A New Coumarin 4 Doped SiO2-PDMS ORMOSIL Bull. Korean Chem. Soc. 2003, Vol. 24, No. 3 299 Influence of pH and Dye Concentration on the Physical Properties and Microstructure of New Coumarin 4 Doped SiO 2 -PDMS ORMOSIL E. O. Oh, R. K. Gupta, N.-H. Cho, Y.-C. Yoo, W.-S. Cho, and C. M. Whang * School of Materials Science and Engineering, and Institute of Advanced Materials, Inha University, 253 Yonghyun-dong, Incheon 402-751, Korea Received September 11, 2002 Physical properties and microstructure of new coumarin 4 doped SiO 2 -PDMS ORMOSILs, synthesized by one-step (OS, acid-catalysis) and two-step (TS, acid-base catalysis) routes of sol-gel method with varying pH (0.6 to 7) and dye content (5 × 10 -4 to 5 × 10 -2 mole), are reported. BET, UV-visible spectroscopy and SEM were used for characterizations. The increase in acid or base concentration increased the size of pores and aggregated silica particles. The samples with pH ≤ 2.5 were transparent and attributed to the small size of pores (~20 Å) and silica particles. The samples with pH > 2.5 were translucent or opaque due to non-uniform pore system formed by voids and large aggregated silica particles. The surface area was found a key factor controlling the interactions between the gel matrix and the dye. The OS samples with the highest dye concentration exhibited the minimal values of pore size, surface area and silica particle size, resulting in the concentration-quenching phenomenon. Key Words : Physical properties, Microstructure, Organic dye, ORMOSIL Introduction The organic dye doped ORMOSILs (organically modified silicates) attract widespread interests in recent years due to their wide application as gain media in laser materials, non- linear optical materials, photochemical hole burning, luminescence solar concentrator, chemical sensor, pH sensor, etc. 1-3 The organic dyes show the high fluorescent quantum yields, large cross-sections for absorption-emission, low threshold power for laser action, attractive gain characteri- stics and tunability in the visible region. 1-3 The sol-gel derived ORMOSIL network contains a significant amount of organic functionalities, which offers great flexibility with respect to the chemical compatibility of the gel with the organic dye. Further, due to the low processing temperature, excellent homogeneity at molecular level, large flexibility in the control of sol-gel processing variables and high compositional purity, the sol-gel derived organic dye doped ORMOSIL showed several advantageous properties over the organic dye as a solute in a fluid media, e.g. elimination of translational freedom by avoiding inter-molecular inter- action, immunization of optical properties from high concent- ration interferences, reduction in rotational relaxation of the excited state of laser dyes, elimination of inherent problems with physical pumping, ease of use and replacement, and fabrication of samples in the desired form and shape. 1 Amongst the several investigated organic dye doped ORMOSILs, 4-12 the chemical bonded SiO 2 -PDMS ORMOSIL is noted as a useful host-matrix for doping a dye. A detailed investigation on the SiO 2 -PDMS ORMOSIL has already been carried out in the present laboratory because the tetraethoxysilane (TEOS, an inorganic precursor for SiO 2 ) shows a controllable hydro- lysis reaction rate and PDMS (poly(dimethylsiloxane)) possesses specific properties like transparency, hydrophobi- city, good chemical and thermal stability and similarity of its backbone structure (-Si-O-) to that of TEOS. 13,14 Recently, we doped coumarin 4 organic dye into the SiO 2 -PDMS ORMOSIL using one-step (acid catalysis) and two-step (acid- base catalysis) routes of sol-gel processing with varying acid (HCl), base (NH 4 OH) and dye concentrations to synthesize OS and TS samples, respectively. 15 Coumarin 4 was chosen as a organic dye because of its high fluorescence efficiency, chemical- and photo-stability. 7-9 It also exhibits specific molecular species associated with protonation-deprotanation reactions. 7-9 It was observed that the increase in the acid or base concentration increased the photo-luminescence peak intensity, which was attributed to an increase in the inter- action between the gel matrix and the dye molecules. 15 Concentration-quenching phenomenon was also noted with increasing dye concentration due to the deactivation process by the intermolecular energy transfer originating from the collision and aggregation of dye molecules. 15 The present paper, in series, reports the effects of pH and dye concentration on the physical properties and microstruc- ture of new coumarin 4 dye doped SiO 2 -PDMS xerogels: OS and TS. BET and scanning electron microscopy (SEM) were used to characterize the physical properties and microstructure, respectively. The transparency of the samples was monitored by UV-visible spectroscopy. Experimental Section Figure 1 shows the schematic flow chart for synthesizing OS and TS samples by acid catalysis (one-step) and acid- base catalysis (two-step) routes of sol-gel processing, respec- * Corresponding author. Fax: +82-32-874-3382, E-mail: cmwhang @inha.ac.kr