For example, Ru(bpy)3 + is a poor choice for a n environmental probe because the promoted electron can distribute itself in a roughly spherical fashion with no overall dipole change and therefore shows almost no solvent effect on its absorption or emission. On the other hand, cisRu(bpy) 2 (CN) 2 h a s a n e n o r m o u s change in the dipole on MLCT excitation because t h e c h a r g e is moved toward the two bpy ligands. cî's-Ru(bpy) 2 (CN) 2 shows enormous solvatochromism in both emission and absorption as well as a large solvent sensitivity to lifetime. Solvatochromism is probably a requirem e n t for a n y s y s t e m t h a t w i l l d e m o n s t r a t e large e n v i r o n m e n t a l sensitivity. Other special probe properties can be built in by suitable ligand modification. For example, 5,6-dihydroxy1,10-phenanthroline complexes of Ru(II) show pH s e n s i t i v i t y , facRe(bpy)(CO) 3 NC(CH 2 ),,CH + (» = 0-17) can provide hydrophobic hooks to attach relatively polar complexes to hydrophobic media such as micelles or cyclodextrins. This can be used to anchor the probe to specific structural features of the target (19). T e m p e r a t u r e sensitivity can be achieved by having two states of different orbital types within kT of each other. Altering populations by temperature will then alter the decay rates, emission intensities, and lifetimes. Thermal activation of a d - d state is one choice, although careful design to minimize permanent decomposition becomes a p r o b l e m . Crosby and coworkers have reported extraordinarily large changes in lifetime versus temperature relationships for Ru(II)L 3 complexes (T < 40 K) with negligible changes in the quantum yields (22). They have suggested the use of these complexes as cryogenic thermometers. For room temperature, some of the Re(I) syst e m s with close π - π * a n d MLCT states are obvious choices; however, temperature dependence studies are just being started. For quenching-based oxygen sen sors the dominant design consider ations appear to be the highest life times and quantum yields (23-26). For a given class of complexes the bimolecular quenching constants are relatively insensitive to structure. Ruthenium(II) complexes have proved to be the most successful to date. Figure 6 shows a Stern-Volm e r q u e n c h i n g plot for R u ( 4 , 7 Ph 2 phen)| + in silicone rubber and a luminescence intensity versus time plot of the same material while being
breathed over. The sensitivity of the luminescence intensity to 0 2 concen t r a t i o n is quite evident. Note the lower 0 2 concentration (higher emis sion intensity) on the initial exhala tion (greater exchange time in the lungs), irregularities of the first few breaths because of the higher C 0 2 levels in the blood, and restoration of the equilibrium concentration. Although general guidance can be given for sensor/complex design, con siderable work is still required to fabricate a useful device. In many cases the support matrix for the com plex will alter its characteristics, sometimes in surprising ways. For example, we recently reported de tailed photophysics and photochem istry of the Ru(II)-based 0 2 sensors. The curvature of the Stern-Volmer quenching plots arises from ground state heterogeneity with different sites having different lifetimes and susceptibilities to oxygen quenching. For t h r e e different complexes, a two-site model gave superb fits to the quenching curves and is thus a potentially useful equation for fun d a m e n t a l studies a n d calibration (26). We also examined oxygen quench ing of Ru(II) complexes bound to a silica surface—a potentially useful sensor support (27). These systems
25.0
(a)
,
/
20.0
/
b 15.0
/
10.0 5.0
/ D
20.0 40.0 60.0 80.0
Oxygen pressure (cm Hg) " ' ' 1.0 • Breath
|
Exhalation * Normal breathing
0.5
0
15
30
45
Time (s)
Figure 6. Quenching of luminescence of Ru(4,7-Ph 2 phen)| + in silicone rubber. (a) Stern-Volmer quenching plot. The solid line is the best fit for a two-site model with different quenching constants. (Adapted from Reference 26.) (b) Luminescence intensity while being breathed over. (Adapted from Reference 25.)
High Performance Dycor™ Quadrupole Mass Spectrometers
The Dycor Quadrupole Mass Spectro meter offers a dynamic range of 7 orders of magnitude along with a high resolution CRT, analog bar, and tabular display modes, with an RS-232 port for computer interface as standard features. The Dycor product line is manufac tured at our facility in the U.S.A. This permits us to offer it at a price which is the most cost-effective in the industry. Whether your need is residual gas analysis, process monitoring, or leak detection, the microprocessor-based models provide you with the ultimate in performance. Applications include: • Residual Gas Analysis • Process Monitoring • Leak Defection • Chemical Vapor Deposition • Fermentation • Sputtering • Plasma Etching • Molecular Beam Epitaxy • Cryogenics • High Energy Physics • Vacuum Furnaces • Evaporation • Ion Beam Milling Features: • 1-100 or 1-200 AMU Range • Faraday Cup and Electron Multiplier • 9" or 12" High Resolution CRT • Analog Bar or Tabular Display • Pressure vs. Time Display • Linear to 4 Decade Log Scale • RS-232 Computer Interface • 10"· Torr Minimum Detectable Partial Pressure • Background Subtraction • Spectral Library • Sample Systems for higher Pressures For literature, contact AMETEK, Process and Analytical Instruments Div. 150 Freeport Road, Pittsburgh, PA 15238, TEL: 412-828-9040, FAX: 412-826-0399.
AMETEK PROCESS * ANALYTICAL INSTRUMENTS DIVISION
CIRCLE 3 ON READER SERVICE CARD
ANALYTICAL CHEMISTRY, VOL. 63, NO. 17, SEPTEMBER 1, 1991 • 835 A
