Triphenylmethyl cations, colors

Carbocations are a class of reactive intermediates that have been studied for 100 years, since the colored solution formed when triphenylmethanol was dissolved in sulfuric acid was characterized as containing the triphenylmethyl cation. In the early literature, cations such as Ph3C and the tert-butyl cation were referred to as carbonium ions. Following suggestions of Olah, such cations where the positive carbon has a coordination number of 3 are now termed carbenium ions with carbonium ions reserved for cases such as nonclassical ions where the coordination number is 5 or greater. Carbocation is the generic name for an ion with a positive charge on carbon. 

The fourth mechanism (4) invokes the electrostatic repulsion between photogenerated charges as the driving force for conformation changes. Triphenylmethane leucoderivatives (72) have been used as photoreceptor molecules. The chromophore dissociates into an ion-pair under ultraviolet irradiation, generating an intensely green colored triphenylmethyl cation. The cation thermally recombines the counter anion as follows. 

Figure 19 A and B show the rate of coloration at 660 nm and the gel expansion rate under continuous light irradiation. The triphenylmethyl cation is weU known to have a very strong absorption at 622 nm. Upon ultraviolet irradiation, the color...

The colors arise because species such as triphenylmethyl cations, (C HjljC, form and remain in the solution as their tetrachloroaluminate, AlCl4, salts ions of this sort are highly colored owing to the extensive delocalization of charge that is possible throughout the three aromatic rings. 

Triphenylmethane leuco derivatives are well-known photochromic molecules that dissociate into ion pairs under UV irradiation, prodncing intensely colored triphenylmethyl cations. The cations thermally recombine with connter ions as above. Triphenylmethane leuco derivatives have been widely used as photoreceptor molecules. 

Von Baeyer (Nobel Prize, 1905) should be credited for having recognized in 1902 the saltlike character of the compounds formed. He then suggested a correlation between the appearance of color and salt formation—the so-called halochromy. Gomberg (who had just shortly before discovered the related stable triphenylmethyl radical), as well as Walden, contributed to the evolving understanding of the structure of related cationic dyes such as malachite green. 

In an attempt to demonstrate the existence of pentavalent nitrogen, Schlenk and Holtz studied the reaction of triphenylmethyl sodium with tetramethylammonium chloride (52). The highly colored material was strongly conducting in polar solvents and could be identified as a salt, the stability of which is due to the resonance stabilization of the triphenyl-methide anion. In the absence of such stabilizing substituent effects (53), as with n-butyl or another alkyllithium reagent, a metalation of the tetramethylammonium cation occurs, which leads to type I products (18) ... 

In a 25-mL Erlenmeyer flask place 8.75 mL of acetic anhydride, cool the tube, and add 0.44 g of fluoboric acid. Add 1 g of triphenylmethanol with thorough stirring. Warm the mixture to give a homogeneous dark solution of the triphenylmethyl fluoborate, then add 0.4 g of cycloheptatriene. The color of the trityl cation should fade during this reaction and the tropilium fluoborate begin to precipitate. Add 10 mL of anhydrous ether to the reaction tube, stir the contents well while cooling on ice, and collect the product by filtration on the Hirsch funnel. Wash the product with 10 mL of dry ether and then dry the product between sheets of filter paper. 

Advantage can be taken of the ionizing and dissociating effects produced by a solvent to activate inert molecules and initiate a polymerization. For instance, triphenylmethyl chloride in pure sulfuric acid solution undergoes an instantaneous ionization and produces a triphenylmethylium cation with a characteristic red color ... 

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