Fluorans

Xanthene dyes (qv) can be either acidic or basic. Acid xanthenes are known to exist in two tautomeric forms. The phenoHc type, or fluorans, are free-acid stmctures such as D C Orange No. 10 (17b) and D C Red No. 21 (17c). Most have poor water solubHity. In contrast to these, the quinoids or xanthenes are usuaHy the highly water-soluble sodium salt counterparts of the fluorans such as D C Orange No. 11 (18) and D C Red No. 22 (21a). Presendy, there are no certifiable basic xanthene colorants. 

Benzoyl leuco Methylene Blue (1), which is a phenothiazine leuco dye, has been known since 1900. The material was developed to extend the range of hues and colors obtainable in such applications as pressure-sensitive carbonless paper and to complement other classes of leuco dyes such as triarylmethanes, crystal violet lactone, and fluorans. Benzoyl leuco Basic Blue 3 (2), which is a phenoxazine leuco dye, is a more recent development. 

Colorless triarylmethane leuco materials 8 can be converted to carbon-ium ion (9)-colored materials, either by hydride abstraction or by chemical or photooxidation. In addition, some leuco compounds such as 11 can be converted to colored materials by treatment with an acid. The latter case is similar to the chemistry observed for fluoran (see Chapter 6) or phthalide (see Chapter 4) leuco compounds (Scheme 1). 

Fluoran (1) is the commonly used name for the spiro [isobenzofuran- 1,9 -xanthen]-3-one. Benzo[a]fluoran (2) has the benzene ring fused to the 1-and 2-positions of the xanthene moiety. Fusion at the 3- and 4-positions gives benzo[c]fluoran (3). Numbering of the atoms is employed as shown in 1-3. 

Fluoran compound used as leuco dye needs to have substituent(s) on the xanthene moiety to develop color, though fluoran 1 itself is prepared as a by-product in the synthesis of phenolphthalein from phenol and phthalic anhydride. 

Fluoran compounds developing colors are not new, having been well known since early times. For example, the Beilstein Handbook of Organic Chemistry, XIX describes many fluoran compounds developing colors from yellow to red. These include 3, 6 -dimethoxyfluoran (4 yellow), 3 -chloro-6 -diethylaminofluoran (5 vermilion), and 9 -diethylaminobenzo[a]fluoran (6 red). 

Fluoran compounds generally lack color stability, and therefore had lost their value as dyestuff for textile finishing. It is, however, very interesting that the old-fashioned fluoran compounds have come around as leuco dyes for use in the new applications. 

A singly black developing leuco dye was ultimately realized by the invention of 2 -anilino-6 -diethylamino-3 -methylfluoran (12).6 Fluoran 12 skillfully utilizes the steric hindrance of a methyl group at 3 -position to develop black color (see discussion below). Practically all black developing fluoran compounds marketed today are derivatives of 12, though each has an individual characteristic, especially for use in thermosensitive recording papers. 

In addition, fluoran compounds such as 6 -[4-(4-anilinoanilino)ani-lino]-2 -chloro-3 -methylfluoran (13)7 giving images readable by near-infrared rays have also been developed for POS (points of sales) labels that are recently being watched with keen interest. 

This chapter describes the properties and syntheses of fluoran compounds, and their applications as well. 

A perspective view of 12 is shown in Figure 6.2. X-ray structure analysis8 on the fluoran shows that the xanthene moiety is slightly bent... 

Figure 6.3. Reversible color-formation reaction between fluoran 9 and Bisphenol A.

Figure 6.4 shows the absorption spectra of the colored form of fluoran 12 developed by tin(IV) chloride in methyl alcohol. It is clear that the... 

Figure 6.4. Absorption spectra of colored fluoran 12 (31.5 pmol/liter in methyl alcohol) developed by tin(IV) chloride (curve 1, 2.5gmol curve 2, 5 gmol curve 3, lOgmol curve 4, 20 pmol).

Fluoran compounds have the remarkable feature of giving a wide variety of colors depending on their substituent(s). 

Fluoran compounds having an unsubstituted amino group at 3 -position also develop reddish yellow color. These include 6 -amino-2 -t-butylfluoran (15 R1, R3 = H, R2 = f-C4H9),12 6 -amino-2 -f-butyl-3 -... 

On the other hand, if fluoran compounds have an unsubstituted amino group at 4-position such as 5 -amino-2, 3, 7 -trimethylfluoran (16),12 they develop greenish blue color. 

In addition, 3 -((V-cyclohexyl-(V-methylamino)-6 -methylfluoran (19)4 develops orange color, whereas fluoran compounds having a disubstituted amino group at 3 -position generally develop red color as described in Section 6.2.2.3. 

Fluoran compounds having a dialkylamino group at 3 -position generally develop color from yellowish red to vermilion. These include... 

Fluoran compounds having an 7V-alkyl-/V-arylamino group at 3 -position such as 2 -chloro-6 -(7V-ethyl-4-methylanilino)fluoran (26 R1 = CH3, R2 = H, R3 = Cl),22 6 -((V-ethyl-4-methylanilino)-2 -methoxyfluoran (26 R1 = CH3, R2 = H, R3 = CH30),22 and 6 -(4-chloro-(V-ethylanilino)-2, 3 -dimethylfluoran (26 R1 = Cl, R2, R3 = CH3)22 also develop vermilion color, but these color tones are more bathochromic because of longer conjugated double bond system. 

Benzofluoran compounds develop more bathochromic color than the corresponding fluoran compounds because of the longer conjugated system. Thus, 9 -diethylaminobenzo[a]fluoran (27 R1, R2 = C2H5),20 9 -(A -ethyl-... 

In addition, 9 -cyclohexylaminobenzo[a]fluoran (28),24 lO -cyclohexyl-benzo[c]fluoran (29),24 etc. develop red color, despite the secondary amino group that contributes generally orange color. 

Benzofluoran compounds having an arylamino group develop much more bathochromic colors. Thus, 9 -(2-methylanilino)benzo[a]fluoran (30),24 10 -(4-methoxyanilino)benzo[c]fluoran (31),24 etc. develop violet color. 

If the arylamino group is substituted with an appropriate group to extend the conjugated double bond system, blue color can also be obtained. For example, 10 -(4-anilinoanilino)benzo[c]fluoran (32 R = C6H5NH)24 and 10 -(4-styrylanilino)benzo[c]fluoran (32 R = C6H5CH=CH)25 develop blue color. 

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