J. Phy. Chem. C. (2016), 120, 22235-22247
Author(s): Kim, H.; Goodson, III. T.; Zimmerman, P. M.
In this combined computational and experimental study, specific chemical interactions affecting the prediction of one-electron and two-electron reduction potentials for anthraquinone derivatives are investigated. For 19 redox reactions in acidic aqueous solution, where AQ is reduced to hydroanthraquinone, density functional theory (DFT) with the polarizable continuum model (PCM) gives a mean absolute deviation (MAD) of 0.037 V for 16 species. DFT(PCM), however, highly overestimates three redox couples with a MAD of 0.194 V, which is almost 5 times that of the remaining 16. These three molecules have ether groups positioned for intramolecular hydrogen bonding that are not balanced with the intermolecular H-bonding of the solvent. This imbalanced description is corrected by quantum mechanics/molecular mechanics (QM/MM) simulations, which include explicit water molecules. The best theoretical estimations result in a good correlation with experiments, V(Theory) = 0.903V(Expt) + 0.007 with an R-2 value of 0.835 and an MAD of 0.033 V. In addition to the aqueous test set, 221 anthraquinone redox couples in aprotic solvent were studied. Five anthraquinone derivatives spanning a range of redox potentials were selected from this library, and their reduction potentials were measured by cyclic voltammetry. DFT(PCM) calculations predict the first reduction potential with high accuracy giving the linear relation, V(Theory) = 0.960V(Expt) – 0.049 with an R-2 value of 0.937 and an MAD of 0.051 V. This approach, however, significantly underestimates the second reduction potential, with an MAD of 0.329 V. It is shown herein that treatment of explicit ion-pair interactions between the anthraquinone derivatives and the cation of the supporting electrolyte is required for the accurate prediction of the second reduction potential. After the correction, V(Theory) = 1.045V(Expt) – 0.088 with an R-2 value 0.910 and an MAD value reduced by more than half to 0.145 V. Finally, molecular design principles are discussed that go beyond simple electron-donating and electron-withdrawing effects to lead to predictable and controllable reduction potentials.
ACS Chemical Biology (2016) 11, 3202-3213
Author(s): Doan, P.; Pitter, D. R. G.; Kocher, A.; Wilson, J. N.; Goodson, III. T.
The classical model for DNA groove binding states that groove binding molecules should adopt a crescent shape that closely matches the helical groove of DNA. Here, we present a new design strategy that does not obey this classical model. The DNA-binding mechanism of small organic molecules was investigated by synthesizing and examining a series of novel compounds that bind with DNA. This study has led to the emergence of structure property relationships for DNA-binding molecules and/or drugs, which reveals that the structure can be designed to either intercalate or groove bind with calf thymus dsDNA by modifying the electron acceptor properties of the central heterocyclic core. This suggests that the electron accepting abilities of the central core play a key role in the DNA-binding mechanism. These small molecules were characterized by steady-state and ultrafast nonlinear spectroscopies. Bioimaging experiments were performed in live cells to evaluate cellular uptake and localization of the novel small molecules. This report paves a new route for the design and development of small organic molecules, such as therapeutics, targeted at DNA as their performance and specificity is dependent on the DNA-binding mechanism.
J. of Am. Chem. Soc., (2016), 138, 16299-16307
Author(s): Abeyasinghe, N.; Kumar, S.; Sun, K.; Mansfield, J. F.; Jin, R.; Goodson, III. T.
New approaches in molecular nanoscopy are greatly desired for interrogation of biological, organic, and inorganic objects with sizes below the diffraction limit. Our current work investigates emergent monolayer-protected gold quantum dots (nanoclusters, NCs) composed of 25 Au atoms by utilizing two photon-excited fluorescence (TPEF) near-field scanning optical microscopy (NSOM) at single NC concentrations. Here, we demonstrate an approach to synthesize and isolate single NCs on solid glass substrates. Subsequent investigation of the NCs using TPEF NSOM reveals that, even when they are separated by distances of several tens of nanometers, we can excite and interrogate single NCs individually. Interestingly, we observe an enhanced two-photon absorption (TPA) cross section for single Au-25 NCs that can be attributed to few atom local field effects and to local field-induced microscopic cascading, indicating their potential for use in ultrasensitive sensing, disease diagnostics, cancer cell therapy, and molecular computers. Finally, we report room-temperature aperture based TPEF NSOM imaging of these NCs for the first time at 30 nm point resolution, which is a similar to 5-fold improvement compared to the previous best result for the same technique. This report unveils the unique combination of an unusually large TPA cross section and the high photostability of Au NCs to (non-destructively) investigate stable isolated single NCs using TPEF NSOM. This is the first reported optical study of monolayer-protected single quantum clusters, opening some very promising opportunities in spectroscopy of nanosized objects, bioimaging, ultrasensitive sensing, molecular computers, and high density data storage.
J. Phy. Chem. C. (2017), 121, 1349-1361
Author(s): Yau, S. H.; Ashenfelter, B. A.; Desireddy, A.; Ashwell, A. P.; Varnavski, O.; Schatz, G. C.; Bigioni, T. P.; Goodson, III. T.
The recent discovery of stable Ag nanoclusters presents new opportunities to understand the detailed electronic and optical properties of the metal core and the ligands using ultrafast spectroscopy. This paper focuses on Ag-32 and Ag-15 (with thiolate ligands), which are stable in solution. The steady state absorption spectra of Ag nanoclusters show interesting quantum size effects, expected for this size regime. Using a simple structural model for Ag-32, TDDFT calculations show absorption at 480 nm and 680 nm that are in reasonable correspondence with experiments. Ag-32(SG)(19) and Ag-15(SG)(11) have quantum yields up to 2 orders of magnitude higher than Au nanoclusters of similar sizes, with an emission maximum at 650 nm, identified as the metalligand state. The emission from both Ag nanoclusters has a common lifetime of about 130 ps and a common energy transfer rate of KEET (3) 9.7 X 10(9) s(1). A dark state competing with the emission process was also observed and was found to be directly related to the difference in quantum yield (QY) for the two Ag clusters. Two-photon excited emission was observed for Ag-15(SG)(11), with a cross-section of 34 GM under 800 nm excitation. Femtosecond transient absorption measurements for Ag-32 recorded a possible metal core state at 530 nm, a metalligand state at 651 nm, and ground state bleaches at 485 and 600 nm. The ground state bleach signals in the transient spectrum for Ag-32 are 100 nm blue-shifted in comparison to Au-25. The transient spectrum for Ag-15 shows a weak ground state bleach at similar to 480 nm and a broad excited state centered at 610 nm. TDDFT calculations indicate that the electronic and optical properties of Ag nanoclusters can be divided into core states and metalligand states, and photoexcitation generally involves a ligand to metal core transition. Subsequent relaxation leaves the electron in a core state, but the hole can be either ligand or core-localized. This leads to emission/relaxation that is consistent with the observed photophysics.
J. Phy. Chem. Lett., (2017), 8, 388-393
Author(s): Varnayski, O.; Pinsky, B.; Goodson, III. T.
We report the fluorescence emission from organic systems selectively excited by entangled pairs of photons. We have demonstrated a linear dependence of this two-photon excited fluorescence on the excitation intensity which is a unique nonclassical feature of two-photon interactions induced by entangled photons. The entangled photon (ETPA) excited fluorescence has been detected in several organic molecules possessing a high entangled photon absorption cross section. The ETPA fluorescence showed a nonmonotonic dependence on the delay between signal and idler beams. The fluorescence signal was detectable within the signal idler relative delay time interval of, similar to 100 fs. This time is comparable with the estimated entanglement time, T-E, making the ETPA-excited fluorescence in organic materials an ideal ultrafast coincidence detector. These results have widespread impact in applications ranging from spectroscopy to chemical and biological sensing, imaging, and microscopy.