Time-resolved fluorescence emission and resonance-enhanced second harmonic generation (SHG) spectra were collected from 4-dimethylaminobenzonitrile (DMABN) adsorbed to the aqueous–silica interface in order to identify how strongly associating solvent–substrate interactions change DMABN’s photoisomerization properties. In bulk polar solution, DMABN forms an excited twisted intramolecular charge-transfer (TICT) state that emits with a distinctive, solvatochromic fluorescent signature. At the silica–aqueous interface, the TICT fluorescence disappears, similar to DMABN’s behavior in nonpolar environments. SHG spectra confirm that the interface is, in fact, polar, and DMABN’s unusual fluorescence emission acquired from the interface is attributed to strong hydrogen bonding associations between the water molecules and the silica surface that prevent adsorbate isomerization. Additionally, SHG spectra show a strong resonance at long wavelengths that is unexpected based on bulk absorbance spectra and selection rules for nonlinear hyperpolarizabilities. Using both Zerner’s INDO semiempirical and TD-DFT calculations, this spectroscopic behavior is attributed to a combination of strong electric fields present at the aqueous–silica interface and surface-induced changes to DMABN’s ground-state molecular structure.

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Second harmonic generation (SHG) was used to measure gold’s second-order nonlinear optical response as a function of temperature and as a function of ambient atmosphere composition. Using a bespoke temperature-controlled assembly, SHG signal was measured from an Au surface between 19 and 260 °C under atmospheres containing varying amounts (0–100%) of O2 with a balance of N2. SHG intensity diminished with increasing temperature, a result that is interpreted in terms of increased electron–phonon scattering at higher temperatures. Surprisingly, SHG intensity also depended on ambient atmosphere, with the SHG signal measured in air (20% O2) being ∼2 times larger than that measured in N2 at room temperature. This dependence of SHG intensity on atmospheric composition disappeared at high temperatures. Systematically titrating O2 into an N2 feed showed that the room-temperature enhancement of SHG signal from Au saturated at ∼15% O2. Despite widespread acceptance that O2 does not adsorb to a bulk Au surface, we propose that these results are consistent with a weakly physisorbed surface excess of O2 at the Au–gas interface. Fitting this O2 enhancement of Au’s SHG response to a Langmuir isotherm implies an adsorption energy on the order of 0.1 eV, consistent with results from previously reported DFT calculations. CO affects the SHG signal from Au in a manner similar to that of O2, implying that the SHG enhancement is due to back-bonding of Au’s conduction band electrons into vacant orbitals on either the O2 or CO adsorbates.