Abstract:
Encrypted markers that are not readily detectable can be revealed by treatment with a specific reagent used as a developer to reveal a readily detectable physical property of the marker, such as a characteristic fluorescence emission after excitation with a particular excitation wavelength, or to reveal a visible color. The encrypted marker can be developed in situ, or a sample can be removed by brushing, scraping, swabbing or scratching the marked object or item and developing the encrypted marker or a sample thereof with the appropriate developer to reveal an overt marker or optical signal. The marker can be revealed by exposure of the encrypted marker or a sample thereof to the developer in any suitable form, such as a solution, a slurry, a swab, a solid (such as in granular form), or a gas or a vapor that includes a developer.

Description:
RELATED APPLICATIONS 
       [0001]    This application claims the benefit of U.S. provisional application Ser. No. 61/954,950 filed Mar. 18, 2014 the disclosure of which is incorporated by reference herein in its entirety. 
     
    
     BACKGROUND 
       [0002]    Fluorescent markers are useful as identification and security markers, but the advantage of their being readily visualized by irradiating with light of the appropriate excitation wavelength is also a disadvantage in that the fluorescent marker can be easily detected, analyzed and copied, rendering them subject to counterfeiting. 
       SUMMARY 
       [0003]    The present invention provides encrypted markers that are not readily detectable, but can be revealed by treatment with a specific reagent used as a developer to reveal a readily detectable physical property of the marker, such as, for instance a characteristic fluorescence emission after excitation with a particular excitation wavelength, or to reveal a visible color. The encrypted marker can be developed in situ, or a sample can be removed by brushing, scraping, or scratching the marked object or item and developing the encrypted marker or a sample thereof with the appropriate developer to reveal an overt marker or optical signal. The marker can be revealed by exposure of the encrypted marker in situ or a sample thereof to the developer in solution, a spray, a vapor or a solid, such as for example, a powder or granules that include the developer. 
         [0004]    Alternatively, there may be sufficient transfer of the encrypted marker molecules from the marked object or item by simply swiping with a swab carrying the developer, e.g. a swab soaked in a solution of the developer or a swab on which the developer is bound or covalently immobilized. The transferred encrypted marker molecules are then developed on or in the swab, or on the marked object or item. The swab, or the marked object or item, can be readily inspected for the presence of the revealed optical characteristics, such as color or fluorescence with emission at a specific wavelength after excitation with the appropriate wavelength of light. 
         [0005]    The developer can be a chemically reactive developer that reacts with the marker to produce a detectable marker product, or the developer can be a chelating agent or other binding agent that in combination with the marker produces a detectable marker product. In another embodiment, the developer can be an ion, such as for instance, a metal ion that is bound by the marker to produce a detectable marker product. 
         [0006]    In one embodiment, the marker of the present invention is an “encrypted fluorophore” (also referred to herein as a “pro-fluorophore”) and does not have the properties of a fluorophore and is thus difficult to recognize as a security marker, but on development or reaction with a developer, which is a specific reagent suitable for development of the pro-fluorophore, is transformed into a fluorophore with a readily detectable signal. Additionally, encryption often improves the physical and chemical stability of the pro-fluorophore. 
         [0007]    In another embodiment, the chromogenic markers (also referred to herein as “chromogens” or “pro-chromophores”) of the present invention are “encrypted chromophores” and are colorless or almost colorless, and are thus difficult to recognize as security markers, but on development or reaction with a developer, a specific reagent, are transformed into a readily detectable chromophore. In one embodiment, the chromophores of the invention are made visible to the naked eye after development of the respective chromogenic markers with the developer reagent specific to the chromogenic marker. 
         [0008]    In another embodiment, the markers of the present invention combine the features of both encrypted fluorophores and encrypted chromophores, wherein the encrypted fluorophores are non-fluorescent or essentially non-fluorescent and the encrypted chromophores are colorless or essentially colorless, making them difficult to detect or recognize as security markers. However, on development or reaction with a developer, these encrypted markers are transformed into a readily detectable fluorophore and a readily detectable chromophore. In one embodiment, such a combination marker can be simultaneously interrogated for fluorescence with UV light of an appropriate excitation wavelength, as well as for color visible to the naked eye by visual inspection of the illuminated marker after development with an appropriate developer. 
         [0009]    In one embodiment, the inventive concept provides a method for cryptically marking an item, wherein the method includes: providing a chromogenic compound as a marker capable of producing a chromophore upon reaction with a developer, and (i) coating the item with a coating comprising the chromogenic marker compound; (ii) attaching a label or indicia comprising the chromogenic marker compound to the item; or (iii) embedding the chromogenic marker compound in at least a portion of the item; and thereby providing a cryptically marked item. 
         [0010]    In another embodiment, the inventive concept provides a method for cryptically marking an item, wherein the method includes: providing a pro-fluorophore (also referred to as a cryptic fluorophore) capable of producing a fluorescent compound upon reaction with a developer, and (i) coating the item with a coating comprising the pro-fluorophore; (ii) attaching a label or indicia comprising the pro-fluorophore to the item; or (iii) embedding the pro-fluorophore in at least a portion of the item; and thereby providing a cryptically marked item. 
         [0011]    In one embodiment of the inventive concept, a pro-fluorophore or chromogenic compound is used as a marker on an object to be tracked or authenticated. The marker may be painted, printed, sprayed, bonded, affixed to or embedded in or on the object or item of interest. 
         [0012]    The item of interest may be swiped, rubbed or treated with a swab soaked in a solution of a developer, which is a specific reagent, capable of transforming the encrypted fluorophore marker or chromogenic marker compound into a fluorophore or chromophore, respectively, with a readily detectable signal. In one embodiment, the developer may be immobilized on the swab, e.g. by adsorption or by covalent bonding. Alternatively, the item of interest may be directly exposed to the developer in the form of a vapor, a spray or a solution. The development of the and authentication of the encrypted fluorophore marker or chromogenic marker compound can be performed by any suitable method, such as for instance and without limitation, in a single step wherein the encrypted fluorophore marker or chromogenic marker compound is developed directly in situ in or on the marked object, or in a two step procedure, wherein the marker is first transferred to a sampling device or swab etc., and then treated with an appropriate developer delivered in the form of a vapor, a spray or a solution, or by sprinkling the sampling device or swab etc. with a powder or granules consisting of or containing the developer. 
     
    
     DETAILED DESCRIPTION 
     Definitions 
       [0013]    Alkyl, as used herein, refers to a saturated branched or straight chain monovalent hydrocarbon radical of a specified number of carbon atoms. Thus, the term alkyl includes, but is not limited to, methyl (C 1  alkyl), ethyl (C 2  alkyl), propyl (C 3  alkyl), isopropyl (also C 3  alkyl), n-butyl(C 4  alkyl), isobutyl (also C 4  alkyl), sec-butyl (also C 4  alkyl), and t-butyl (also C 4  alkyl). 
         [0014]    Alkenyl refers to branched or straight chain hydrocarbon radical having at least one double bond between two carbon atoms. 
         [0015]    Alkynyl refers to branched or straight chain hydrocarbon radical having at least one triple bond between two carbon atoms. 
         [0016]    Cycloalkyl as used herein means a saturated monocyclic, polycyclic or bridged hydrocarbon ring system substituent or linking group. In a substituted cycloalkyl ring, the substituent is bonded to ring carbon atom replacing a hydrogen atom. For example, C 3 -C 10  cycloalkyl designates a ring of three to ten carbon atoms, or a ring of three or more carbon atoms wherein the remaining carbon atoms forming one or more alkyl substituents of the ring. 
         [0017]    Heterocyclyl as used herein means a saturated, partially unsaturated or unsaturated monocyclic, polycyclic or bridged hydrocarbon ring system substituent or linking group, wherein at least one ring carbon atom has been replaced with a heteroatom such as, but not limited to nitrogen, oxygen, sulfur, selenium, boron or phosphorus. A heterocyclyl ring system can be a ring system having one, two, three or four nitrogen ring atoms, or a ring system having zero, one, two or three nitrogen ring atoms and one oxygen or sulfur ring atom. The heterocyclic ring system can include more than one ring heteroatom. A heterocyclyl substituent is derived by the removal of one hydrogen atom from a single carbon or nitrogen ring atom. Heterocyclyl includes, but is not limited to, furyl, thienyl, 2H-pyrrole, 2-pyrrolinyl, 3-pyrrolinyl, pyrrolidinyl, pyrrolyl, 1,3-dioxolanyl, oxazolyl, thiazolyl, imidazolyl, 2-imidazolinyl, imidazolidinyl, 2-pyrazolinyl, pyrazolidinyl, pyrazolyl, isoxazolyl, isothiazolyl, oxadiazolyl, triazolyl, thiadiazolyl, tetrazolyl, 2H-pyranyl, 4H-pyranyl, pyridinyl, piperidinyl, 1,4-dioxanyl, morpholinyl, 1,4-dithianyl, thiomorpholinyl, pyridazinyl, pyrimidinyl, pyrazinyl, piperazinyl, azepanyl, diazepinyl, indolizinyl, indolyl, isoindolyl, 3H-indolyl, indolinyl, benzo[b]furyl, benzo[b]thienyl, 1H-indazolyl, benzimidazolyl, benzothiazolyl, purinyl, 4H-quinolizinyl, quinolinyl, isoquinolinyl, cinnolinyl, phthalzinyl, quinazolinyl, quinoxalinyl, 1,8-napthyridinyl, pteridinyl, quinuclidinyl. 
         [0018]    As noted above, heterocyclyl also includes aromatic heterocycles, such as pyrrolyl, pyrazolyl, imidazolyl, triazolyl, oxazolyl, isoxazolyl, thiazolyl, isothiazolyl, furyl, thienyl, pyridyl, pyrazinyl, pyrimidinyl, and can be optionally substituted, for instance with alkyl. Heterocyclyl also includes bicyclic heterocyclyls with one or both rings having a heteroatom, e.g. imidazopyrazinyl, benzofuranyl, benzodioxolyl, benzothiophenyl, and quinolinyl. 
         [0019]    Arylalkyl means an aryl group attached to the end carbon atom of an alkyl group such as, for instance C 1 -C 4  alkyl. 
         [0020]    Aryl means an aromatic, unsaturated it-electron conjugated monocyclic or polycyclic hydrocarbon ring system substituent or linking group of six, eight, ten or fourteen carbon atoms. An aryl group is derived by the removal of one hydrogen atom from a carbon ring atom. Aryl includes, but is not limited to, phenyl, naphthalenyl, azulenyl and anthracenyl. 
         [0021]    Halo- means fluoro (—F), chloro (—Cl), bromo (—Br) or iodo (—I). 
         [0022]    Carboxyl means a substituent of the formula —COOH. 
         [0023]    Hydroxyl means a substituent of the formula —OH. 
         [0024]    Cyano means a substituent of the formula —C≡N. 
         [0025]    Nitro means a substituent of the formula —NO 2 . 
         [0026]    Oxo means a substituent of the formula ═O in which the oxygen atom is double bonded. 
         [0027]    Amino means a substituent of formula —NH 2  or a linking group having the formula —NH—. 
         [0028]    Alkylamino or dialkylamino means a substituent of the formula —NH-alkyl or —N(alkyl) 2 , respectively. 
         [0029]    Azido means a substituent of the formula —N 3  also represented as —N═N + ═N − . 
         [0030]    Compounds useful in the practice of the present invention as marker pro-fluorophores or chromogenic compounds include pro-fluorophore forms of xanthane dyes such as rhodamines, rhodols and fluoresceins, as well as derivatives of coumarin, cyanine and oxazine. 
         [0031]    Scheme I shows the general case of a profluorophore (Pro-FL) also interchangeably referred to herein as a fluorogenic compound, treated with a developer to produce a fluorescent compound (FL*) having a characteristic emission wavelength when stimulated with light of the appropriate excitation wavelength, wherein the emitted light is readily detectable either by eye or by spectroscopic measurement. 
         [0000]    
       
                 
         
             
             
         
       
     
         [0032]    Scheme II shows the general case of a chromogenic compound (CHR-gen) also interchangeably referred to herein as a leuco-form of a chromophore (i.e. a chromogen) treated with a developer to produce a chromophore or dye (CHR*) having a characteristic absorption that is readily detectable either by eye or by spectroscopic measurement. 
         [0000]    
       
                 
         
             
             
         
       
     
         [0033]    Examples of fluorescein derivatives useful in the practice of the present invention as marker pro-fluorophores or chromogenic compounds are derivatives of each of the following compounds: Fluorescein, 5-(and-6-)-Carboxyfluorescein (FAM), 5-(and-6)-4,7,2′,7′-Tetrachlorofluorescein (TET), 5-(and-6)-2′,4′,4,5′,7′,7-hexachlorofluorescein (HEX), 5-(and-6)-Carboxy-4′,5′-Dichloro-2′,7′-Dimethoxyfluorescein (JOE), Eosin Y (2′,4′,5′,7′-Tetrabromofluorescein), Eosin B (4′,5′-Dibromo-2′,7′-dinitrofluorescein), Rose Bengal (4,5,6,7-Tetrachloro-2′,4′,5′,7′-tetraiodofluorescein), Erythrosin (2′,4′,5′,7′-Tetraiodofluorescein), 2,7-Dichlorofluorescein, Yakima Yellow, VIC, NED, and many more well known to those of skill in the art of fluorescent compounds and dyes. These derivatives are generally colorless, or essentially colorless and lack appreciable fluorescence; and can be treated with a suitable developer to produce the fluorescent compound or the dye itself. 
         [0034]    Examples of rhodamines useful as marker pro-fluorophores or chromogenic compounds include for instance: derivatives of Tetramethylrhodamine (TRITC), 5-(and-6)-Carboxytetramethylrhodamine (TAMRA), 5-(and-6)-carboxy-X-rhodamine (ROX), Rhodamine 110 (Xanthylium, 3,6-diamino-9-(2-carboxyphenyl)-, salts), Rhodamine B, Rhodamine 6G, etc. 
         [0035]    Examples of oxazines useful as marker pro-fluorophores or chromogenic compounds include derivatives of: Nile Red, Nile Blue, Cresyl Violet and Oxazine 170, etc., which can be treated with a developer to reveal the underivatized Nile Red, Nile Blue, Cresyl Violet or Oxazine 170 marker. 
         [0036]    For instance, in one embodiment, the pro-fluorophore can be a xanthane, such as for instance: 
         [0000]    
       
                 
         
             
             
         
       
     
         [0000]    wherein R 1 , R 2 , R 3 , R 4 , R 5 , R 6 , R 7 , R 8 , R 9  and R 10  are each independently selected from hydrogen, halogen (F, Cl, Br, I), nitro (NO 2 ), cyano (CN), carbonyl (CHO or C(O)R), C 1 -C 8  alkyl, aryl, and C 3 -C 8  cycloalkyl, each optionally substituted with one or more functional groups such as carboxyl, carbonyl, amino, cyano, nitro, alkyl, alkenyl, alkynyl or azido. 
         [0037]    Alternatively, a pair of R groups independently selected from R 1 , R 2 , R 3 , R 4 , R 5 , R 6 , R 7 , R 8 , R 9  and R 10  can form a ring between one another or between R n  (one of R 1 -R 10 ) and either X or Y or both; X and Y are substituted oxygen, X═OR or nitrogen, Y═NRR′ wherein substituents R and R′ independently selected from hydrogen, C 1 -C 8  alkyl, aryl, C 1 -C 8  acyl (each alkyl, aryl and acyl being optionally substituted with one to three substituents independenly selected from halo, nitro (NO 2 ), and C 1 -C 3  alkyl), alkylsulfonyl (RSO 2 —) and arylsulfonyl (ArSO 2 —), (wherein the alkylsulfonyl and arylsulfonyl are optionally substituted with one to three substituents independently selected from halo, nitro (NO 2 ), and C 1 -C 3  alkyl), trialkyl and triarylsilyl; and Z represents oxygen (O), sulfur (S), selenium (Se) or substituted nitrogen (N—R″), wherein R″ is defined as for R above; or R″ is an amino group, NR′R′″, hydroxyl group or OR′″, wherein R′″ is independently selected from the options for R as defined above. 
         [0038]    In one embodiment, the pro-fluorophore (Pro-FL) useful in the practice of the present invention can be prepared from the fluorophore (FL*) by chemical conversions, known in the art, and as depicted in the general Scheme III, as shown below. 
         [0000]    
       
                 
         
             
             
         
       
     
         [0039]    In another embodiment, the said chromogen (CHR-gen) can be prepared from the chromophore (CHR*) by chemical conversions, known in the art, and as depicted in the general Scheme IV, shown below. 
         [0000]    
       
                 
         
             
             
         
       
     
         [0040]    Examples of the above conversions of fluorogen to pro-fluorogen or chromophore to chromogen according to the present invention are depicted in Examples 1, 2 and 6 below. 
         [0041]    In another embodiment, the pro-fluorophore can be an oxazine such as for instance: 
         [0000]    
       
                 
         
             
             
         
       
     
         [0000]    wherein R 1 , R 2 , R 3 , R 4 , R 5  and R 6  are hydrogen, halogen (F, Cl, Br, I), nitro (NO 2 ), cyano (CN), carbonyl (CHO or C(O)R), C 1 -C 8  alkyl, C 3 -C 8  cycloalkyl or aryl. Each of R 1 , R 2 , R 3 , R 4 , R 5  and R 6  can also form an optionally substituted ring between one another or between R n  and either X or Y, or both. X and Y are substituted oxygen, X═OR or nitrogen, Y═NRR′ wherein substituents R, R′ are independently selected from hydrogen, C 1 -C 8  alkyl, aryl and C 1 -C 8  acyl, trialkyl and triarylsilyl; each alkyl, aryl and acyl being optionally substituted with one to three substituents independently selected from halo, nitro (NO 2 ) and C 1 -C 3  alkyl), alkylsulfonyl (RSO 2 —) and arylsulfonyl (ArSO 2 —), wherein the alkylsulfonyl and arylsulfonyl are each optionally substituted with one to three substituents independently selected from halo, nitro (NO 2 ) and C 1 -C 3  alkyl. 
         [0042]    In another instance, in one embodiment, the pro-fluorophore can be a coumarin such as: 
         [0000]    
       
                 
         
             
             
         
       
     
         [0000]    wherein R 1 , R 2 , R 3 , R 4  and R 5  are each independently, hydrogen, halogen (F, Cl, Br, I), nitro (NO 2 ), cyano (CN), carbonyl (CHO or C(O)R), C 1 -C 8  alkyl, C 3 -C 8  cycloalkyl, or aryl; wherein each alkyl, aryl and acyl is optionally substituted with one to three substituents independently selected from halo, nitro (NO 2 ) and C 1 -C 3  alkyl), alkylsulfonyl (RSO 2 —) and arylsulfonyl (ArSO 2 —), wherein the alkylsulfonyl and arylsulfonyl are each optionally substituted with one to three substituents independently selected from halo, nitro (NO 2 ) and C 1 -C 3  alkyl. R 1 , R 2 , R 3 , R 4  and R 5  can also form a ring between one another or between R n  and X; wherein X represents substituted oxygen, X═OR or substituted nitrogen, X═NRR′ wherein substituents R, R′ are independently hydrogen, alkyl, aryl, or C 1 -C 8  acyl, (each alkyl, aryl and acyl being optionally substituted with one to three substituents independently selected from halo, nitro (NO 2 ) and C 1 -C 3  alkyl), alkylsulfonyl (RSO 2 —) and arylsulfonyl (ArSO 2 —), wherein the alkylsulfonyl and arylsulfonyl are optionally substituted with one to three substituents independently selected from halo, nitro (NO 2 ), and C 1 -C 3  alkyl groups; trialkyl and triarylsilyl, and Y represents either oxygen (O) or NH. 
         [0043]    In another embodiment, the pro-fluorophore can be a compound of Formula IV, as shown below: 
         [0000]    
       
                 
         
             
             
         
       
     
         [0000]    wherein R 1 , R 2 , R 3 , R 4  and R 5  are each independently, hydrogen, halogen (F, Cl, Br, I), nitro (NO 2 ), cyano (CN), C 1 -C 8  alkyl, C 3 -C 8  cycloalkyl, or aryl, each alkyl, cycloalkyl and aryl being optionally substituted with one to three substituents independently selected from halo, nitro (NO 2 ), and C 1 -C 3  alkyl. R 1 , R 2 , R 3 , R 4  and R 5  can also form a ring between any two of R 1 , R 2 , R 3 , R 4  and R 5 , or between any one of R 1 , R 2 , R 3 , R 4  and R 5  and the N of Formula IV; and wherein R is hydrogen, C 1 -C 8  alkyl, C 3 -C 8  cycloalkyl, arylalkyl (such as benzyl), or aryl wherein each of the C 1 -C 8  alkyl, C 3 -C 8  cycloalkyl, arylalkyl and aryl are optionally substituted with one to six substituents independently selected from carboxyl (COOH), sulfonate (SO 3 H) and amino (NH 2 ). X in Formula IV represents a counter-ion exemplified by but not limited to: chloride, bromide, iodide, tosylate, mesylate and perchlorate. 
         [0044]    In another embodiment, the pro-fluorophore can be a compound of Formula V, as shown below: 
         [0000]    
       
                 
         
             
             
         
       
     
         [0000]    wherein R 1 , R 2  and R 3  are hydrogen, halogen (F, Cl, Br, I), nitro (NO 2 ), C 1 -C 8  alkyl, C 3 -C 8  cycloalkyl, or aryl, each independently optionally substituted with one to three halo, nitro (NO 2 ), C 1 -C 3  alkyl groups. R 1 , R 2  and R 3  can also form a cyclic structure between any two of R 1 , R 2  and R 3 , or between any one of R 1 , R 2  and R 3  and N or Y of the heterocycle of formula V; and
 
wherein R is hydrogen, or C 1 -C 8  alkyl, C 3 -C 8  cycloalkyl, arylalkyl (such as benzyl), or aryl, each alkyl, cycloalkyl and arylalkyl being independently optionally substituted. Suitable substituents include but are not limited to carboxyl (COOH), sulfonate (SO 3 H), and amino (NH 2 ) etc. X represents a suitable counter-ion such as for instance, but not limited to: chloride, bromide, iodide, tosylate, mesylate and perchlorate. Y is oxygen, nitrogen, sulfur, selenium or C(CH 3 ) 2 .
 
         [0045]    In another embodiment of the invention, the pro-fluorophore can be any of the moieties A, L 1 , L 2 , L 3 , L 4  in Formula VI (below): 
         [0000]    
       
                 
         
             
             
         
       
     
         [0046]    Wherein the sum of m+n+p+q is a positive integer between 1 and 12; m is at least one; and n, p and q are each independently zero or a positive integer. In one embodiment, the complex is formed from A, L 1 , L 2 , L 3 , L 4  in Formula VI with or without the assistance of a catalyst or activator as explained below. 
         [0047]    Furthermore, L 1 , L 2 , L 3 , L 4  can each be different from each other (L 1 ≠L 2 ≠L 3 ≠L 4 ); or two or more of the moieties L 1 , L 2 , L 3  and L 4  can be the same. In one particular embodiment the moieties L 1 , L 2 , L 3  and L 4  are all the same (L 1 =L 2 =L 3 =L 4 ). 
         [0048]    The structure represented by Formula VI is a coordination compound such as, for instance, an organometallic compound, where A is a central ion which is chelated by ligands L 1 , L 2 , L 3 , L 4 . Central ion A and ligands L 1 , L 2 , L 3 , L 4  are chosen in such a way that only the combination of A coordinated by ligands L 1 , L 2 , L 3 , L 4  will generate a discreet spectroscopic signal manifested by color or fluorescence, or both. 
         [0049]    L 1 , L 2 , L 3 , L 4  can be any chemical moiety capable of forming coordination bonds with central ion A, via electron pairs available on a nitrogen, oxygen, sulfur or selenium atom of the ligand. One category of the molecules useful in the present invention as a ligand incorporates a heterocyclic compound and derivatives thereof, which may include, but is not limited to a heterocycle selected from: furane, thiophene, pyrrole, oxazole, thiazole, imidazole, pyrazolidinyl, pyrazole, isoxazole, isothiazole, oxadiazole, triazole, thiadiazole, tetrazole, 2H-pyrane, 4H-pyrane, pyridine, bipyridyl, terpyridine, triazine, piperidine, pyrrolidine, 1,4-dioxane, morpholine, 1,4-dithiane, thiomorpholine, pyrazine, pyridazine, pyrimidine, piperazole, azepane, diazepine, indolizine, indole, isoindole, benzimidazole, benzoxazole, benzothiazole, purine, quinolizidine, quinoline, isoquinoline, diazanaphthalenes, pteridine and phenanthroline. 
         [0050]    Another category of the ligands L includes polydentate chelators and their derivatives, such as ethylenediaminetetraacetic acid (EDTA), 1,2-bis(2-aminophenoxy)ethane-N,N,N′,N′-tetraacetic acid (BAPTA), diethylenetriaminepentaacetic acid (DTPA), triethylenetetramine-N,N,N′,N′″,N′″-hexaacetic acid (TTHA), N,N,N′,N′-tetrakis(2-pyridylmethyl)ethylene-diamine (TPEN), ethylene glycol-bis(2-aminoethylether)-N,N,N′,N′-tetraacetic acid (EGTA), 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA), 1,4,8,11-tetraazacyclo-tetradecane-5,12-dione dehydrate (5,12-Dioxocyclam), calixarenes (e.g. meso-octamethyl-calix(4)pyrrole, calix[4]arene-25,26,27,28-tetrol, calix[6]arene), crown ethers (e.g. dibenzo-15-crown-5, cyclen), cyclodextrins (e.g. α-cyclodextrin). 
         [0051]    Another category of ligands, L, includes but is not limited to β-diketones (e.g. acetylacetone and the derivatives thereof, including hexafluoroacetylacetone; 4,4,4-trifluoro-1-phenyl-1,3-butanedione; 1,3-diphenyl-1,3-propanedione); Phosphine oxides (e.g. triphenylphosphine oxide, n-trioctylphosphine oxide); aromatic acids and phenols and their derivatives (e.g. tiron, catechol, salicylic acid); Schiff bases and their derivatives (e.g. the 2-phenolsalicylimine depicted in Example 5 below). 
         [0052]    Another class of ligands, L, includes solvent molecules. This class is chemically very diverse, and includes several of the compounds mentioned above, such as heterocyclic compounds (pyridine, collidine, furan, tetrahydrofurane (THF) etc.) and crown ethers (15-crown-5 etc.), as well as other molecules representing a broad selections of chemical functionalities. These include, for instance, the following molecules and their derivatives: water, alcohols (methanol, ethanol etc.), amines (triethylamine, diisopropylethylamine, morpholine, dimethylamine, N,N-dimethylaminopyridine (DMAP) etc.), ethers (diethyl ether etc.), polar aprotic solvents (dimethyl sulfoxide (DMSO), dimethylformamide (DMF), dimethylacetamide (DMA), hexamethylphosphoramide (HMPA), 1,3-dimethyl-3,4,5,6-tetrahydro-2(1H)-pyrimidinone (DMPU), 1,3-dimethyl-2-imidazolidinone (DMI), 1-methyl-2-pyrrolidinone (NMP), acetonitrile (AcCN), sulfolane, tetraethylene glycol dimethyl ether (tetraglyme), etc.). 
         [0053]    In embodiments of the present invention, central ion A can be any positively charged ion capable of forming a coordination compound with one or more of the ligands, L. One category of the ions, A useful in the practice of the present invention is the category of metal ions, including, but are not limited to, transition metal ions, alkaline and alkaline earth metal ions, lanthanides and actinides. 
         [0054]    Central ion A can be provided in a form of an inorganic or organic salt. Suitable inorganic salts include, but are not limited to mineral acid salts such as, sulfates, sulfites, sulfides, nitrates, nitrites, carbonates, borates, phosphates, selenates, fluorides, chlorides, bromides, iodides, chlorates, perchlorates, etc. Suitable organic salts include, but are not limited to organic acid salts such as, monocarboxylic acid salts (e.g. salts of formic, acetic, propionic, butyric, pivalic, 2-ethylhexanoic, palmitic, stearic, oleic, benzoic, salicylic, 4-sulfosalicylic etc. acid), polycarboxylic acid salts (e.g. salts of oxalic, malonic, succinic, glutaric, adipic, citric, trimesic, mellitic etc. acids). 
         [0055]    Another class of central ion A includes oxygen atom-containing moieties such as hydroxides and oxides etc. 
         [0056]    The choice of the supplied form of central ion A for any particular embodiment of the present invention is dictated by the application (i.e. the object or item to be marked and its environment) and the carrier properties (i.e. the nature of the medium that includes the marker, such as for instance a coating, a paint or a varnish layer). 
         [0057]    In one embodiment, the complex is formed from A, L 1 , L 2 , L 3 , L 4  in Formula VI with the assistance of a catalyst or activator. The catalysts and activators useful in the practice of the present invention include basic and nucleophilic agents, exemplified but not limited to, simple organic and inorganic bases (hydroxides, amines, alkoxides, phenoxides and the like). This category also comprises inorganic or organic salts of weak acids and strong bases generating upon solvolysis alkaline solutions, e.g. ammonium, alkaline and alkaline earth metal carbonates, bicarbonates, borates, phosphates, acetates etc. Exemplary hydroxides are alkaline and alkaline earth hydroxides such as ammonium, lithium, sodium, potassium, rubidium, cesium, calcium, strontium or barium hydroxide as well as tetraalkylammonium hydroxides, such as tetramethylammonium or tetrabutylammonium hydroxide. Examples of amines useful as catalysts and activators include but are not limited to: triethylamine, diisopropylethylamine, benzylamine, morpholine, pyrrolidine, piperidine, dimethylaminoaniline etc. Examples of alkoxides and phenoxides useful as catalysts and activators include but are not limited to: alkaline alkoxides and phenoxides, such as: sodium or potassium methoxide, ethoxide or phenoxide. 
         [0058]    In another embodiment, the catalyst is an acidic substance, as defined by either Brønsted-Lowry or Lewis theory, and can be represented by simple organic or inorganic acid, as well as any inorganic or organic salt of weak base and strong acid generating acidic conditions upon solvolysis, e.g. metal chloride, bromide, nitrate, sulfate etc. 
         [0059]    Compounds useful as developers, i.e. decrypting agents in the practice of the present invention include compounds known in the art as good chelators or complexing agents, such as for instance and without limitation:
       Heterocyclic aromatic compounds, e.g. imidazoles, phenanthrolines, pirydines, thiazoles and the derivatives thereof;   Polydentate chelators (EDTA, EGTA, BAPTA, DOTA, DTPA);   β-diketones (e.g. acetylacetone and the derivatives thereof, including but not limited to hexafluoroacetylacetone; 4,4,4-trifluoro-1-phenyl-1,3-butanedione; 1,3-diphenyl-1,3-propanedione);   Phosphine oxides (trioctylphosphine oxide; triphenylphosphine oxide);   Aromatic acids and polyphenols (e.g. salicylic acid, catechol, Tiron);   Aromatic aldehydes (e.g. 4-(N,N-dimethylamino)benzaldehyde, 4-(N,N-dimethylamino)cinnamaldehyde, p-anisaldehyde, 4-hydroxybenzaldehyde, etc.)   Schiff bases and derivatives (e.g. 2-phenolsalicyl imine depicted in Example 5 below).       
 
         [0067]    Compounds useful as developers, i.e. decrypting agents in the practice of certain embodiments of the present invention include compounds known in the art as nucleophilic species, involved in solvolysis or deprotection, such as for instance, simple organic and inorganic bases (hydroxides, amines, alkoxides, phenoxides and the like); simple organic and inorganic salts (acetates, sulfides, iodides, fluorides, oxalates, citrates and the like). 
         [0068]    Compounds useful as developers, i.e. decrypting agents in the practice of certain embodiments of the present invention also include compounds known in the art as acidic species, involved in solvolysis or deprotection such as simple organic and inorganic acids. In other embodiments, the developer can be an electrophilic species for solvolysis or deprotection, such as for instance, an aldehyde compound, e.g. benzaldehyde or salicyladehyde, or derivatives thereof. 
         [0069]    In some specific embodiments the pro-fluorophore or chromogen can be used as the developer and any of the chemical compounds listed above can be used in the coating or embedded in the material marked. The chromogen or pro-fluorophore can then be used as the developer. 
         [0070]    In some specific embodiments the developer may require the presence of the catalyst to speed up the decryption reaction of the encrypted fluorophore (which may also be referred to as a pro-fluorophore) or the encrypted chromophore (which may also be referred to as a prochromophore, or a chromogen) and to render the marker useful as a rapid in-field authentication system. The catalysts useful in the practice of the invention include for instance, basic and nucleophilic agents, exemplified but not limited to, simple organic and inorganic bases (hydroxides, amines, alkoxides, phenoxides and the like. In another embodiment, the catalysts can be a simple organic or inorganic acid. 
       EXAMPLES 
     1. Pro-Fluorophores 
       [0071]    Encrypted pro-fluorophores are hidden/occult fluorophores that are revealed by development with a specific reagent that converts the pro-fluorophore or leuco-form that may be colorless and shows little or no fluorescence to the active fluorophore which may also be intensely colored under visible light. 
         [0072]    For instance the compound, rhodamine B hydrazide, a colorless compound can be developed with Cu 2+  metal ions under acidic conditions to yield a magenta compound with intense orange-red fluorescence. See the reaction illustrated above, showing the conversion of the colorless hydrazide to the magenta copper-coordinated compound by the copper ion, which in turn is slowly hydrolyzed to the colorless carboxylate. 
         [0000]    
       
                 
         
             
             
         
       
     
         [0073]    In other examples of profluorophores, the natural properties of susceptibility to a first order hydrolysis reaction can be used as a “time stamp” wherein the rate of decay in air or in a specific carrier under normal conditions is characteristic of the particular encrypted fluorophore. For example, the compound can be a leuco form chosen to be relatively stable in a liquid, such as an ink and decay over a period of weeks to produce a colored and intensely fluorescent form. 
       2. Two Part Fluorophores 
       [0074]    The two part fluorophores are assembled from two non-fluorescent, or very weakly fluorescent components (A) and (B), in some cases in the presence of a catalyst. Only upon reaction of both (A) and (B) components is the actual fluorophore produced. The reaction between both components may also generate colorimetric changes in addition to developing fluorescence. Component (A) of the two part fluorophore can be used as the hidden security/authentication marker and the verification can be achieved by adding the second component (B) to component (A) to produce the active fluorophore. In certain embodiments this process may be enhanced by the addition of a catalyst. 
         [0075]    For instance, substituted benzazolium salts can be used as the component (A) and developed with a specific reagent (and an optional catalyst) as exemplified below: 
         [0000]    
       
                 
         
             
             
         
       
     
         [0076]    These non-fluorescent components (A) are stable and only revealed by exposure to the appropriate second components (B) and do not spontaneously decrypt over time. 
         [0077]    Alternative substituents of the aromatic compounds can be used to change the spectral properties or solubility profile of the fluorophore at the option of the chemist or the designer of the pro-fluorophore as a cryptic marker. 
       3. Thermostable Pro-Fluorophores 
       [0078]    Inorganic elements can be used as thermostable or even fire resistant markers that can be revealed by development with a specific reagent, which can be an organic component. For instance aluminum chloride (AlCl 3 ) can be used as a covert marker that may be incorporated into for example an ink, a lacquer or a varnish and revealed by addition of a specific chelating agent which becomes intensely fluorescent by forming a coordination compound with new spectral properties. In some cases a catalyst may be required to speed the reaction. An example of the formation of an aluminum-based fluorophore from an aluminum chloride (AlCl 3 ) marker is shown below: 
         [0000]    
       
                 
         
             
             
         
       
     
         [0079]    Other inorganic elements or ions useful as thermostable pro-fluorophores include anions such as phosphate ions, iodide ions, fluoride ions and acetate ions; and cations such as metal ions. Appropriate choice of alternative substituents of the aromatic coordination compounds or chemosensors can be used to change the spectral and other physicochemical properties of the fluorophore at the option of the designer. 
       4. Polymerizable Encrypted Pro-Fluorophores 
       [0080]    Encrypted pro-fluorophores can be rendered polymerizable by addition of appropriate functional groups. Such encrypted pro-fluorophores can be based on excellent fluorophores (bright and with high quantum yields) that are readily and inexpensively synthesized. Pro-chromophores and pro-fluorophores with chromophoric and fluorescence emission properties can be provided, depending on the chromophore or fluorophore template chosen. 
         [0000]    
       
                 
         
             
             
         
       
     
         [0081]    The chemical stability and hydrophobicity of the pro-fluorophores can be controlled by choice of the appropriate acyl groups or aromatic ring substituents. 
         [0082]    In one embodiment, the encrypted fluorophores can also be incorporated into nanoparticles containing DNA, such as for instance, and without limitation, lyophilized or encapsulated DNA. Polymerization of the above exemplified polymerizable pro-fluorophore in a methyl-methacrylate polymer is shown below: 
         [0000]    
       
                 
         
             
             
         
       
     
         [0083]    Such polymers can be used to encapsulate security markers, such as DNA or other biomolecules, or can be assembled as microparticles or nanoparticles for use as transferable markers, or for incorporation into polymers or other materials with or without entrapped DNA or other detectable marker(s). 
         [0084]    The following examples 1-6 are embodiments of the above described compounds useful as encrypted optical markers of the invention. 
       Example 1 
     Synthesis of Rhodamine B Hydrazide 
       [0085]    
       
                 
         
             
             
         
       
     
         [0086]    Rhodamine B (479 mg) was dissolved in ethanol and excess of 65% aqueous hydrazine was added to this mixture under vigorous stirring. After refluxing for 2 hours the reaction mixture was cooled and off-white precipitate was collected. The solid was extensively washed with ethanol: water mixture and the 2-amino-3′,6′-bis(diethylamino)spiro[isoindoline-3,9′-xanthene]-1-one product was dried in vacuo. 
       Example 2 
     Synthesis of Eosin Y Diacetate 
       [0087]    
       
                 
         
             
             
         
       
     
         [0088]    Eosin Y (648 mg) was suspended in acetic anhydride and the mixture was stirred at reflux for 5 hours. Then the reaction mixture was slowly added to ice-water mixture. The resulting light brown solid was collected and washed with several portion of water. Then the product was dissolved in dimethylformamide (DMF) and precipitated by adding to water. After water wash the solid -acetoxy-2′,4′,5′,7′-tetrabromo-3-oxo-spiro[isobenzofuran-1,9′-xanthene]-3′-yl) acetate product was dried in vacuo. 
       Example 3 
     Synthesis of 1,2-Dimethylbenzothiazolium iodide 
       [0089]    
       
                 
         
             
             
         
       
     
         [0090]    To 2-methylbenzothiazole (2.54 ml) in anhydrous toluene iodomethane (2.5 ml) was added in portions. The mixture was refluxed for 6 hours, cooled down to room temperature and then left for several hours at 4° C. Resulting yellow precipitate was collected and washed repeatedly with acetone. The solid 2,3-dimethyl-1,3-benzothiazole hydroiodide product was then dried in vacuo. 
       Example 4 
     Synthesis of a Fluorescent Dysprosium Complex 
       [0091]    
       
                 
         
             
             
         
       
     
         [0092]    4,4,4-Trifluoro-1-phenyl-1,3-butanedione (649 mg) and 1,10-phenanthroline were dissolved in absolute ethanol (3 ml). To this solution aqueous sodium hydroxide (1 M) was added dropwise and the reaction mixture was stirred for 10 min. Separately, dysprosium nitrate hydrate (456 mg) was dissolved in water (1 ml) and the solution was added to the alkaline dione. The reaction mixture was stirred at 60 C for 5 hours to give a white solid form of the dimethyl-dysprosium; methyl; [(E)-[3-oxo-3-phenyl-1-(trifluoromethyl)propylidene]-3-oxidanyl]; 1,10-phenanthroline product. 
       Example 5 
     Synthesis of 2-Phenolsalicylimine 
       [0093]    
       
                 
         
             
             
         
       
     
         [0094]    A solution of 2-hydroxyaniline (2.84 g) in ethanol was added to a solution containing 2-hydroxybenzaldehyde (3.4 g) in ethanol. The mixture was refluxed for 2 hours under nitrogen and then it was cooled to room temperature. The orange 2-[(E)-(2-hydroxyphenyl)iminomethyl]-phenol precipitate was washed with ice cold ethanol and dried in vacuo. 
       Example 6 
     Synthesis of Fluorescein Dimethacrylate 
       [0095]    
       
                 
         
             
             
         
       
     
         [0000]    Fluorescein sodium salt (376 mg) was suspended in methacrylic anhydride and the mixture was stirred at reflux for 5 hours. Then the reaction mixture was slowly added to ice-water mixture. Resulting light brown solid was collected and washed with several portion of water. Then the product was dissolved in dimethylformamide and precipitated by adding to water. After water wash the yellow solid 6′-(2-methylprop-2-enoyloxy)-3-oxo-spiro[isobenzofuran-1,9′-xanthene]-3′-yl]2-methylprop-2-enoate product was dried in vacuo.