Unpolished specimens with a more developed surface have lower color stability. Specimens stored in water develop some changes in their visual appearance. The presently proposed methods are effective in evaluating the luminescence of dental materials. Luminescence needs to be tested in addition to color, as the two characteristics are uncorrelated. It is important to further improve the color and luminescence stability of dental materials.
A novel fluorescent (p-phenylene ethynylene)-calix[4]arene-based polymer (CALIX-PPE) has been successfully synthesized by cross-coupling polymerization of bis-calix[4]arene 1 with 1,4-diethynylbenzene. The polycondensation was carried out in toluene/NEt 3 at 35 8C for 24 h, using PdCl 2 (PPh 3) 2 /CuI as the catalytic system, furnishing CALIX-PPE in excellent isolated yields (higher than 95%, several runs). The yellow polymer is freely soluble in several nonprotic organic solvents. The GPC trace of the isolated polymer showed a monomodal distribution and a number-average molecular weight of 23,300 g mol À1 (M w /M n ¼ 2.05). No evidence was found in the structural analysis (FTIR and 1 H/ 13 C NMR) regarding the formation of alkyne homocoupled segments along the polymer chain. For comparative purposes, the synthesis of an analogous poly(p-phenylene ethynylene) containing p-t-butyl-phenoxymethyl side chains (TBP-PPE) was also undertaken. A great similarity was found between the photophysical properties of CALIX-PPE and TBP-PPE in solution (UV-vis and laser induced luminescence), clearly demonstrating their unique dependence on the structure and conformation of the conjugated PPE backbone. The fluorescence spectra of polymers are of nearly identical shape, displaying their maximum emission around 420 nm. The calculated solution photoluminescence quantum yields of CALIX-PPE and TBP-PPE are of similar magnitude (/ F(CALIX-PPE) ¼ 0.43; / F(TBP-PPE) ¼ 0.51). V
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