Cyanine radical

Furthermore, an intramolecular PET in ion-pairs involving dye cations is possible. So, cyanine borates exist in ester solution as ion pairs [145]. The irradiation of that species initially creates cyanine radicals and alkylboranyl radicals. Latter species fragment to triphenylborane and alkyl radicals see Eq. (13). In support of the assumption of an intra-ion-pair electron transfer, no cyanine radicals have been obtained by irradiation in acetonitrile solution. 

Figure 3. The absorption of cyanine dye (Cy) radicals monitored at 430 nm following excitation of a benzene solution with an 18 ps laser pulse. The time dependence of the absorption changes of cyanine radical for the benzyltriphenylborate case is faster than its decay. For the vinyltriphe-nylborate, back electron transfer and the reaction that follows electron transfer have competitive rates. For the tetraphenylborate salt, the back electron transfer process dominates after electron transfer, therefore the boron-carbon bond cleavage does not occur and almost no cyanine dye radical formation is observed (data adapted from [25]).

Table 1. Oxidation and reduction potential data, rate constants for electron transfer for cyanine borates in acetonitrile and benzene solution, and efficiency of cyanine radical formation.

An intrinsic semiconductor is characterized by a small band gap and a low density of highly mobile intrinsic charge carriers. Electrons as well as holes contribute to the conductivity which increases with temperature. Phthalo-cyanine radicals such as the sandwich type PC2LU or PcLi carry intrinsic charges. Their facile oxidation and reduction suggests that intrinsic conductivity should be possible. The electrical properties of these materials, especially as thin films incorporated in various devices, have been studied [32]. 

Substituents on the methine chain can stabilize the dye radical cation if the substituent (like methyl) is located on the high electron density carbons. However, no significant stabilization occurs when alkyl groups are on the alternate positions (like 9, 11 for the dication in Fig. 9). Current results for several dyes including die arbo cyanines and carbocyanines indicate that electronic stabilization of the dication radical lengthens the radical lifetime and also enhances the reversibiUty of the dimerization process (37). 

Dicarbocyanine and trie arbo cyanine laser dyes such as stmcture (1) (n = 2 and n = 3, X = oxygen) and stmcture (34) (n = 3) are photoexcited in ethanol solution to produce relatively long-Hved photoisomers (lO " -10 s), and the absorption spectra are shifted to longer wavelength by several tens of nanometers (41,42). In polar media like ethanol, the excited state relaxation times for trie arbo cyanine (34) (n = 3) are independent of the anion, but in less polar solvent (dichloroethane) significant dependence on the anion occurs (43). The carbocyanine from stmcture (34) (n = 1) exists as a tight ion pair with borate anions, represented RB(CgH5 )g, in benzene solution photoexcitation of this dye—anion pair yields a new, transient species, presumably due to intra-ion pair electron transfer from the borate to yield the neutral dye radical (ie, the reduced state of the dye) (44). 

Photopolymerization reactions are widely used for printing and photoresist appHcations (55). Spectral sensitization of cationic polymerization has utilized electron transfer from heteroaromatics, ketones, or dyes to initiators like iodonium or sulfonium salts (60). However, sensitized free-radical polymerization has been the main technology of choice (55). Spectral sensitizers over the wavelength region 300—700 nm are effective. AcryUc monomer polymerization, for example, is sensitized by xanthene, thiazine, acridine, cyanine, and merocyanine dyes. The required free-radical formation via these dyes may be achieved by hydrogen atom-transfer, electron-transfer, or exciplex formation with other initiator components of the photopolymer system. 

Various hybrid compounds comprised of two types of nitroxide radicals and either a pentamethine (Cy5) or trimethine cyanine (Cy3) were synthesized by Sato and co-workers [32]. These compounds seem to be promising fluorescent chemo-sensors for the measurement of reducing species such as Fe2+, ascorbic acid, and hydroxyl radicals. 

Sato S, Tsunoda M, Suzuki M, Kutsuna M, Takido-uchi K, Shindo M, Mizuguchi H, Obara H, Ohya H (2009) Synthesis and spectral properties of polymethine-cyanine dye-nitroxide radical hybrid compounds for use as fluorescence probes to monitor reducing species and radicals. Spectrochim Acta A 71 2030-2039... 

Ehlenfeldt MK and Prior RL. 2001. Oxygen radical absorbance capacity (ORAC) and phenolic and antho-cyanin concentrations in fruit and leaf tissues of highbush blueberry. J Agric Food Chem 49(5) 2222— 2227. 

The general structure with a cyanine unit at one terminus is represented in Figure 16. Two-electron transfer of the hybrid system produces another cyanine substructure via neutral radical state. In this case, a two step redox reaction is expected, because the neutral radical state is stabilized by the capto-dative substituents effect (19). Therefore, three colored sates will be achieved by the hybrid system. We call this system a cyanine-cyanine hybrid. 

Di(l-azulenyl)(6-azulenyl)methyl cation (24+) represented in Figure 17 exemplifies the cyanine-cyanine hybrid (20). Di(l-azulenyl)methylium unit in 24+ acts as a cyanine terminal group. The tropylium substructure stabilizes the cationic state (24+). Reduction of 24+ should afford the neutral radical 24, which is stabilized by capto-dative substitution effect, because 24 is substituted with azulenes in the donor and acceptor positions. The anionic state (24") is also stabilized by contribution of the cyclopentadienide substructure, which should exhibit the third color change in this system. 

In recent years visible photoinitiators for the formation of polymers via a radical chain reaction have also been developed. These absorb light which is blue, green, or red and also cause the polymerization of polyolacrylates, in some instances, such as encapsulated systems, with speed which is near photographic. Some of these photoinitiators provide the photochemical backbone of the nonsilver, near-photographic speed, imaging processes such as the Cycolor processes invented by the Mead Corporation. Cycolor initiators are cyanine dye, borate ion salts (4)—so-called ( +, —) ion pair... 

The dye radical formed by reduction of the dye molecule would have an additional electron, would not have the same electronic configuration, and possibly not the same geometric configuration compared to the excited dye molecule. Moreover, the electrochemical measurements contain contributions from solvation energy differences between the parent dye and its reduced or oxidized radicals (43). These contributions do not appear in the dye s optical transition energy. In addition, many cyanine dyes undergo irreversible redox reactions in solution and the potentials, as commonly measured, are not strictly reversible. Nevertheless, Loutfy and Sharp (260) showed that the absorption maxima of more than 50 sensitizing dyes in solution conformed approximately to the equation... 

Aryliodonium salts have been found to be coinitiators for photooxidizable dye sensitization (105). Smith polymerized aerylamide-bis(acrylamide) mixtures using acridine, xanthene, or cyanine dyes and, for example, diphenyllodonium chloride as an electron acceptor. Reduction of the salt results in the formation of phenyl radicals. 

More recently, Schuster [25] has demonstrated that cyanine dyes, i.e., cyanine borates or cyanine dye-borate mixtures, provide visible light activated initiation of free radical polymerization [26]. The photoexcited cyanine dye oxidizes alkyltriphenylborates by PET to produce the bleached reduced cyanine along with an alkyl radical. The alkyl radical can then initiate free radical polymerization [27], This visible light activated PET bond cleavage is of considerable importance in photoimaging and photocuring [28]. 

Apart from iodide ion, radicals are efficient quenchers of excited states of molecules [16] the processes of quenching of excited states of various molecules by radicals were studied earlier in detail [17 - 19]. It was shown that the triplet states of usual cyanine dyes are mainly quenched by the mechanism of acceleration of the intersystem crossing to the ground state (T-So). In this case, the quenching process is described by the following scheme ... 

In summary, the triplet decay kinetics of KI-K4 in the presence of DNA are biexponential the two observed components are attributed to two different complexes between the dye and DNA. Using nitroxyl radical and iodide ion as quenchers, we have shown that cyanine dyes form with DNA two types of complexes formed by binding of the dye in the groove of a DNA molecule and by intercalation of the dye between base pairs. 

Wang, H., Cao, G., and Prior, R.L., The oxygen radical absorbing capacity of antho-cyanins, J. Agric. Food Chem., 45, 304—309, 1997. 

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