Photoluminescence and quantum yields of urea and urethane cross-linked nanohybrids derived from carboxylic acid solvolysis

Urea and urethane cross-linked hybrids, classed as di-ureasils and di-urethanesils, were prepared through sol-gel derived carboxylic acid solvolysis. The resulting nanohybrids were characterized by X-ray diffraction, mid-infrared spectroscopy, Si-29 and C-13 nuclear magnetic resonance, and photoluminescence spectroscopy and results were compared with those of similar hybrid materials obtained from the conventional sol-gel route. The results indicate a similar structure for the hybrids, independent of the synthesis process used. All the hybrids are efficient room-temperature white-light emitters with emission quantum yields between 6 and 20%. The emission quantum yields of hybrids prepared through carboxylic acid solvolysis are 27-35% higher than those calculated for the di-ureasils and di-urethanesils synthesized via the conventional sol-gel technique. This is attributed to the presence of a larger number of nonbonded NH urea- and urethane-groups in the hybrids prepared by carboxylic acid solvolysis, illustrating the key role played by the synthetic method on the extent and magnitude of hydrogen bonding involving urea and urethane linkages.