BACKGROUND OF THE INVENTION Photochromism is defined as"a reversible transformation of a single chemical species being induced in one or both directions by absorption of electromagnetic radiation, with two states having different distinguishable absorption spectra. "Thus, photochromic compounds are compounds that possess at least two isomeric forms which have different physical properties, such as absorption properties, refractivity, and the like, and can be transformed from one form to another by light excitations at prescribed wavelengths.

Photochromism has been intensively studied due to its potential use for optical recording and other optical functioning devices. To be practically used as optical recording materials, both isomeric forms must be thermally stable and possess excellent durability for reversible photochromic reactivity. Diarylethene is one class of photochromic compounds which possesses these properties, and therefore is a suitable class of compounds for the construction of optical functioning devices. The cis-configuration of both aryl groups in the diarylethenes studied is generally fixed by an upper cycloalkene structure, such as fluorinated alicyclic group, aromatic group, anhydride and maleimide group. Apart from the difference in absorption characteristics and the like between the two forms and their thermal stabilities, the availability of desirable excitation wavelengths that can be tuned and selected for the photochromic reactions also represents an important aspect in the design of materials for optical functioning devices. It has been shown that with the more 1l-conjugated upper cycloalkene structures, such as maleimide derivatives, in the diarylethene compounds, the

photocyclization proceeded with lower energy excitation in the visible region.

Further information can be found in U. S. patents 5,175, 079,5, 183,726, 5,443, 940, 5,622, 812, and 6,359, 150; Japanese patents JP 2-250877, JP 3-014538, JP 3-261762, JP 3- 261781, JP 3-271286, JP 4-282378, JP 5-059025, JP 5-222035, JP 5-222036, JP 5-222037, JP 6-199846, JP 10-045732, JP 2000-072768, JP 2000-344693, JP 2001-048875, JP 2002-226477, JP 2002-265468 and JP 2002-293784 ; and in Irie et al.,"Thermally Irreversible Photochromic Systems. Reversible Photocyclization of Diarylethene Derivatives", Journal of Organic Chemistry, 1988,53, 803-808, Irie et al. ,"Thermally Irreversible Photochromic Systems. A Theoretical Study", Journal of Organic Chemistry, l988, 53, 6136-6138, and Irie, "Diarylethenes for Memories and Switches", Chemical Review, 2000,100, 1685-1716. The photochromic compounds of this invention can be used in the same way as described in these references.

The present invention relates to the use of coordination compounds to perturb the properties of the diarylethenes in photochromic compounds. Described below is a report of the design, synthesis and studies of cis-diarylethene-containing ligands, with the upper cycloalkene being part of a mono-or poly-cyclic ring structure that contains one or more donor heteroatom (s), such as phenanthrolines, pyridines, diazines, triazines, polypyridines, porphyrins and phthalocyanines and the like, for coordination compound formation.

SUMMARY OF THE INVENTION The object of the present invention is to provide a new class of diarylethene-containing coordination compounds capable of displaying perturbed and sensitized photochromic properties. The invented photochromic compound is a coordination compound that contains a diarylethene with one or more donor atoms coordinated to an acceptor atom of the coordination compound. Any diarylethene in which the ethene group in a heterocyclic moiety, monocyclic or polycyclic, with any donor atom (s) capable of forming a coordination compound can be used in the present invention. There is no restriction on the nature of the aryl groups and they can be heteroaryl groups such as, for instance, thienyl groups. Likewise, any acceptor atoms which can be coordinated with the ethene-containing heterocyclic ligand moiety can be employed.

In a preferred form, the photochromic coordination compound is expressed by the following general formula (I) :

where unit B represents a mono-or poly-cyclic ring structure, such as phenanthroline, pyridine, diazine, triazine, polypyridine, porphyrin and phthalocyanine and the like, that contains one to four donor heteroatom (s) X, such as nitrogen, oxygen, sulfur, phosphorus, selenium, i. e. , n is integer from 0 to 3, [M] represents the coordination unit containing an acceptor atom M, such as rhenium (I), zinc (II), ruthenium (II), osmium (II), rhodium (III), iridium (III), gold (III), copper (I), copper (II), platinum (II), palladium (II), iron (II), cobalt (III), chromium (III), cadmium (II), boron (III) and the like, Ri and R6 individually represent alkyl groups and alkoxy groups, and R2 to Rs individually represent atoms or groups selected from the group of hydrogen atom, halogen atom, hydroxyl group, alkyl group, alkoxy group, cyano group, nitro group, alkylcarbonyl group, alkoxycarbonyl group, perfluoroalkyl group, aryl group, cycloalkyl group, arylcarbonyl group, aryloxycarbonyl group, mono-or dialkylaminocarbonyl group, alkylcarbonyloxy group, arylcarbonyloxy group, aryloxy group, alkoxycarbonyl group, aryloxycarbonyloxy group, and the like. In general, any alkyl or alkoxy group contains 1 to about 20 carbon atoms, any cycloalkyl group contains 3 to 8 carbon atoms, and any aryl group contains 6 to about 20 carbon atoms.

A non-limiting list of examples of diarylethene compounds containing a heterocyclic ethene-containing ligand moiety includes 5, 6-dithienyl-l, 10- phenanthroline, 2,3, 7,8, 12, 13, 17, 18-octathienyl-5, 10,15, 20- tetraphenyl porphyrin, 6, 7- dithienyl- dipyrido [3, 2-a: 2', 3'-c] phenazine and the like.

A non-limiting list of coordination units includes chlorotricarbonylrhenium (I), dithiolatozinc (II), dihaloplatinium (II), bipyridylplatinium (II) bis [bipyridyl]-ruthenium (II), diphosphinocopper (I), bipyridylcopper (I) and the like.

One of the advantages of the formation of coordination compounds from their pure organic counterparts (free ligands) in this invention is the extension of the excitation wavelength for the photocyclization of the diarylethene moiety from k zu 340 nm to lower

energy, so that the photochromic forward reaction can proceed with visible light excitation by utilization of the low-energy absorptions characteristic of coordination compounds. In addition, the photochromic reactions can be utilized to switch the photoluminescence properties characteristic of the coordination compounds.

BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 shows a representative synthetic route for a diarylethene-containing ligand and its coordination compounds using 5, 6-dithienyl-1, 10-phenanthroline and its chlorotricarbonylrhenium (I) and dithiolatozinc (II) compounds as illustrative examples.

Fig. 2 shows the overlaid electronic absorption spectra of the open form (-) and the close form (--) of a diarylethene-containing nitrogen donor ligand (LI).

Fig. 3 shows the overlaid electronic absorption spectra of the open form (-) and the close form (--) of a diarylethene-containing coordination compound (1).

Fig. 4 shows the photochromic reactions of (a) a diarylethene-containing ligand and (b) its coordination compound using 5, 6-dithienyl-1, 10-phenanthroline and its chlorotricarbonylrhenium (I) compound as illustration.

Fig. 5 shows the overlaid corrected emission spectra of the open form (-) and the close form (--) of (1) in benzene solution at 298 K.

Fig. 6 shows the overlaid corrected emission spectra of the open form (-) and the close form (--) of (1) in EtOH-MeOH (4: 1 v/v) glass at 77 K.

Fig. 7 shows the absorption spectral changes of complex (4) in benzene upon excitation at X = 300 nm.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Example 1 The ligand (LI) is synthesized by the Suzuki cross-coupling reactions of 2.5 equivalents of 2, 5-dimethyl-3-thienylboronic acid and 5, 6-dibromo-1, 10- phenanthroline in the presence of the palladium catalyst, Pd (PPh3) 4, and sodium carbonate in a heterogeneous mixture of water and THE'according to the synthetic route depicted in Fig. 1. Excitation of (L1) with 313 nm light resulted in the formation of the close form, corresponding to the photocyclization product. The overlaid electronic absorption spectra of the open and close forms of (LI) in benzene solution are shown in Fig. 2.

Upon coordination to a chlorotricarbonylrhenium (I) complex, the open form of the corresponding complex (1) undergoes photocyclization with the excitation of both the intraligand absorptions at k < 340 nm and the metal-to-ligand charge transfer (MLCT) absorption characteristic of this coordination compound up to k zu 480 nm. The electronic absorption data of (LI) and complex (1) are summarized in Table 1. The close forms of these compounds are found to undergo thermal backward reactions. The half-lives of the close forms have been determined and summarized in Table 2. The quantum yields for both photocyclization and photo-cycloreversion of (L1) and its rhenium complex (1) are summarized in Table 3.

TABLE 1 Compound Configuration Absorption (in benzene) #abs/nm (# /dm3 mol-1 cm-1) Open form 304 (8670) Close form 366 (24340), 510 (4050), 540 (3860) (1) Open form 338 (4930), 396 (4690) (1) Close form 390 (36670), 546 (5390), 580 (5050) TABLE 2 Compound Half-lives (t..) at 20 OC Half-lives (t..) at 60 °C (LI) 143 hours 222 mins (1) 77.7 hours 79.3 mins TABLE 3 Compound Photochemical Quantum Yield/+ <BR> <BR> <BR> <BR> Photocyclization° Photo-cycloreversion <BR> <BR> <BR> <BR> <BR> <BR> 313 #440 #365 #510 0. 486 0 0.123 0.029 (1) 0.552 0.648 0. 028 0.009 'Values reported are corrected to the ratio of the photochromic active conformation, i. e. with respect to the anti-parallel configuration The photoluminescence properties of both the open and close forms were measured.

Fig. 5 and 6 display the overlaid emission spectra of the open form and the close form of complex (1) in benzene at 298 K and in EtOH-MeOH glass (4: 1 v/v) at 77 K. The emission of complex (1) was found to change from metal-to-ligand charge transfer (MLCT) phosphorescence to ligand-centered (LC) phosphorescence upon photocyclization of the open form to the close form. These demonstrate the change of emission properties upon photochromic reactions. Table 4 summarized the emission data of ligand (LI) and complex (1).

TABLE 4 Compound Medium (T/K) Emission #ema/nm (#o/µs) Open form Close form Benzene (298) 383 (<0. 1) 644 (<0. 1) Glassb (77) 577 (5.2) (1) Benzene (298) 595 (0.26) 626 (<0. 1) Glassb (77) 535 (7.2) 620 (6.4) 'Excitation wavelength at ca. 355 nm. Emission maxima are corrected values. bEtOH-MeOH (4: 1, v/v) CNon-emissive Example 2 Upon coordination of (LI) to a dithiolatozinc (II) complex, the open forms of the corresponding complexes (2), (3) and (4) undergo photocyclization with excitation at X < 340 nm. Fig. 7 shows the absorption spectral changes of complex (4) upon excitation at k = 300 nm. The electronic absorption maxima of both the open and the close forms of complexes (2), (3) and (4) are summarized in Table 5.

TABLE 5 Complex Configuration Absorption maximum (in benzene) #abs/nm (2) Open form 302,326, 378 (2) Close form 366, 382, 536,576 (3) Open form 302,326, 382 (3) Close form 366,382, 538 (4) Open form 302,336, 396 (4) Close form 366,384, 542, 584 Those skilled in the art will recognize that various changes and modifications can be made in the invention without departing from the spirit and scope thereof. The various embodiments described were for the purpose of further illustrating the invention and were not intended to limit it.