TICT compounds

TICT probes can be used as polarity probes (Rettig and Lapouyade, 1994). The classical TICT compound exhibiting dual fluorescence, DMABN (see Section 3.4.4),... 

W. Rettig, Charge separation in excited states of decoupled systems-TICT compounds and implications regarding the development of new laser dyes and the primary processes of vision and photosynthesis, Angew. Chem. Int. Ed. Engl. 25, 971-988(1986). 

Some typical structural elements that occur in TICT compounds are collected in Figure 5.3. 

Figure 5.3. Common structural elements active as donors and acceptors in TICT compounds. The relevant linking positions are marked by arrows.

Figure 5.7, (a) Widely used biological probes of the TICT type. 4, s4 s7> (b) TICT compounds with dual fluorescence and interesting properties amenable to tailor-making of fluorescence probes. Functionalization and inclusion in larger molecular assemblies is easily possible, e.g., at the positions indicated by R. This holds especially for esters and amides which have been used as probes in large polymers 97-1011 or in TICT-labeled cyclodextrins.(l49)... 

TICT compounds may thus be useful as anionic-like photoreducers. This reactivity is, however, modest and will, of course, depend on the energy gap between the initial locally excited state and the TICT state itself. As far as the reactivity is concerned, these states may be considered as a contact ion pair in the excited state. 

The important classes of TICT compounds have been reviewed previously [2,8,15,16,17,18], and only the most remarkable features and latest developments will be reported here. 

The majority of TICT compounds investigated up to now contain (substituted) amino groups. In some cases, these are directly TICT-active (i.e. the bond linking the amino group twists). In other cases, the amino group merely helps to enhance the donor character of the donor moiety, and twisting occurs around a different (usually C-C) bond. The first set of compounds will be grouped under aromatic amines , the second one under aryl-anilines (Sect 4.2). 

By changing the acceptor system, a large variety of TICT compounds is available, only few of which have been studied more closely, e.g. CIN and C9A [54,55]. 

The original study was done using CIN as TICT catalyst [13,14]. A later study [138] compared the reactivity of CIN with the related TICT compounds C9A and CBN which possess a differently sized acceptor group. Donors were used as quenchers, and the good correlation of the quenching rate constants k, with the donor oxydation potential establish the expected electron transfer... 

Fig. 19. Possible scheme for use of TICT compounds as catalysts in photoreduction (e.g. photodechlorination of organic compounds ArCl). >, is a sacrificial donor to reduce the TICT cation radical back to the starting material in the ground state, TICT compounds can also be used as photooxydation catalysts as shown for the example of norbornadiene(Dq)-isomerization [13]. In this case, direct back electron transfer in the ground state takes place as shown in the figure...

An interesting new approach of using TICT compounds in intermolecular interactions are molecules with a crown ether function. These molecules can form complexes with cations, e.g. Ca +, and the complex can have different emission properties in the complexed and the uncomplexed state. In this way, specific ion probes usable in medicine, biology and analytical chemistry can be developed. The mechanism of cation sensing and the TICT-crown molecules studied to date [145-147] are collected in Fig. 20. 

As a fluorescence probe, a TICT compound possesses several merits. It can probe both microviscosity and micropolarity and furthermore, the mode of molecular motion necessary to the TICT phenomenon is well defined and limited to the rotation around a particular bond. This is a difference from intramolecular exciplex or excimer formation in which multiple modes of bond rotation and bending are compounded [7]. Fluorescence polarization studies also can not be free from ambiguity of the mode of rotation [8]. 

Absorption and emission spectra of radical copolymer of methyl methacrylate wi h N,N-dimethylaminobenzoyloxyethylmethacrylate ( < 0.017=, Mn = 3 X 10 )(l) and ethyl N,N-dlmethylaminobenzoate (ll) are compared in Figure 2. Except for a small red shift ( 3 nm) in I, the absorption spectra are almost identical. Although the presence of a ground state dimer or solute-solvent complex is known for N,N-dimethylaminobenzonitrile, the best studied TICT compound, there seems to be no such complications for the present studies. This is a favorable condition to study the TICT chromophore in polymers since polymers tend to enhance complicated molecular association even in extremely dilute solution [12] which overshadows the aimed photophysical phenomenon. 

Time-resolved fluorescence spectra of TICT compounds bonded to polymers are exemplified by studies on dimethylaminobenzene derivatives. 

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