other and each is a condensed aromatic hydrocarbon or a derivative thereof. 
 
         [0025]     Also, a 4,4′-bipyridine derivative represented by the following formula (3) falls under the photochromic material according to the present invention.  
                         
 
 wherein R 11  and R 12 , may be the same or different from each other and each is an alkyl group having 1 to 10 carbon atoms or a derivative thereof, and X −  is selected from among Cl − , BF 4   − , PF 6   − , AsF 6   − , ClO 4   − , and NO 3   − . 
 
         [0026]     The photochromic composition according to the present invention comprises a solution having the photochromic compound according to the present invention dissolved in one solvent or a mixed solvent selected from among dimethylformamide (DMF), dimethylacetamide, propylene carbonate, acetonitrile, gamma-butyllactone, and butanol.  
         [0027]     A photochromic film according to the present invention may comprise a film having the photochromic compound according to the present invention contained in a dissolved form.  
         [0028]     By such a configuration, a photochromic composition having the photochromic compound according to the present invention dispersed in a prescribed solvent or a polymer film having the photochromic compound according to the present invention dispersed therein may be composed, which makes it easy to realize various functional elements applied to the photochromic compound according to the present invention such as display elements. What is more, the use of the photochromic compound according to the present invention meets a production of functional elements satisfying various needs including its function, its performance, and a cost.  
         [0029]     It is noted that the photochromic film according to the present invention may be formed, for example, by a process comprising adding PVP (polyvinyl pyrolidone), polymethyl metacrylate (PMMA) or such to be thicken, followed by film formation. Also, it can be formed by a process comprising adding the photochromic compound according to the present invention to a composition whose monomer component is polymerizable by vinyl-addition polymerization, ring-opening addition polymerization or a combination thereof (such as epoxy, acrylic and urethane monomers), followed by polymerization. Furthermore, it can be formed by the addition of the photochromic compound according to the present invention, utilizing any of the known resin formation techniques such as ultraviolet curing and thermal curing. It should be noted that kinds of the polymers making up a matrix for compounding the photochromic compound according to the present invention are not specifically restricted, and various polymers are applicable as long as they can highly disperse the photochromic compound according to the present invention and they are chemically stable.  
         [0030]     As described above, the functional element according to the present invention comprises a photochromic which sensitizes a wavelength of not less than 700 nm or a specific wavelength within an infrared region, and which exhibits absorbance within a visible region, and a light source which has an energy strength at a wavelength region of not less than 700 nm or a specific wavelength within an infrared region enough for being sensitized by the photochromic compound, wherein a photochromic phenomenon, which sensitizes a wavelength of not less than 700 nm or a specific wavelength within an infrared region, and which exhibits absorbance within a visible region, is utilized.  
         [0031]     The functional element constructed as described above basically possesses an effect that it sensitizes a light (for example, a xenon light source) having a prescribed energy at a wavelength region of not less than 700 nm, particularly at a specific wavelength within an infrared region of not less than 750, nm to exhibit a photochromic phenomenon. Specifically, a photochromic display element can be realized, which reversibly brings out coloration within a visible light region and bleaching by irradiation of a light (for example, a xenon light source) having a prescribed energy at a wavelength region of not less than 700 nm, particularly at a specific wavelength within an infrared region of not less than 750 nm.  
         [0032]     The functional element according to the present invention may be constructed to possess a ultraviolet shielding member, which shield an incident ultraviolet light entering in the functional element.  
         [0033]     In the functional element constructed as described above, since a member for shielding an ultraviolet light is provided at a side where a light enters in the photochromic layer, the deterioration of the photochromic layer due to a ultraviolet light can be prevented.  
         [0034]     Also, since a member for shielding an ultraviolet light is provided at a side where a light enters in the photochromic layer, even if the photochromic layer is to sensitize a light within a ultraviolet region to exhibit a photochromic phenomenon (for example, in the case where the layer is composed of the photochromic compound represented by the formula (1)), the photochromic phenomenon exhibited by being sensitized to a light within a ultraviolet region can be avoided.  
         [0035]     The functional element according to the present invention may be constructed such that the photochromic layer containing the photochromic compound at least contains a ultraviolet absorber, which absorbs a ultraviolet light.  
         [0036]     Similarly, the photochromic composition and the photochromic film according to the present invention may have a construction at least comprising the photochromic compound and a ultraviolet absorber, which absorbs a ultraviolet light.  
         [0037]     In the functional element, the photochromic composition, and the photochromic film according to the present invention, the deterioration of the photochromic layer due to a ultraviolet light can be prevented.  
         [0038]     Also, even if the photochromic layer is to sensitize a light within a ultraviolet region to exhibit a photochromic phenomenon (for example, in the case where the layer is composed of the photochromic compound represented by the formula (1)), the photochromic phenomenon exhibited by being sensitized to a light within a ultraviolet region can be avoided by the ultraviolet absorber.  
         [0039]     An example of the functional element includes a photochromic display element.  
         [0040]     As the display elements utilizing an emitter, various industrial products such as automobile meters, and display portions of cellular phones have been put into practical use. In these display element, when a light source (such as a xenon light source) having a prescribed energy at a specific wavelength region within a wavelength region of not less than 700 nm is used as a light source, and when the photochromic materials according to the present invention, which can be sensitive to the light source, is used, a display element can be made with no complicated control circuit. Also, the display element according to the present invention can be jointly used as a dimming element together with any of the conventional techniques.  
         [0041]     Especially, as in the display element, another example of the functional element includes an anti-glare mirror.  
         [0042]     Being applied to an automobile mirror, the present invention can solve the problem in terms of complicated configuration due to control circuits for controlling the actuation of an electrochromic element, and the like as in an anti-glare mirror utilizing the conventional electrochromic element. Also, in the present invention, there is no need for separately providing a light source for irradiating a ultraviolet light in order to exhibit a photochromic phenomenon as in the conventional anti-glare mirror utilizing a photochromic material. Consequently, an anti-glare mirror, which can solely realize an anti-glare function without any sensor and control circuit can be provided, having extremely high practical application. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0043]      FIG. 1  is a cross-sectional view schematically showing a configuration of a photochromic display element according to a first embodiment of the present invention.  
         [0044]      FIG. 2  is a cross-sectional view schematically showing a configuration of a photochromic display element according to a second embodiment of the present invention.  
         [0045]      FIG. 3  is a cross-sectional view schematically showing a configuration of a photochromic display element according to a third embodiment of the present invention.  
         [0046]      FIG. 4  is a cross-sectional view schematically showing a configuration of a photochromic display element according to a fourth embodiment of the present invention.  
         [0047]      FIG. 5  is a drawing for explaining a process for synthesizing a photochromic compound according to the present invention.  
         [0048]      FIG. 6  is a graph showing spectral characteristics of a light with which the photochromic compound according to the present invention is irradiated.  
         [0049]      FIG. 7  is a graph showing a change in absorbance when a light having spectral characteristics shown in  FIG. 6  with which the photochromic compound according to the present invention is irradiated.  
         [0050]      FIG. 8  is a plane view schematically showing a configuration of a reflectance-variable, automobile mirror, which is an automobile mirror utilizing the conventional electrochromic material. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0051]     Embodiments of the present invention will now be described by referring to the drawings.  
       Photochromic Element According to First Embodiment  
       [0052]      FIG. 1  is a cross-sectional view schematically showing a configuration of a photochromic display element  1  according to a first embodiment of the present invention. As shown in  FIG. 1 , a photochromic element  1  is composed of a transparent substrate  12  placed on a front surface side, and a substrate  14  placed on a rear surface side between which a photochromic layer  13 , which contains a photochromic compound and which exhibits a photochromic phenomenon, is intervened.  
         [0053]     The transparent substrate  12  at the front surface side and the substrate  14  at the rear surface side are both made of glass. The photochromic layer  13  is composed, for example, of a photochromic compound represented by the formula (1). At a front surface side of the transparent substrate  12  at the front surface side, a ultraviolet light shielding member  15  for shielding a ultraviolet light is provided.  
         [0054]     The present invention is not specifically restricted to the substrates making up the substrate  12  and the substrate  14 , respectively, any of the conventional transparent substrates and substrates can be applied as long as they have a transparency and/or strength required in the present invention. For example, glass such as a soda lime glass and a transparent resin such as an acrylic plate may be used. Also, in the present invention, the ultraviolet light shielding member  15  is not specifically restricted, and any of the conventional ultraviolet light shielding members is applicable as long as they can prevent the photochromic compound according to the present invention from being deteriorated through a ultraviolet light. For example, a ultraviolet cutting film may be applied to the transparent substrate  12  via an adhesive. Also, a thin film for shielding a ultraviolet light may be formed onto the transparent substrate  12  by any of various methods. Alternatively, the transparent substrate  12  itself may be made of ultraviolet absorbing glass or colored glass.  
         [0055]     The photochromic element  1  according to the first embodiment of the present invention constructed as described above basically possesses an effect that it sensitizes a light (for example, a xenon light source) having a prescribed energy at a wavelength region of not less than 700 nm or a specific wavelength within an infrared region to exhibit a photochromic phenomenon. Specifically, a photochromic display element can be realized, which reversibly brings out coloration within a visible light region and bleaching by irradiation of a light (for example, a xenon light source) having a prescribed energy at a wavelength region of not less than 700 nm or a specific wavelength within an infrared region.  
         [0056]     In the functional element constructed as described above, since a member for shielding an ultraviolet light is provided at a side where a light enters in the photochromic layer, the deterioration of the photochromic layer due to a ultraviolet light can be prevented.  
         [0057]     Also, since a member for shielding an ultraviolet light is provided at a side where a light enters in the photochromic layer, even if the photochromic layer is to sensitize a light within a ultraviolet region to exhibit a photochromic phenomenon (for example, in the case where the layer is composed of the photochromic compound represented by the formula (1)), the photochromic phenomenon exhibited by being sensitized to a light within a ultraviolet region can be avoided.  
         [0058]     In the first embodiment described above, the photochromic element  1  may be constructed so as to possess a light reflective layer at a front surface of the substrate  14  placed at the rear surface side (a surface at the side of the photochromic layer or a surface at the opposite side). Also, the substrate  14  itself may be composed of a light-reflective substrate comprising a light-reflective material.  
         [0059]     In such a configuration, a mirror serving as the function of the photochromic element can be realized. This configuration is suitably for an automobile anti-glare mirror.  
         [0060]     In the present invention, materials for the light-reflective layer and the light-reflective substrate are not specifically restricted, and various materials having a light-reflective function conventionally known are applicable, as long as they exhibits the effects of the present invention. Amongst them, from the viewpoints of a light-reflective function, chemical stability, productivity, and a cost, the light-reflective layer and the light-reflective substrate may be composed of a thin metal film comprising Cr as a main component. Also, the production of the light-reflective layer is not specifically restricted in the present invention, and the light-reflective layer may be produced by any of various known methods in which various kinds of known light-reflective substances (such as Al, Cr, Ni, Ag, and Rh) may be formed into a layer having a prescribed thickness including, but being not restricted to, wet processes such as application methods, plating methods, and electrophoresis methods as well as dry processes such as deposition methods and sputtering methods. Also, it is possible to obtain a given color and a given reflecting ratio by a combination of an optical thin film with a reflective metal.  
       Photochromic Element According to Second Embodiment  
       [0061]      FIG. 2  is a cross-sectional view schematically showing a configuration of a photochromic element  2  according to a second embodiment of the present invention. As shown in  FIG. 2 , a photochromic element  2  is composed of a transparent substrate  22  placed on a front surface side, and a substrate  24  placed on a rear surface side between which a photochromic layer  23 , which contains a photochromic compound and which exhibits a photochromic phenomenon, is intervened.  
         [0062]     The transparent substrate  22  at the front surface side and the substrate  24  at the rear surface side are both made of glass. The photochromic layer  23  is composed, for example, of a photochromic compound represented by the formula (1). An ultraviolet absorber, which absorbs a ultraviolet light, is compounded in the photochromic layer  23 .  
         [0063]     The ultraviolet absorber to be used in the present invention are not specifically restricted, and various ultraviolet absorbers known in the art may be applied as long as they can prevent the photochromic compound according to the present invention from being deteriorated. In the present invention, for example, a benzophenone type ultraviolet absorber (2,4-dihydroxybenzophenone) or such commercially available from Sumitomo Chemicals Co., Ltd., Dainippon Ink and Chemicals Inc., or Dow Chemicals Inc. may be compounded in a photochromic solution making up the photochromic layer or a polymer film containing the photochromic compound, after confirming that the ultraviolet absorber to be used can be well dissolved in a solvent composition.  
         [0064]     The photochromic element  2  according to the first embodiment of the present invention constructed as described above basically possesses an effect that it sensitizes a light (for example, a xenon light source) having a prescribed energy at a wavelength region of not less than 700 nm or a specific wavelength within an infrared region to exhibit a photochromic phenomenon. Specifically, a photochromic display element can be realized, which reversibly brings out coloration within a visible light region and bleaching by irradiation of a light (for example, a xenon light source) having a prescribed energy at a wavelength region of not less than 700 nm or a specific wavelength within an infrared region.  
         [0065]     The ultraviolet absorber in the photochromic element  2  can prevent the photochromic layer or the composition from being deteriorated through an ultraviolet light.  
         [0066]     Also, even if the photochromic layer is to sensitize a light within a ultraviolet region to exhibit a photochromic phenomenon (for example, in the case where the layer is composed of the photochromic compound represented by the formula (1)), the photochromic phenomenon exhibited by being sensitized to a light within a ultraviolet region can be avoided by means of the ultraviolet absorber.  
         [0067]     In the second embodiment described above, the photochromic element  2  may be constructed so as to possess a light reflective layer at a front surface of the substrate  24  placed at the rear surface side (a surface at the side of the photochromic layer or a surface at the opposite side).  
         [0068]     In such a configuration, a mirror serving as the function of the photochromic element can be realized. This configuration is suitably for an automobile anti-glare mirror.  
         [0069]     For example, as shown in  FIG. 3  (third embodiment), a photochromic display element  3  may be composed of a transparent substrate  32  placed on a front surface side, and a light reflective film  35  placed on a front surface side of a substrate  34  between which a photochromic layer  33 , which contains a photochromic compound and which exhibits a photochromic phenomenon, is intervened. The photochromic layer  33  has a ultraviolet absorber which absorbs a ultraviolet light compounded therein. This configuration is suitable as an automobile anti-glare mirror.  
         [0070]     In the second embodiment described above, the substrate  24  placed at the rear surface side may be composed of a light-reflective substrate.  
         [0071]     In such a configuration, a mirror having the function of the photochromic display element can also be realized.  
         [0072]     For example, as shown in  FIG. 4  (fourth embodiment), a photochromic display element  4  may be composed of a transparent substrate  42  placed on a front surface side, and a light-reflective substrate  45  placed on a rear surface side between which a photochromic layer  43 , which contains a photochromic compound and which exhibits a photochromic phenomenon, is intervened. The photochromic layer  43  has a ultraviolet absorber which absorbs a ultraviolet light compounded therein. This configuration is suitable as an automobile anti-glare mirror.  
         [0073]     In the present invention, materials for the light-reflective substrate placed on the rear side are not specifically restricted, and various materials having light-reflective characteristics, which can exhibit the effects of the present invention are applicable. Examples which can be used in the present invention include substrates having a layer of any of various light-reflective substances (such as Al, Cr, Ni, Ag, and Rh) formed on the surface, substrates made of the light-reflective substances just mentioned or aluminum alloy plates or stainless steel plates having a prescribed glossiness.  
       EXAMPLES  
       [0074]     Photochromic compounds and photochromic elements utilizing the photochromic compounds according to the present invention will now be specifically described.  
         [0075]     4,4′-Bipyridine derivative represented by the following formula (1) was synthesized by a method shown below.  
                         
 
         [0076]     To a solution of a compound shown in  FIG. 5  (1) [3,5-dibenzyloxybenzyl bromide, alias: 3,5-bis(benzyloxy)benzyl bromide] in dry acetonitrile, a solution of a compound shown in  FIG. 5  (2) [4,4′-bipyridine, alias: 4,4′-bipyridyl]] in dry acetonitrile was added dropwise under a reflux condition over a period of several ten minutes, and the mixture was refluxed over a period of several days. After the reaction was completed, the solution was allowed to cool down to a room temperature. This gave a yellow crystal. This was washed with acetonitorile and with ether and recrystallized from methanol to obtain 4,4′-bipyridine derivative represented by the formula (1).  
         [0077]     The resulting 4,4′-bipyridine derivative (12 mg) and 50 mg of polyvinylpyrolidone ultraviolet absorber were weighted and they were dissolved in 0.5 ml of DMF dimethylformamide) to prepare a solution of 4,4′-bipyridine derivative in DMF.  
         [0078]     Subsequently, the solution od 4,4′-bipyridine derivative in DMF was applied to a soda lime glass, dried in vacuo at 60° C. to obtain an approximately 0.1 mm thick film. This film was then sandwiched between ultraviolet light shielding films (UV cutting films available from Achilles Co., Ltd. under a trade name of Achilles Vinylus), irradiated with a light from a xenon lamp having spectral characteristics shown in  FIG. 6 , and was measured for distribution of absorbance before and after the irradiation. As a result, the absorbance of the film containing the 4,4′-bipyridine derivative was changed as shown in  FIG. 7 .  
         [0079]     As shown in  FIG. 6 , this xenon lamp has peak of emission spectrum (bright line spectrum of xenon) at approximately 830 nm, which is within a wavelength region of not less than 700 nm and within an infrared region.  
         [0080]     As shown in  FIG. 7 , it can be understood that in the film containing the 4,4′-bipyridine derivative, the absorbance is instantly increased at approximately 610 nm as a center upon the irradiation with a light of the xenon lamp. Also, it can be understood that as the time elapse after stopping the irradiation with a light of the xenon lamp is increased from 5 minutes, to 20 minutes, to 30 minutes, and to 90 minutes, the absorbency at approximately 610 nm as a center is gradually decreased, and is returned to be its original state.  
         [0081]     As described above, the 4,4′-bipyridine derivative sensitizes a light containing an infrared of 830 nm whereby the absorbance at approximately 610 nm as a center, which is within a visible light region, is increased to have a blue color. In addition, when the irradiation with a light containing the infrared light is stopped (or is placed in a dark portion), the derivative becomes colorless. Accordingly, the derivative has been found to exhibit a photochromic phenomenon in which a colored state of a blue color and colorless bleached state are reversibly brought about.  
         [0082]     While the present invention has been described by referring to the embodiments, the present invention is not restricted thereto, and various modification and alternation can be made without departing from the scope and the sprits of the present invention.