Pyrylium dye

The ability of a range of 2,4,6-triaryl thio-, seleno- and telluro- pyrylium dyes to generate singlet oxygen and hence to function as sensitiseis for photodynamic therapy has been examined <99JMC3942,3953>. 

Pyruvic carboxylase Pyrylium dyes Pyrylocyanme radicals Pyrylocyamnes Py-Sal 25 Pythium spp. 

Since the initial report, photoinitiators containing other cationic dyes have appeared. These include indolenine dyes [38, 39], azulene dyes [40], pyrylium dyes, and... 

In a related phenomenon, the spectral shifts which accompany aggregation-deaggregation have been reported as a basis of yet another erasable medium.24 The active layer consists of discrete particles of a cocrystalline complex of a pyrylium dye with a polymer such as polycarbonate. Pulsed irradiation of this aggregated film resulted in an instantaneous deaggregation process which was accompanied by a hypsochromic shift of about 100 nm. The recording mechanism is believed to be thermal and a thermal erasure (reaggregation of the dye-polymer complex) was also demonstrated. 

The addition of a photoconductive dye or pigment to a polymer such as PVK is another way of extending the spectral response to the visible range. When sensitized by pyrylium dyes [16], PVK has a quantum efficiency of photogeneration as high as 0.56 at 10 V m ... 

Matsui, M. Kawamura, S. Shibata, K. Muramatsu, H. Synthesis and characterization of mono-, bis-, and tris-substituted pyridinium and pyrylium dyes. Bull. Chem. Soc. Jpn. 1992, 65, 71-74. 

FIGURE 11. (a) A chromogenic hybrid material containing a pyrylium dye anchored on the zeolite Beta walls, (b) The color changes observed in this hybrid material upon addition of certain amines. 

Dyes pyrylium also used for thiapyrylium and benzothiapyrylium. 

A novel indolinospirothiopyran dye, 2-A, A -dimethylaminoethenyl-(l",3",3"-trimethylindoline-2"-spiro-2 -2 /7-l -thiopyrano)[5, 6 -dbenzo[3]pyrylium iodide, has been prepared by the condensation of 1,2,3,3-tetramethylindolium iodide and 2-T,A-dimethylaminoethenyl-3-formyM/7-l-benzopyran -thione (Scheme 28) <1999JMC2991, 2000S247>. 

Condensation occurs in a basic, neutral or acidic medium and high yields are often obtained at moderate temperatures. An electron-releasing group in the aryl ring(s) attached to the pyrylium ion decreases the reactivity of the methylene group (66JGU1724). Similar reactions occur in the benzopyrylium and xanthylium series and the products sometimes have potential as dyes. 

The a- and y-methylene groups of pyrylium salts condense with pyranones and ben-zopyranones when the salts are heated in acetic anhydride or phosphorus oxychloride. Some of the resulting compounds are potentially interesting as dyes, for example the tripyran (110) (68JOC4418). In hot acetic anhydride equimolar amounts of the salt (111) and flavone condense to give a green-coloured salt (112) (78AP236). 

Methylene groups of pyrylium cations also react under mild conditions with nitrosoben-zenes in the presence of acetic anhydride and an alkali metal salt, which plays an essential role in the formation of Schiff s bases (116). The latter are hydrolyzed to the carbonyl compound (117) (71BSF3603). It is possible to form potential dyes by treatment of pyrylium salts with ortho esters such as triethyl orthoformate. The reactants are heated together in acetic anhydride or pyridine and trimethine dyes of type (118) and (119) are formed (70JHC1395, 71JOC600, 59CB2309). 

The reaction of croconic acid with 2,6-di-f-butyl-4-methylthiopyrylium salts yields the croconate dye 623 which absorbs in the infrared at 950 nm compared with 845 nm for the pyrylium analogue. This croconate dye is quite soluble in dichloromethane (180 mg ml-1), a feature that renders it attractive for thin film applications <2000JOC2236>. 

Methylideneseleno- and telluropyrans 53a and 53b are air oxidized to the corresponding bipyranylidenes 54a and 54b (Equation 18) <1995JOC6631 >. This reaction is involved in the chalcogen scrambling observed in the preparation of unsymmetrical pyrylium trimethine dyes (see Section 7.11.5.6). 

Ikeda et a/.121 122 who studied sensitization of PVK by benzopyrylium and pyrylium salts provided evidence that the sensitization of the photocurrent in PVK and fluorescence quenching involve an electron transfer from PVK to an excited dye. The lower than expected quantum yield of photocarrier generation (<0.1)122 is explained by the need for the thermal dissociation of the photogenerated charge transfer complex into free holes and electrons. 

Such compounds as polynuclear aromatics, heteroaromatics, ketones, quinones and dyes can serve as donors. Both excited singlet and triplet-states of these products can be involved in the PET. Diaryliodonium salts, triarylsulfonium salts, phosphonium salts, ammonium salts, pyrylium and thiapyrylium salts possess enough thermal stability and corresponding reduction potential to function as electron acceptors (R X+). In order to select suitable photoinitiator systems based on compounds discussed, the Weller-Eq. (5) can be employed. 

Fig. 3. Selected bond lengths (in A) and bond angles (in degrees) for a thiopyrylium-thiopyrylium (A) and a telluropyrylium-pyrylium (B) monomethine dye.

Other studies that have been reported regard spectral and luminescent properties of some pyrylium and thiopyrylium salts (86MI1), emission properties of -y,y -chalcogenopyrylotrimethine cyanine dyes (90JA3845), and the effect of the polymethine chain length on the fluorescence spectra of symmetrical chalcogenopyrylocyanine dyes (92MI3). 

Symmetrical tetra-rert-butyl-substituted pyrylium and thiopyrylium mo-nomethine dyes have been shown by H NMR to be mainly present in solution in the conformation E,Z (29) and Z,Z (30), respectively (88KGS167) (Section II,B). Conformational analysis of monomethine and trimethine cyanine dyes containing pyrylium and thiopyrylium nuclei has been also carried out using nuclear Overhauser effect (89MI1). 

Diphenylformamidine reacts with 2 equiv. of 2,6-di-ter/-butyl-4-methyl-thiopyrylium cation (62) to yield the trimethine dye 225 presumably through the Intermediacy of the anilinovinyl derivative 239 (81JAP81-30465). Analogous anilinovinyl derivatives (240 and 241) have been prepared by reaction of 4-methyl-2,6-diphenylthiopyrylium (204) and seleno-pyrylium (205) ions, respectively, with ethyl A -phenylformimidate (PhN=CHOEt) (74KGS53). 

Figure 1 The absorption spectra of (4-(4-dimethylaminophenyl)-2,6-diphenyl-thia-pyrylium perchlorate) in dichoromethane, a homogeneous film of the perchlorate compound in bisphenol-A polycarbonate, and the aggregated dye produced by exposing the homogeneous film to dichloromethane vapors.

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