The chemical nature of surface adsorbates affects the localized surface plasmon resonance of metal nanoparticles. However, classical electromagnetic simulations are blind to this effect, whereas experiments are typically plagued by ensemble averaging that also includes size and shape variations. In this work, we are able to isolate the contribution of surface adsorbates to the plasmon resonance by carefully selecting adsorbate isomers, using single-particle spectroscopy to obtain homogeneous linewidths, and comparing experimental results to high-level quantum mechanical calculations based on embedded correlated wavefunction theory. Our approach allows us to indisputably show that nanoparticle plasmons are influenced by the chemical nature of the adsorbates 1,7-dicarbadodecaborane(12)-1-thiol (M1) and 1,7-dicarbadodecaborane(12)-9-thiol (M9). These surface adsorbates induce inside the metal electric dipoles that act as additional scattering centers for plasmon dephasing. In contrast, charge transfer from the plasmon to adsorbates—the most widely suggested mechanism to date—does not play a role here.

Nanoparticles synthesized from plasmonic metals can absorb low-energy light, producing an oscillation/excitation of their valence electron density that can be utilized in chemical conversions. For example, heterogeneous photocatalysis can be achieved within heterometallic antenna-reactor complexes (HMARCs), by coupling a reactive center at which a chemical reaction occurs to a plasmonic nanoparticle that acts as a light-absorbing antenna. For example, HMARCs composed of aluminum antennae and palladium (Pd) reactive centers have been demonstrated recently to catalyze selective hydrogenation of acetylene to ethylene. Here, we explore within a theoretical framework the rate-limiting step of hydrogen photodesorption from a Pd surface-crucial to achieving partial rather than full hydrogenation of acetylene-to understand the mechanism behind the photodesorption process within the HMARC assembly. To properly describe electronic excited states of the metal-molecule system, we employ embedded complete active space self-consistent field and n-electron valence state perturbation theory to second order within density functional embedding theory. The results of these calculations reveal that the photodesorption mechanism does not create a frequently invoked transient negative ion species but instead enhances population of available excited-state, low-barrier pathways that exhibit negligible charge-transfer character.

In this work, we study the photophysical properties of an adenine-based oligonucleotide using an ensemble of about 200 configurations obtained from molecular dynamics simulations. Specifically, a QM/MM approach is used to obtain the excited-state energies and properties of (dA)20(dT)20 with a dimer of π-stacked adenine bases included in the quantum region. The absorption and circular dichroism spectra are computed and analyzed using the algebraic diagrammatic construction through second order level of theory method (ADC(2)) combined with classical mechanics. We find that the experimentally observed red-shifted shoulder in the absorption spectrum is due to excitonic interactions, while charge-transfer states are present within the absorption band at the higher-energy end of the spectrum. More importantly, low-energy states with charge-transfer mixing exist, which could lead to excimers and bonded excimers. These observations suggest that mixing between charge-transfer and excitonic states plays an important role in the photophysics of oligonucleotides. They also highlight the importance of taking into account the conformational flexibility of the oligonucleotide when investigating photophysical properties.

Substitution can be used to efficiently tune the photophysical properties of chromophores. In this study, we examine the effect of substituents on the absorption and fluorescence properties of anthracene. The effects of mono-, di-, and tetrasubstitution of electron-donating and -withdrawing functional groups were explored. In addition, the influence of a donor-acceptor substituent pair and the position of substitution were investigated. Eleven functional groups were varied on positions 1, 2, and 9 of anthracene, and on position 6 of 2-methoxyanthracene and 2-carboxyanthracene. Moreover, the donor-acceptor pair NH/COH was added on different positions of anthracene for additional studies of doubly substituted anthracenes. Finally, we looked into quadruple substitutions on positions 1,4,5,8 and 2,3,6,7. Vertical excitation energies and oscillator strengths were computed using density functional theory with the hybrid CAM-B3LYP functional and 6-311G(d) basis set. Correlations between the excitation energies or oscillator strengths of the low-lying bright L state and the Hammett sigma parameter, σ, of the substituents were examined. The energy is red-shifted for all cases of substitution. Oscillator strengths increase when substituents are placed along the direction of the transition dipole moment of the bright L excited state. Substitution of long chain conjugated groups significantly increases the oscillator strength in comparison to the cases for other substituents. In addition, the results of quadruply substituted geometries reveal symmetric substitution at the 1,4,5,8 positions significantly increases the oscillator strength and can lower the band gap compared to that of the unsubstituted anthracene molecule by up to 0.5 eV.

A lot has been learned about the physical and chemical transformations that originate from the absorption of light by DNA, and computational chemistry has played a critical role in revealing the mechanisms of how these transformations occur. Nucleic acids consist of chromophores interacting via π stacking and hydrogen bonding. The fate of these systems after they absorb light is determined by the interplay and competition between pathways involving one chromophore or interacting chromophores. This Perspective highlights the role of π stacking in photophysical and photochemical processes in oligonucleotides and reveals the importance of excimers and exciplexes. Special types of excimers/exciplexes, characterized as bonded excimers/exciplexes, are also found to be important.