Novel optical properties of n-alkylammonium dihydrogen phosphates under ultraviolet excitation and first principle study

n-Alkylammonium dihydrogen phosphates are the compounds comprised of organic alkyl ammonia cation and dihydrogen phosphate anion. These compounds can absorb ultraviolet and emit white visible light predominated by blue emission. Even afterglow as long as 4 min will be observed when they are exposed to 254 nm ultraviolet radiation. For the compounds without π orbits no metal ions luminescence center, it is difficult to explain these luminescence phenomena through normal theory. Therefore, the first principle studies were introduced to explore the electronic structures of these compounds. The calculational results demonstrate that the orbits energy levels of these compounds do not depend on the number of carbon atoms and ultraviolet absorption is supposed to be not derived from intrinsic orbits but some extrinsic levels. Combining with the optical spectrums, it is quite likely that the novel optical properties are linked with the hydrogen defects from dihydrogen phosphate anions. Furthermore, our results suggest that the generalized gradient approximation (GGA) methods are more appropriate than local density approximation (LDA) to calculate these compounds own hydrogen bonds.