**4. Solid state structure**

#### **4.1. LTAM molecules**

The X-ray crystal structure of LTAM **12,** as a representative example, displays an orthorhombic crystal system with space group *Pna21,* with a residual factor of *R1 = 0.0517.*  ORTEP diagrams of LTAM **12**, showing atom numbering, are provided in Fig. 20(a).

For LTAM **12**, the C7-C8 and C7-C24 distances are 1.512 and 1.513 Å, respectively, *i.e.*, typical lengths for C-C single bonds, and the enamine C8-C9 and C24-C25 bonds are 1.335 and 1.335 Å, respectively, which are typical lengths for C=C bonds. The LTAM **12** molecules possess three-bladed propeller conformations, similar to earlier reports for various nonhetaryl LTAM dyes. The inter-plane angles between the aromatic rings A-B, A-C, and B-C in LTAM **12** are 81.29, 87.79 and 86.02°, respectively, as shown in Fig. 20(b).

442 Advanced Aspects of Spectroscopy

respectively.

various TAM+ dyes are shown in Fig. 19.

In the UV-Vis spectral data of Table 6, MG and crystal violet dyes show absorption maxima at 620 and 430 nm for the x- and y-band, respectively, whereas the absorption maxima of the vinyl-log of MG are red-shifted for both the x- and y-bands, *i.e.*, 651 and 488 nm, respectively. This suggests that the vinyl effects of a vinyl unit may, to a large extent, behave like extended conjugation for both the x- and y-bands. Chemical skeletons for the N~N+ and C(phenyl)~N+ responsible for the x- and y-band, respectively, in the absorption spectra of

Structurally, the FB analogs of symmetric and unsymmetric TAM+ dyes in this work can be characterized as Cy3 and Cy5 dyes, respectively, as closed-chain cyanines.(Ernst et al., 1989) It was reported that Cy3 is maximally excited at 550 nm and maximally emits at 570 nm in the orange-red part of the spectrum, whereas Cy5 is maximally excited at 649 nm and maximally emits at 670 nm, which is in the red part of the spectrum. Therefore, the x-band of the Un-TAM+ are expected to be higher than 650 nm and 550 nm, for the y- and x-band,

**Figure 19.** Chemical skeleton for the N~N+ and C(phenyl)~N+ responsible for the x- and y-band,

From the reaction of Un-LTAM **4** with HClO4, the decomposed product {5-chloro-1,3,3 trimethyl-2-((1E,3E)-4-(4-nitrophenyl)buta-1,3-dienyl)indolium perchlorate} was isolated, brown, yield 57%, M.p.= 257–258 °C, IR (KBr) 3072, 2984, 2934, 1707, 1596, 1340, and 1086 cm−1, 1H NMR (DMSO-*d*6) δ 1.76 (6H, s), 4.03 (3H, s), 7.37 (1H, d, *J* = 15.3 Hz), 7.66 (1H, dd, *J*  = 10.2, 15.3 Hz), 7.73 (1H, d, *J* = 9.0 Hz), 7.79 (1H, d, *J* = 15.3 Hz), 7.92 (2H, d, *J* = 6.9 Hz), 7.95 (1H, d, *J* = 9.0 Hz), 8.09 (1H, s), 8.33(1H, dd, (*J* = 10.2, 15.3 Hz), and 8.33(2H, d, *J* = 6.9 Hz).

The X-ray crystal structure of LTAM **12,** as a representative example, displays an orthorhombic crystal system with space group *Pna21,* with a residual factor of *R1 = 0.0517.* 

For LTAM **12**, the C7-C8 and C7-C24 distances are 1.512 and 1.513 Å, respectively, *i.e.*, typical lengths for C-C single bonds, and the enamine C8-C9 and C24-C25 bonds are 1.335 and 1.335 Å, respectively, which are typical lengths for C=C bonds. The LTAM **12** molecules possess three-bladed propeller conformations, similar to earlier reports for various non-

ORTEP diagrams of LTAM **12**, showing atom numbering, are provided in Fig. 20(a).

respectively, in the absorption spectra of various TAM+ dyes.

**4. Solid state structure** 

**4.1. LTAM molecules** 

**Figure 20.** ORTEP diagrams with atom numbering scheme (a) and the propeller shape (b) of LTAM **12**, showing the inter-plane angles.


Selected bond lengths and bond angles for the LTAM molecules are listed in Table 7.

**Table 7.** Selected bond lengths and bond angles of LTAM molecules.

The dihedral angles H8-C8-C7-H7 and H24-C24-C7-H7 in LTAM **12** are 172.06° (θ1) and 176.41° (θ2), respectively. The inter-plane angles and dihedral angles for the LTAM molecules are given in Table 8.


aSymbols (A-B) and numbering systems are as indicated in Fig. 20(b).

bSymbols (θ1 and θ2) are the dihedral angles H(8)-C(8)-C(7)-H(7) and H(24)-C(24)-C(7)-H(7), respectively.

**Table 8.** Inter-plane angles and dihedral angles for LTAM molecules in the solid state.

The C(7)=C(8) double bonds of the two 5-chloro Fischer's base moieties have *EE* configurations in **1**. In contrast, in **2**, the C(14)=C(15) and C(2)=C(3) double bonds of the two 5-chloro Fischer's base moieties belong to the *ZE* configuration. The *EE* (for **1**) and *ZE* (for **2**) isomers formed as the sole product in each case, despite the fact that three isomers, namely *ZE, EE,* and *ZZ*, are possible for these dyes which result from the reaction of excess 5-chloro Fischer's base and 4- and 3-pyridine carboxaldehyde.

Compound **12** is stacked so that a dimer is formed in the unit cell of the crystal. The packing in the unit cell of LTAM **12** is distinct, as can be seen in Fig. 21.

**Figure 21.** Molecular packing of LTAM **12**, showing the formation of a dimer.

#### **4.2. Un-LTAM molecules**

The Un-LTAM **4** was only successfully crystallized from acetone. Unfortunately, crystal growth was unsuccessful for the remainder of the Un-LTAM molecules. Selected bond lengths and bond angles are listed in Table 9.

Novel Fischer's Base Analogous of Leuco-TAM and TAM+ Dyes – Synthesis and Spectroscopic Characterization 445


**Table 9.** Selected bond lengths and bond angles of Un-LTAM **4**.

444 Advanced Aspects of Spectroscopy

aSymbols (A-B) and numbering systems are as indicated in Fig. 20(b).

Fischer's base and 4- and 3-pyridine carboxaldehyde.

in the unit cell of LTAM **12** is distinct, as can be seen in Fig. 21.

**Figure 21.** Molecular packing of LTAM **12**, showing the formation of a dimer.

**4.2. Un-LTAM molecules** 

lengths and bond angles are listed in Table 9.

Compound Interplane anglesa (º) Dihedral anglesb (º)

LTAM **1** 79.8 74.9 84.8 164.1 149.8 LTAM **4** 60.1 75.7 83.2 178.9 158.9 LTAM **7** 77.9 85.6 80.2 152.9 139.8 LTAM **8** 135.4 72.8 72.8 163.2 163.2 LTAM **9** 60.0 77.3 83.5 156.1 179.0 LTAM **11** 44.83 74.32 72.92 174.62 151.68 LTAM **12** 81.29 86.02 87.79 172.06 176.41

bSymbols (θ1 and θ2) are the dihedral angles H(8)-C(8)-C(7)-H(7) and H(24)-C(24)-C(7)-H(7), respectively. **Table 8.** Inter-plane angles and dihedral angles for LTAM molecules in the solid state.

The C(7)=C(8) double bonds of the two 5-chloro Fischer's base moieties have *EE* configurations in **1**. In contrast, in **2**, the C(14)=C(15) and C(2)=C(3) double bonds of the two 5-chloro Fischer's base moieties belong to the *ZE* configuration. The *EE* (for **1**) and *ZE* (for **2**) isomers formed as the sole product in each case, despite the fact that three isomers, namely *ZE, EE,* and *ZZ*, are possible for these dyes which result from the reaction of excess 5-chloro

Compound **12** is stacked so that a dimer is formed in the unit cell of the crystal. The packing

The Un-LTAM **4** was only successfully crystallized from acetone. Unfortunately, crystal growth was unsuccessful for the remainder of the Un-LTAM molecules. Selected bond

ring A-B ring B-C ring A-C <sup>1</sup> <sup>2</sup>

The X-ray crystal structure of Un-LTAM **4** shows a triclinic crystal system with space group *P-1*. An ORTEP diagram of Un-LTAM **4**, including the atom-numbering scheme, is shown in Fig. 22(a).

**Figure 22.** ORTEP diagrams with atom numbering scheme (a) and the propeller shape (b) of Un-LTAM **4**, showing the inter-plane angles.

The C9''-C10'' and C10''-C11''" distances are 1.516 and 1.509 Å, respectively, typical for C–C single bonds. The C7''-C8'' single bond distance, however, was 1.443 Å, which is shorter than a typical single bond and longer than a typical double bond. This is perhaps due to conjugation since the length (1.47 Å) of the central single bond of 1,3-butadiene is approximately 6 ppm shorter than that of the analogous single bond (1.53 Å) in butane. The two enamine C2=C7'' and C2'=C11'', and C8''=C9'' double bonds were 1.354, 1.324, and 1.328 Å, respectively, which are typical C=C bond lengths. In the crystal, the three aromatic rings of **4** are linked to three different layers, *viz.* a vinyl FB, a FB, and a phenyl group. The unsymmetrical molecule is a distorted version of the well-known three-bladed propeller conformation. (Keum et al., 2011). The inter-plane angles of the aromatic rings A–B, A–C, and B–C in Un-LTAM **4** are 87.4°, 67.5°, and 61.5°, respectively (Fig. 22(b)).

**Figure 23.** Chemical structures of Un-LTAM , with *ZEE* and *ZEZ* configurations.

The double bonds C2=C7'', C8''=C9'', and C2'=C11'' of Un-LTAM **4** have *EEE* configurations. The *EEE* isomers of these LTAM dyes are formed as the sole product in all cases, even though there are three possible isomers, the two other diastereomers being the *ZEE* and *ZEZ* isomers, as shown in Fig. 23. Generally, the central carbon-carbon double bond of these LTAM dyes is expected to have an *E* configuration.

Although the presence of the *ZEE* and *ZEZ* diastereomers was generally expected to be found in organic solvents, none of these isomers were detected, unlike for the LTAM molecules examined previously.

Fig. 24 shows the molecular packing diagram of Un-LTAM **4**, showing the formation of the dimer, which is stacked in an alternating fashion in the unit cell of the crystal. The intermolecular distances in the dimer are 8.53 and 9.40 Å, for the FB and phenyl rings, respectively.

**Figure 24.** Molecular packing diagram of Un-LTAM **4**, showing formation of the dimer.

#### **5. Conclusion**

Novel Fischer's base analogs of LTAM and Un-LTAM molecules and their corresponding TAM+ dyes have been successfully developed. 1H and 13C NMR assignments for the prepared LTAM molecules have been completed by 1D and 2D NMR experiments, including DEPT, COSY, HSQC, HMBC, and NOESY. The geometry of the double bond was Z*E* in most cases, as measured directly by NOESY. The *EE* and *ZZ* isomers have *C*<sup>2</sup> symmetry, and hence, the two FB rings of these isomers are identical. Therefore, the 1H NMR spectra of the *EE* and *ZZ* isomers are expected to be relatively simple compared to those of the *ZE* isomer. The novel LTAM molecules exist as a single isomer (*ZE* or *EE*) in the solid phase and they are equilibrated with other isomers in organic solvents. The percent ratios among the diastereomeric isomers of LTAM derivatives in the thermal equilibrium states vary according to the molecules examined and solvents used.

UV-Vis spectral data shows various structural forms of the LTAM and Un-LTAM molecules, such as (a) leuco-, (b) colored TAM+, (c) carbinol-, and (d) decomposed-forms, similar to the commercially known TAM+ dyes, such as MG, crystal violet, etc. Particularly, UV-Vis spectroscopic data for the Un-TAM+ dyes showed absorptions in the near-IR region.

X-ray crystal analysis showed that the *ZE* isomers were generally formed with a so-called three-bladed propeller conformation. These isomers stacked to form a dimer or double dimer. However, the *EE* isomers were also formed specifically for the LTAMs **3**, **5**, **8**, and **11**, which have a resonance-electron withdrawing (-R) group at the para-position of the phenyl ring. Further analysis of a variety of substituted LTAM molecules is required to determine what makes the diastereomer structures change in the solid state.
