Fluoran compounds

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. 

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-... 

Fluoran compounds having two diarylamino groups at 3 - and 6 -positions generally develop blue tone colors. For example, 3, 6 -bis-(diphenylamino)fluoran (34 R1, R2 = H)29 develops reddish blue color, and 3 -diphenyl amino-6 -di-/ -tolylaminofluoran (34 R1 = H, R2 = CH3)29 and 3, 6 -bis(di-/i-tolylamino)fluoran (34 R1, R2 = CF13)29 blue color. 

The most remarkable feature of fluoran compounds is producing singly black color, which can hardly be attained by any other class of leuco dyes. 

Today, many black developing fluoran compounds having a wide variety of substituents on the amino group at 6 -position are available. These include 2 -anilino-3 -methyl-6 -(Af-methyl-Af- -propylamino)fluoran (53 ... 

In addition, dimer-type black developing fluoran compounds such as 2,2-bis(4-[6 -(/V-cyclohexyl-/V-methylamino)-3 -methylfluoran-2 -yl-amino]phenyl propane (59)60 are also proposed. Fluoran 59 has much lower solubility in organic solvents to improve image stability to plasticizer for use in thermosensitive recording label paper. 

Extension of conjugated double bond system at 2 - and/or 6 -positions makes it possible for fluoran compounds to have an absorption in the near infrared region up to 1200 nm. These include 2 -anilino-6-(4-anilinoanilino)-3 -methylfluoran (62),63 6 -[4-(4-anilinoanilino)anilino]-2 -chloro-3 -methylfluoran (63),7 6 -[4-(4-dimethylaminoanilino)anilino]-2 -methylflu-... 

Fluoran compounds have an optically active spiro-carbon atom. Consequently, some fluoran compounds, especially those having an alkylamino group of four or more carbon atoms at 3 -position, have been found to exhibit crystal modifications as determined by X-ray diffraction. Each crystal modification reveals different physical properties such as melting point, solubility, and affinity with acidic compounds, resulting in different characteristics regarding use for carbonless copying papers, thermosensitive recording papers, and the like. 

Table 1 shows a few examples of fluoran compounds having crystal modifications. 

The reaction of keto acids with phenols is mainly used to prepare fluoran compounds developing colors from orange to red. 

The reaction of the keto acids (66) with 4-alkoxydiphenylamines (75 R = CH3, C2H5) is widely used to prepare fluoran compounds developing green or black colors. The reaction in concentrated sulfuric acid gives intermediate phthalide compounds (76), which are then treated with base to convert into 2 -anilino-6 -aminofluorans (77) (Eq. 5). 

The reaction of the keto acids with 4-hydroxydiphenylamines (75 R = H) gives directly the fluoran compounds (77), not via phthalide intermediate. The yields, however, are much lower than those using 4-alkox ydiphen ylamines. 

Some fluoran compounds in Table 6 are found to form adducts with solvent. For example, when 6 -diethylamino-2 -(2,4-dimethylanilino)-3 -methylfluoran (77e) is recrystallized from toluene, it forms an adduct, mp 137-139°C, having 0.5mol of toluene of crystallization per mol of the fluoran the toluene of crystallization liberates on treatment with boiling n-hexane or isopropanol or on heating in vacuo. 2 -Anilino-6 -(N-cyclo-hexyl-N-methylaminoj-3 -methyl fluoran (77c) forms an adduct with... 

The reaction of keto acids (79) having no amino group with 3-alkoxy-diphenylamines (80) is used to synthesize 3 -anilinofluorans (81), especially near-infrared-absorbing fluoran compounds (Eq. 6). 

Table 7 shows melting points of a few near-infrared-absorbing fluoran compounds (81) thus prepared. 

Table 7. Melting Points of NIR-Absorbing Fluoran Compounds (81)...

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