Luminescence Effects in Gels Containing Organo-Aluminum Complexes
- PDF / 363,329 Bytes
- 10 Pages / 414.72 x 648 pts Page_size
- 113 Downloads / 206 Views
Introduction
The fluorescence behavior of metalorganic complex-doped sol-gel media is an area of great interest to those conducting research related to opto-chemical sensors (1) and materials associated novel photonic device architectures (2, 3). The sol-gel method comprises a low temperature preparative approach which is readily used to synthesize glassy materials containing chromophoric dopants. Di-(sec-butoxy)aluminoxytriethoxysilane (DBATES) is an especially useful sol-gel precursor, as it represents a double alkoxide which can be used to synthesize compositionally novel alumina-silica gel "copolymers". Hydrolyzed DBATES is an amphoteric species which tends to buffer precursor sols over a moderate (near 7) pH range. Acid and/or base catalysts are typically not used in the preparation of these systems. Thus, DBATES-derived gels containing flourescent dopants tend to maintain exceptional fluorophoric activity, as problems associated with chromophoric decomposition upon exposure to pH extremes are obviated. DBATES-derived gels offer high optical quality solid state media which are typically prepared under synthetically mild conditions. Derivatives of 8-hydroxyquinoline, such as 8-hydroxyquinoline-5-sulfonic acid (HQS) (see fig 1), have been used as fluorometric analytical reagents for the detection of metal ions such as aqueous A13 + (4). The unmodified HQS molecule is, itself, virtually non-luminescent. According to Schulman (5), the lowest excited state of HQS is the n-+ 7t* singlet, which generally possesses a very small radiative probability due, at least in part, to poor orbital overlap. The inherent structural flexibility of thesemolecules also tends to reduce the fluorescence yield. Upon complexation with species such as A13+, however, HQS-metal complexes become highly
569 Mat. Res. Soc. Symp. Proc. Vol. 346. 01994 Materials Research Society
SO 3 H
OH
8-Ilydroxyquinoline-5-sulfonic acid Figure 1
S0 31-1
10.2:1 water:isopropanol
I
N H add dropwise with vigorous stirring: 8-1 lydroxyquinoline-5-sulfoiuc acid
S1:1 Isopropanol:DBATES I
Cast into cuvettes and allow to gel DBATES Figure 2 570
luminescent. Tris(8-hydroxyquinolinolato)aluminum(mll)
(Alq3), for example, has been shown
to be a metalorganic complex which exhibits blue-green fluorescence and has a quantum efficiency of approximately 8%. The luminescence phenomenon in such chelates is reported to arise occurs when the non-bonding electrons on the HQS molecule interact with the metal center, and the energy of these nonbonding levels is concomitantly reduced. Thus, the lowest excited state for the HQS molecule is no longer the n---** singlet, but becomes instead a it---** singlet. The tr-4ir* singlet transition, which is highly allowed, has a very large oscillator strength. Such transitions are typically associated with fluorphores exhibiting high quantum yields such as the rhodamine and coumarin laser dyes. In addition, the rigidity of HQS-metal molecular complexes are greatly increased with respect to the noncomplexed parent structure. T
Data Loading...
