Evidence of Organic Luminescent Centers in Sol–Gel‐Synthesized Yttrium Aluminum Borate Matrix Leading to Bright Visible Emission

Abstract

Yttrium aluminum borate (YAB) powders prepared by sol-gel process have been investigated to understand their photoluminescence (PL) mechanism. The amorphous YAB powders exhibit bright visible PL from blue emission for powders calcined at 450 °C to broad white PL for higher calcination temperature. Thanks to C labelling, NMR and EPR studies show that propionic acid initially used to solubilize the yttrium nitrate is decomposed into aromatic molecules confined within the inorganic matrix. DTA-TG-MS analyses show around 2 wt % of carbogenic species. The PL broadening corresponds to the apparition of a new band at 550 nm, associated with the formation of aromatic species. Furthermore, pulsed ENDOR spectroscopy combined with DFT calculations enables us to ascribe EPR spectra to free radicals derived from small (2 to 3 rings) polycyclic aromatic hydrocarbons (PAH). PAH molecules are thus at the origin of the PL as corroborated by slow afterglow decay and thermoluminescence experiments.

Time-gated triplet-state optical spectroscopy to decipher organic luminophores embedded in rigid matrices

“…These a-YAB micrometer-sized powders prepared by modified Pechini and sol-gel methods give intense visible emissions with potential for developing metal ion-free luminescent materials, particularly phosphors for solid-state lighting. [4][5][6] Such intense visible luminescence has also been reported in many other inorganic materials obtained by various wet-chemical routes. [7][8][9][10][11] During the synthesis process, the metal-organic precursors undergo a complex chain of reactions, including polyesterification (Pechini route) or polycondensation (sol-gel chemistry) reactions for the preparation of initial resins or gels, followed by degradation and the associated structural transformation during the thermal treatments performed under controlled atmospheric conditions (pyrolysis or calcination).…”

“…In an earlier report, we identified the presence of certain organic compounds confined in the a-YAB matrix using solidstate NMR and EPR spectroscopies. 4 The pulsed-ENDOR associated DFT calculations revealed that the small (2-3 rings) PAH molecules exhibit good correlation with the PL properties of a-YAB powders. 4 The electronic structures and S-S/S-T excitation energies of these small PAH molecules are calculated here using TD-DFT methods (ESI, † Table S1).…”

“…4 The pulsed-ENDOR associated DFT calculations revealed that the small (2-3 rings) PAH molecules exhibit good correlation with the PL properties of a-YAB powders. 4 The electronic structures and S-S/S-T excitation energies of these small PAH molecules are calculated here using TD-DFT methods (ESI, † Table S1). Among the several selected compounds, the electronic structure of the phenalene molecule possesses a significant resemblance with the allowed transition of a luminophores presented in Fig.…”

“…However, the first theoretical triplet-state energy of phenalene is lower than the experimental results. It is to be noted that the calculated molecules are just the simplest working models (based on the radical species identified by magnetic resonance techniques), 4 while the true trapped molecule(s) will most probably be chemically related, exhibiting additional pending functional group(s), which may cause the favorable lift in the triplet-state energy. 21 The solid host matrix due to its polarization influence also brings a marked reduction in DE ST , associated with the hybrid charge transfer type characters of the singlet-triplet energy levels.…”

“…Indeed, we showed by differential thermal analysis and thermogravimetry, coupled with mass spectrometry, that the a-YAB powders contain significant amounts of carbonaceous (organic) compounds (200-10 000 ppm) arising from precursors and solvents. 4,5 Moreover, we were able to make progress in establishing a correlation of photoluminescence (PL) emission with the presence of aromatic compounds such as small polycyclic aromatic hydrocarbon (PAH) molecules based on the observations of solid-state NMR, EPR-ENDOR and associated DFT calculations. 4 However, still precise information on luminescent compounds is not fully at reach, which is crucial in improving the performance of these promising metal ion-free luminescent materials.…”