Behavior of organic compounds confined in monoliths of sol-gel silica glass. Effects of guest-host hydrogen bonding on uptake, release, and isomerization of the guest compounds

Various proteins, catalysts, and other compounds can be encapsulated or diffused into porous sol-gel glasses, but little is known about their interactions with the glass matrix. We report unexpectedly large effects that hydrogen bonding between organic compounds and sol-gel silica has on equilibria and reactions involving these guest compounds. Silica monolith immersed in a solution takes up the organic solute. Styrene, which is incapable of hydrogen bonding, becomes evenly distributed between external solution and the glass. Aniline and N,N-diethyl-p-methoxybenzamide, which are capable of hydrogen bonding, become extracted into the glass when the solvent (neat CCl4) does not interfere with their hydrogen bonding with silica. They become evenly distributed between the solution and the glass when a component of the solvent (DMF added to CCl4) or chemical modification (trimethylsilylation) of the silica surface suppresses hydrogen bonding of the guests with the surface. Ultraviolet spectra show that silica-guest interactions are present when the guest uptake is excessive and absent when this uptake is balanced. Ultraviolet spectra of aniline show that the hydrogen atoms are donated by silica to the guest. Not only the extent, but also the rate, of uptake is enhanced when the guest makes hydrogen bonds to the silica matrix; suppression of these bonds lowers the uptake rate. Silica monolith extracts trans-3,3'-diacetylazobenzene from a CCl4 solution 1250-fold. Upon addition of DMF, hydrogen bonds are broken, and the monolith completely releases the solute into the external solution. Five derivatives of azobenzene (3,3'-dimethyl-, 3-acetyl-, 3,3'-diacetyl-, 3,5-diacetyl-, and 3,3',5,5'-tetraacetylazobenzene), which differ in the propensity for accepting hydrogen atoms, served as photochromic probes and showed the effect of hydrogen bonding on reactivity. Both the photoinduced (trans-to-cis) and the subsequent thermal (cis-to-trans) isomerizations of the five derivatives obey the first-order law in glass as well as in free solution. When the solvent (neat CCl4) allows hydrogen bonding, the proportion of the isomers in the photostationary state differs between the glass and solution, and the rate constant for the thermal reaction is two to four times (in different derivatives) smaller in the glass than in solution. Evidently, hydrogen bonding retards the rearrangement of the probe molecules in the silica matrix. When hydrogen bonding is abolished (by addition of DMF to CCl4), the compositions of the photostationary state in the glass and in solution become equal, and so do the rate constants. Effects of hydrogen bonding on enzymes encapsulated in sol-gel glass and on the distribution of analytes between the glass monolith and the sample solution should be taken into consideration when designing accurate biosensors.