Enhanced detection of explosives by turn-on resonance Raman upon host-guest complexation in solution and the solid state.

The recognition of nitroaromatic explosives by a tetrakis-tetrathiafulvalene-calix[4]pyrrole receptor (TTF-C[4]P) yields a "turn on" and fingerprinting response in the resonance Raman scattering observed in solution and the solid state. Intensity changes in nitro vibrations with analyte complexation occur via a mechanism of resonance between the 785 nm laser line and the strongly absorbing charge-transfer chromophore arising from the complex between electron-donating TTF-C[4]P and electron-accepting nitroaromatic explosives. The addition of chloride forms the Cl-·TTF-C[4]P complex resetting the system for reuse.

Surface-Enhanced Spectroscopy for Higher-Order Light Scattering: A Combined Experimental and Theoretical Study of Second Hyper-Raman Scattering.

Motivated to explore the ultimate limits of surface-enhanced nonlinear spectroscopies, we report on the first observation of molecular second hyper-Raman scattering with the aid of surface enhancement and provide a new theoretical framework for first-principles calculations of the second hyper-Raman effect. Second hyper-Raman enhancement factors, determined to be a minimum of 10(5) times stronger than those in Raman scattering, demonstrate a clear trend toward larger enhancements for nonlinear phenomena, and the nearly quantitative agreement between simulation and experiment provides a unique spectroscopic window into higher-order molecular responses.

Multiscale Reactive Molecular Dynamics for Absolute pKa Predictions and Amino Acid Deprotonation.

Accurately calculating a weak acid's pKa from simulations remains a challenging task. We report a multiscale theoretical approach to calculate the free energy profile for acid ionization, resulting in accurate absolute pKa values in addition to insights into the underlying mechanism. Importantly, our approach minimizes empiricism by mapping electronic structure data (QM/MM forces) into a reactive molecular dynamics model capable of extensive sampling. Consequently, the bulk property of interest (the absolute pKa) is the natural consequence of the model, not a parameter used to fit it. This approach is applied to create reactive models of aspartic and glutamic acids. We show that these models predict the correct pKa values and provide ample statistics to probe the molecular mechanism of dissociation. This analysis shows changes in the solvation structure and Zundel-dominated transitions between the protonated acid, contact ion pair, and bulk solvated excess proton.

Non-Condon Effects on the Doubly Resonant Sum Frequency Generation of Rhodamine 6G.

We report first-principles simulations of the doubly resonance sum-frequency generation (DR-SFG) spectrum for rhodamine 6G (R6G). The simulations are done using a time-dependent formalism that includes both Franck-Condon (FC) and Herzberg-Teller (HT) terms in combination with time-dependent density functional theory (TDDFT) calculations. The simulated spectrum matches experiments, allowing a detailed assignment of the DR-SFG spectrum. Our work also shows that non-Condon effects are important and the DR-SFG spectrum of R6G is highly dependent on both FC and HT modes. This is surprising as R6G is known to be a strong FC resonant Raman scatterer. The simulations predict an orientation where the xanthene plane of R6G is perpendicular to the surface with binding through one of the ethyl amine groups. Our results show the importance of first-principles simulations for providing a detailed assignment of DR-SFG experiments, especially for large molecules where such an assignment is complicated due close-lying vibrational modes.

Simulating One-Photon Absorption and Resonance Raman Scattering Spectra Using Analytical Excited State Energy Gradients within Time-Dependent Density Functional Theory.

A parallel implementation of analytical time-dependent density functional theory gradients is presented for the quantum chemistry program NWChem. The implementation is based on the Lagrangian approach developed by Furche and Ahlrichs. To validate our implementation, we first calculate the Stokes shifts for a range of organic dye molecules using a diverse set of exchange-correlation functionals (traditional density functionals, global hybrids, and range-separated hybrids) followed by simulations of the one-photon absorption and resonance Raman scattering spectrum of the phenoxyl radical, the well-studied dye molecule rhodamine 6G, and a molecular host-guest complex (TTF⊂CBPQT(4+)). The study of organic dye molecules illustrates that B3LYP and CAM-B3LYP generally give the best agreement with experimentally determined Stokes shifts unless the excited state is a charge transfer state. Absorption, resonance Raman, and fluorescence simulations for the phenoxyl radical indicate that explicit solvation may be required for accurate characterization. For the host-guest complex and rhodamine 6G, it is demonstrated that absorption spectra can be simulated in good agreement with experimental data for most exchange-correlation functionals. However, because one-photon absorption spectra generally lack well-resolved vibrational features, resonance Raman simulations are necessary to evaluate the accuracy of the exchange-correlation functional for describing a potential energy surface.

Vibronic coupling simulations for linear and nonlinear optical processes: simulation results.

A vibronic coupling model based on time-dependent wavepacket approach is applied to simulate linear optical processes, such as one-photon absorbance and resonance Raman scattering, and nonlinear optical processes, such as two-photon absorbance and resonance hyper-Raman scattering, on a series of small molecules. Simulations employing both the long-range corrected approach in density functional theory and coupled cluster are compared and also examined based on available experimental data. Although many of the small molecules are prone to anharmonicity in their potential energy surfaces, the harmonic approach performs adequately. A detailed discussion of the non-Condon effects is illustrated by the molecules presented in this work. Linear and nonlinear Raman scattering simulations allow for the quantification of interference between the Franck-Condon and Herzberg-Teller terms for different molecules.

Vibronic coupling simulations for linear and nonlinear optical processes: theory.

A comprehensive vibronic coupling model based on the time-dependent wavepacket approach is derived to simulate linear optical processes, such as one-photon absorbance and resonance Raman scattering, and nonlinear optical processes, such as two-photon absorbance and resonance hyper-Raman scattering. This approach is particularly well suited for combination with first-principles calculations. Expressions for the Franck-Condon terms, and non-Condon effects via the Herzberg-Teller coupling approach in the independent-mode displaced harmonic oscillator model are presented. The significance of each contribution to the different spectral types is discussed briefly.

Probing two-photon properties of molecules: large non-Condon effects dominate the resonance hyper-Raman scattering of rhodamine 6G.

Experimentally measured resonance hyper-Raman (RHR) spectra spanning the S(1) ← S(0), S(2) ← S(0), and S(3) ← S(0) transitions in rhodamine 6G (R6G) have been recorded. These spectra are compared to the results of first-principles calculations of the RHR intensity that include both Franck-Condon (A-term) and non-Condon (B-term) scattering effects. Good agreement between the experimental and theoretical results is observed, demonstrating that first-principles calculations of hyper-Raman intensities are now possible for large molecules such as R6G. Such agreement indicates that RHR spectroscopy will now be a routine aid for probing multiphoton processes. This work further shows that optimization of molecular properties to enhance either A- or B-term scattering might yield molecules with significantly enhanced two-photon properties.

Molecular logic gates using surface-enhanced Raman-scattered light.

A voltage-activated molecular-plasmonics device was created to demonstrate molecular logic based on resonant surface-enhanced Raman scattering (SERS). SERS output was achieved by a combination of chromophore-plasmon coupling and surface adsorption at the interface between a solution and a gold nanodisc array. The chromophore was created by the self-assembly of a supramolecular complex with a redox-active guest molecule. The guest was reversibly oxidized at the gold surface to the +1 and +2 oxidation states, revealing spectra that were reproduced by calculations. State-specific SERS features enabled the demonstration of a multigate logic device with electronic input and optical output.

Assessment of the accuracy of long-range corrected functionals for describing the electronic and optical properties of silver clusters.

The absorption spectra and ionization potentials of silver clusters Ag(n) (n=4-20) are examined in the framework of density-functional theory (DFT) using several different methods of representing the exchange-correlation functional. Three different types of exchange-correlation functionals are used: those including gradient corrections to the density in the generalized gradient approximation, global hybrid functionals mixing in a portion of the Hartree-Fock exchange, and long-range-corrected (LC-) functionals. Comparison of ionization potentials calculated using DFT with those derived from experiments demonstrates that LC-functionals more accurately represent the electronic structure of the silver clusters studied. Absorption spectra are compared with both experimental spectra and those derived using higher level theoretical calculations showing that the LC-functionals appear to correctly describe the optical transitions in the gas phase, particularly when a small redshift in the experimental spectrum is accounted for due to matrix effects. It is also demonstrated that the LC-hybrid functionals significantly reduce the occurrence of spurious states in the optical absorbance spectrum while maintaining the intensity of plasmon like features of the spectra for larger silver clusters.

Understanding the Resonance Raman Scattering of Donor-Acceptor Complexes using Long-Range Corrected DFT.

The optical properties involving charge-transfer states of the donor-acceptor electron-transfer complexes carbazole/tetracyanoethylene (carbazole/TCNE) and hexamethylbenzene/tetracyanoethylene (HMB/TCNE) were investigated by utilizing the time-dependent theory of Heller to simulate absorbance and resonance Raman spectra. Excited-state properties were obtained using time-dependent density functional theory (TDDFT) using the global hybrid B3LYP and the long-range corrected LC- ωPBE functionals and compared with experimental results. It is shown that, while reasonable simulations of the absorbance spectra can be made using B3LYP, the resonance Raman spectra for both complexes are poorly described. The LC-ωPBE functional gives a more accurate representation of the excited-state potential energy surfaces in the Franck-Condon region for charge-transfer states, as indicated by the good agreement with the experimental resonance Raman spectrum. For the carbazole/TCNE complex, which includes contributions from two overlapping excited states on its absorbance spectrum, interference effects are discussed, and it is found that detuning from resonance with an excited state results in interference along with other factors. Total vibrational reorganization energy for both complexes is discussed, and it is found that both B3LYP and LC-ωPBE yield reasonable estimates of this quantity compared with experiment.