**2.1. General leuco-TAM and TAM+ molecules**

Generally, the Friedel-Crafts-type catalytic alkylation of aromatic rings with aromatic aldehydes is an effective method for TAM+ formation. (Li et al., 2008; Kraus et al., 2008) Several mild and efficient triaryl- and triheteroarylmethanes formations using [Ir(COD)Cl]2- SnCl4, AuCl3, Cu(OTf)2, and Sc(OTf)3 as catalysts have also been reported. Grignard reagents or n-butyl lithium compounds have also been used for their preparation. A brief summary of the preparation methods of general LTAM/TAM+ molecules is shown in Fig. 3.

**Figure 3.** The synthetic procedures for the commercially well-known LTAM molecules.

Although a number of methods are available for the synthesis of triarylmethanes, most are multistep processes and/or require harsh reaction conditions.

## **2.2. Fischer's base analogs of Leuco-TAM**

Fischer's base analogs of LTAM molecules can be obtained from a reaction of a molar excess of Fischer's base and substituted aryl aldehydes. The prepared LTAM dyes consist of two FB rings on the central carbon, where a substituted phenyl ring is located. The LTAM molecules can be symmetric or unsymmetric, depending on the identity of the two FB rings. They are the precursors of the TAM+ dyes, which are structurally close to the polymethine dyes (*e.g.*, Cy3, Cy5, *etc.*). (Ernst et al., 1989)

## *2.2.1. Symmetric LTAM FB analogs*

The FB analogs of symmetric leuco-TAM molecules {2,2′-(2-phenylpropane-1,3 diylidene)bis(1,3,3-trimethylindoline)} derivatives were obtained from the reaction of 5 substituted benzaldehyde and excess (2- to 3-fold) FB in ethanol at room temperature for 2–4 h, as shown in Fig. 4. The white precipitate was filtered from the reaction mixture and washed thoroughly with cold ethyl alcohol. Purification was carried out through

precipitation from acetone. TAM+ dyes were then obtained from a reaction of LTAM molecule with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) in the presence of HCl, followed by separation of the deep blue form from the product mixtures by column chromatography in MC/MeOH (9:1).

Melting points, yields, and other characteristic data of the prepared LTAM molecules are summarized in Table 1.

**Figure 4.** Synthetic scheme for symmetrical LTAM molecules.

#### *2.2.2. Unsymmetric LTAM FB analogs*

Unsymmetric LTAMs (Un-LTAM) were obtained from a reaction of excess Fischer's base with the substituted cinnamaldehydes, as shown in Fig. 5. The Un-LTAMs have two different FB skeletons on the central carbon, 1,3,3-trimethyl-2-methyleneindoline and 2 allylidene-1,3,3-trimethylindoline groups. The symmetric TAM+ dyes with styryl-ring pendants are expected to possess elongated conjugation from the N+ center of the FB ring to the phenyl ring. However, these LTAM molecules were not successfully obtained from the reactions of Fischer's base with the substituted cinnamaldehydes.

**Figure 5.** Synthetic scheme for unsymmetrical LTAM molecules.

Experimentally, Un-LTAM molecules were formed as the sole product and no symmetrical LTAM dyes were formed. This suggests that the Michael-type addition of the second molecule of FB occurs on the δ-carbon and not on the β-carbon of the extended ,βunsaturated iminium salts that were formed from the reaction of FB and cinnamaldehydes. The mechanistic processes for the formation of Un-LTAM molecules are shown in Fig. 6.

426 Advanced Aspects of Spectroscopy

summarized in Table 1.

chromatography in MC/MeOH (9:1).

**Figure 4.** Synthetic scheme for symmetrical LTAM molecules.

reactions of Fischer's base with the substituted cinnamaldehydes.

**Figure 5.** Synthetic scheme for unsymmetrical LTAM molecules.

*2.2.2. Unsymmetric LTAM FB analogs* 

precipitation from acetone. TAM+ dyes were then obtained from a reaction of LTAM molecule with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) in the presence of HCl, followed by separation of the deep blue form from the product mixtures by column

Melting points, yields, and other characteristic data of the prepared LTAM molecules are

Unsymmetric LTAMs (Un-LTAM) were obtained from a reaction of excess Fischer's base with the substituted cinnamaldehydes, as shown in Fig. 5. The Un-LTAMs have two different FB skeletons on the central carbon, 1,3,3-trimethyl-2-methyleneindoline and 2 allylidene-1,3,3-trimethylindoline groups. The symmetric TAM+ dyes with styryl-ring pendants are expected to possess elongated conjugation from the N+ center of the FB ring to the phenyl ring. However, these LTAM molecules were not successfully obtained from the

**Figure 6.** Mechanistic processes of the Michael-type addition of a FB molecule to the β and δ carbon of the α, β, γ, and δ-unsaturated iminium salts to form symmetrical and unsymmetrical LTAM dyes, respectively.

Melting points, yields, and other characteristic data of the prepared LTAM and Un-LTAM molecules are summarized in Table 1.


**Table 1.** Melting points (M.p.), yields, and colours of the prepared LTAM and Un-LTAM molecules.
