TICT Probes

In a series of papers, Diphant has been used to probe the microviscous properties of various polymer oils, and free volume parameters have been extracted. 4,88 90) In a comparative study of Excimer and TICT probes, it could be shown that the response of these probes is frozen out at lower temperatures, as can be expected from the large reaction volume necessary, whereas the TICT reaction still shows sizable rates at these high-viscosity conditions. (26) Moreover, this study also showed that the free volume fraction measured by the TICT probes is larger than that measured by Excimer probes. 

The twisting motion in the TICT mechanism is also a large-amplitude motion which is easily stopped by very high-viscosity conditions, and therefore TICT molecules with various rotating moieties can be used in the development of free volume 

The dual fluorescing type 2 compounds DMABN and its ester derivatives have been used to extract detailed free volume parameters, especially for the case where the probes are covalently linked to the polymeric backbone, e.g., by an oxymethylene chain of variable length/97-101 As the TICT reaction requires a comparatively large reaction volume, the measurable effects are especially strong in solutions of these labeled polymers but they tend to disappear for the pure polymer due to its too-large 

DMABN suffers from the fact that dual fluorescence is only observable for polar media. Therefore, the pretwisted ester DMPYRBEE has been developed which shows dual fluorescence also in alkane solvents. 9 This probe allowed measurement of nonpolar polymeric siloxane oils and a comparison with the corresponding measurements using an EXCIMER probe. As expected from the decreased reaction volume necessary for the TICT photoreaction, the latter is usable down to much lower temperatures (higher viscosities) and probes a larger fraction of free volume. 26  

---

Molecular rotors with a dual emission band, such as DMABN or A/,A/-dimethyl-[4-(2-pyrimidin-4-yl-vinyl)-phenyl]-amine (DMA-2,4 38, Fig. 13) [64], allow to use the ratio between LE and TICT emission to eliminate instrument- and experiment-dependent factors analogous to (10). One example is the measurement of pH with the TICT probe p-A,A-dimethylaminobenzoic acid 39 [69]. The use of such an intensity ratio requires calibration with solvent gradients, and influences of solvent polarity may cause solvatochromic shifts and adversely influence the calibration. Probes with dual emission bands often have points in their emission spectra that are independent from the solvent properties, analogous to isosbestic points in absorption spectra. Emission at these wavelengths can be used as an internal calibration reference. 

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),... 

Figure 5.10. TICT probes used in probing of polymers.125,26 91 l03)...

Since the dynamics of the twisted intramolecular charge transfer (TICT) process is very sensitive to the polarity of the medium, the local polarity of an organized medium may also be determined from the rate of the HCT process. For TNS, which is nearly nonfluorescent in water ((j)f = 10 and Xf = 60 ps), the emission quantum yield and lifetime increases nearly 50 times on binding to cyclodextrins and more than 500 times on binding to a neutral micelle, TX -100 [86]. Such a dramatic increase in the emission intensity and lifetime arises because of the marked reduction of the nonradiative HCT process inside the less polar microenvironment of the cyclodextrins and the micelle. Determination of the micropolarities of various organized assemblies using TICT probes has been surveyed quite extensively in several recent reviews [5b-d,f,86]. Therefore, in this chapter we will focus only on some selected works not covered in the earlier reviews. 

Energy diagram of the photophysical processes involved upon excitation of TICT probes... 

In the ground state, the TICT probes (25) to (27) adopt a planar conformation. Photoexcitation can result in the formation of two excited states (see Diagram in Table... 

Molecular rotors are useful as reporters of their microenvironment, because their fluorescence emission allows to probe TICT formation and solvent interaction. Measurements are possible through steady-state spectroscopy and time-resolved spectroscopy. Three primary effects were identified in Sect. 2, namely, the solvent-dependent reorientation rate, the solvent-dependent quantum yield (which directly links to the reorientation rate), and the solvatochromic shift. Most commonly, molecular rotors exhibit a change in quantum yield as a consequence of nonradia-tive relaxation. Therefore, the fluorophore s quantum yield needs to be determined as accurately as possible. In steady-state spectroscopy, emission intensity can be calibrated with quantum yield standards. Alternatively, relative changes in emission intensity can be used, because the ratio of two intensities is identical to the ratio of the corresponding quantum yields if the fluid optical properties remain constant. For molecular rotors with nonradiative relaxation, the calibrated measurement of the quantum yield allows to approximately compute the rotational relaxation rate kor from the measured quantum yield [Pg.284]

Some TICT-forming fluorescent probes containing the / j - /V. /V - cl i a I k I a m i n o benzylidene malononitrile motif (usually related to 30 in Fig. 11) have been applied to monitor and quantify polymerization reaction, crosslinking, chain relaxation,... 

Rettig W, Lapouyade R (1994) Fluorescence probes based on twisted intramolecular charge transfer (TICT) states and other adiabatic photoreactions. Topics in fluorescence spectroscopy 4 109-149... 

Rettig W. and Lapouyade R. (1994) Fluorescent Probes Based on Twisted Intramolecular Charge Transfer (TICT) States and Other Adiabatic Reactions, in Lakowicz J. R. (Ed.), Topics in Fluorescence Spectroscopy, Vol. 4, Probe Design and Chemical Sensing, Plenum Press, New York, pp. 109-149. 

Fluorescence Probes Based on Twisted Intramolecular Charge Transfer (TICT) States and Other Adiabatic Photoreactions... 

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)... 

Various photochemical reaction mechanisms lend themselves for variation of the above parameters, and the more important ones are summarized above. In general, one can expect for the volume necessary for the reaction to occur that it decreases in the sense Excimer > TICT > Butterfly > ESIPT mechanism. Therefore, for a given size distribution of microscopic free volume voids, the fraction of the total free volume usable for the reaction and thus amenable to probing increases in the same sense with the ESIPT mechanism being the outstanding extreme, because this reaction cannot even be stopped in a rigid matrix at very low temperature. 39 ... 

---