Mechanism of Ultrafast Triplet Exciton Formation in Single Cocrystals of π-Stacked Electron Donors and Acceptors.

Ultrafast triplet formation in donor-acceptor (D-A) systems typically occurs by spin-orbit charge-transfer intersystem crossing (SOCT-ISC), which requires a significant orbital angular momentum change and is thus usually observed when the adjacent π systems of D and A are orthogonal; however, the results presented here show that subnanosecond triplet formation occurs in a series of D-A cocrystals that form one-dimensional cofacial π stacks. Using ultrafast transient absorption microscopy, photoexcitation of D-A single cocrystals, where D is coronene (Cor) or pyrene (Pyr) and A is N,N-bis(3'-pentyl)-perylene-3,4:9,10-bis(dicarboximide) (C5PDI) or naphthalene-1,4:5,8-tetracarboxydianhydride (NDA), results in formation of the charge transfer (CT) excitons Cor•+-C5PDI•-, Pyr•+-C5PDI•-, Cor•+-NDA•-, and Pyr•+-NDA•- in <300 fs, while triplet exciton formation occurs in τ = 125, 106, 484, and 958 ps, respectively. TDDFT calculations show that the SOCT-ISC rates correlate with charge delocalization in the CT exciton state. In addition, time-resolved EPR spectroscopy shows that Cor•+-C5PDI•- and Pyr•+-C5PDI•- recombine to form localized 3*C5PDI excitons with zero-field splittings of |D| = 1170 and 1250 MHz, respectively. In contrast, Cor•+-NDA•- and Pyr•+-NDA•- give triplet excitons in which |D| is only 1240 and 690 MHz, respectively, compared to that of NDA (2091 MHz), which is the lowest energy localized triplet exciton, indicating that the Cor-NDA and Pyr-NDA triplet excitons have significant CT character. These results show that charge delocalization in CT excitons impacts both ultrafast triplet formation as well as the CT character of the resultant triplet states.

Photon Upconversion in a Glowing Metal-Organic Framework.

The interaction of low-energy light with matter that leads to the production of high-energy light is known as photon upconversion. This phenomenon is of importance because of its potential applications in optoelectronics, energy harvesting, and the biomedical arena. Herein, we report a pillared-paddlewheel metal-organic framework (MOF), constructed from a tetrakis(4-carboxyphenyl)porphyrin sensitizer and a dipyridyl thiazolothiazole annihilator, designed for efficient triplet-triplet annihilation upconversion (TTA-UC). Single-crystal X-ray diffraction studies reveal that the Zn-metalated sensitizers are coordinated to Zn2 nodes in a paddlewheel fashion, forming 2D sheets, to which are linked annihilators, such that each sensitizer is connected to five of them. The precise arrangements of sensitizers with respect to annihilators, and the high annihilator-to-sensitizer ratio, facilitate Dexter energy transfer. This level of organization in an extended structure leads to a high TTA-UC efficiency of 1.95% (theoretical maximum = 50%) at an excitation power density of 25 mW cm-2.

Symmetry-Breaking Charge Separation in the Solid State: Tetra(phenoxy)perylenediimide Polycrystalline Films.

Generation of electron-hole pairs via symmetry-breaking charge separation (SB-CS) in photoexcited assemblies of organic chromophores is a potentially important route to enhancing the open-circuit voltage of organic photovoltaics. While most reports of SB-CS have focused on molecular dimers in solution where the environmental polarity can be manipulated, here, we investigate SB-CS in polycrystalline thin films of 1,6,7,12-tetra(phenoxy)perylene-3,4:9,10-bis(dicarboximide) having either n-octyl groups (octyl-tpPDI) or hydrogen atoms (H-tpPDI) attached to its imide nitrogen atoms. Structural analyses using various X-ray techniques reveal that while both compounds show π-π stacking in thin films, H-tpPDI is more slip-stacked than octyl-tpPDI and has intermolecular hydrogen bonds to its neighboring molecules. Transient absorption spectroscopy shows that octyl-tpPDI exhibits strong mixing between its singlet excited state and a charge transfer state, yielding an excimer-like state, while H-tpPDI undergoes nearly quantitative SB-CS, making the latter a promising candidate for use in organic photovoltaic devices.

Charge-transfer biexciton annihilation in a donor-acceptor co-crystal yields high-energy long-lived charge carriers.

Organic donor-acceptor (D-A) co-crystals have attracted much interest due to their important optical and electronic properties. Co-crystals having ⋯DADA⋯ π-stacked morphologies are especially interesting because photoexcitation produces a charge-transfer (CT) exciton, D˙+-A˙-, between adjacent D-A molecules. Although several studies have reported on the steady-state optical properties of this type of CT exciton, very few have measured the dynamics of its formation and decay in a single D-A co-crystal. We have co-crystallized a peri-xanthenoxanthene (PXX) donor with a N,N-bis(3-pentyl)-2,5,8,11-tetraphenylperylene-3,4:9,10-bis(dicarboximide) (Ph4PDI) acceptor to give an orthorhombic PXX-Ph4PDI ⋯DADA⋯ π-stacked co-crystal with a CT transition dipole moment that is perpendicular to the transition moments for S n ← S0 excitation of PXX and Ph4PDI. Using polarized, broadband, femtosecond pump-probe microscopy, we have determined that selective photoexcitation of Ph4PDI in the single co-crystal results in CT exciton formation within the 300 fs instrument response time. At early times (0.3 ≤ t ≤ 500 ps), the CT excitons decay with a t -1/2 dependence, which is attributed to CT biexciton annihilation within the one-dimensional ⋯DADA⋯ π-stacks producing high-energy, long-lived (>8 ns) electron-hole pairs in the crystal. These energetic charge carriers may prove useful in applications ranging from photovoltaics and opto-electronics to photocatalysis.

Three-Dimensional Characterization of Layers of Condensed Gas Molecules Forming Universally on Hydrophobic Surfaces.

Understanding the solvation layer of hydrophobic surfaces is essential for elucidating the interaction between hydrophobic surfaces in aqueous solutions. Despite their importance, little is known on these layers due to the lack of lateral resolution in spectroscopic or scattering experiments and probe instability in the static scanning probe methods used in most experiments. Using a high-resolution FM-AFM with stiff cantilevers and hydrophilic tips, we overcome this instability to provide the first detailed 3d maps of the solvation/hydration layer of two archetypal hydrophobic surfaces: graphite (HOPG) and self-assembled fluoro-alkane monolayer (FDTS). In degassed solutions we find different tip-surface interactions for the two surfaces; hydration oscillations superimposed on van der Waals attraction with HOPG and electrostatic repulsion with FDTS. Both are similar to interactions observed with hydrophilic surfaces. In solutions equilibrated with atmospheric air or high-pressure nitrogen, the tip-surface interaction changes dramatically, disclosing the formation of a 2-5 nm thick layer of condensed gas molecules adsorbed to the hydrophobic surfaces. This layer leads to strikingly similar tip-surface interactions for HOPG and FDTS with only weak dependence upon the concentration of dissolved gas molecules, indicating universality in the way hydrophobic surfaces present themselves to nondegassed aqueous solutions. Measurements at low cantilever oscillation amplitudes reveal the inner structure of the layer of condensed gas molecules with an average distance between its constituents, 0.5-0.8 nm, agreeing with recent molecular dynamics calculations. In addition to the uniform condensed layers, we probe sparse nanobubbles found on the surface. Those show distinct interaction with the tip, different from that with the flat layer.

Regulation of Surface Charge by Biological Osmolytes.

Osmolytes, small molecules synthesized by all organisms, play a crucial role in tuning protein stability and function under variable external conditions. Despite their electrical neutrality, osmolyte action is entwined with that of cellular salts and protons in a mechanism only partially understood. To elucidate this mechanism, we utilize an ultrahigh-resolution frequency modulation-AFM for measuring the effect of two biological osmolytes, urea and glycerol, on the surface charge of silica, an archetype protic surface with a pK value similar to that of acidic amino acids. We find that addition of urea, a known protein destabilizer, enhances silica's surface charge by more than 50%, an effect equivalent to a 4-unit increase of pH. Conversely, addition of glycerol, a protein stabilizer, practically neutralizes the silica surface, an effect equivalent to 2-units' reduction of pH. Simultaneous measurements of the interfacial liquid viscosity indicate that urea accumulates extensively near the silica surface, while glycerol depletes there. Comparison between the measured surface charge and Gouy-Chapman-Stern model for the silica surface shows that the modification of surface charge is 4 times too large to be explained by the change in dielectric constant upon addition of urea or glycerol. The model hence leads to the conclusion that surface charge is chiefly governed by the effect of osmolytes on the surface reaction constants, namely, on silanol deprotonation and on cation binding. These findings highlight the unexpectedly large effect that neutral osmolytes may have on surface charging and Coulomb interactions.

New Information on the Hydrophobic Interaction Revealed by Frequency Modulation AFM.

Using ultrahigh resolution atomic force microscopy (AFM) operated in frequency modulation mode, we extend existing measurements of the force acting between hydrophobic surfaces immersed in water in three essential ways. (1) The measurement range, which was previously limited to distances longer than 2-3 nm, is extended to cover all distances, down to contact. The measurements disclose that the long-range attraction observed also by conventional techniques, turns at distances shorter than 1-2 nm into pronounced repulsion. (2) Simultaneous measurements of the dissipative component of the tip-surface interaction reveal an anomalously large dissipation commencing abruptly at the point where attraction begins. The dissipation is more than 2 orders of magnitude larger than expected from bulk water viscosity or from similar measurements between hydrophilic surfaces. (3) The short-range repulsion is oscillatory, indicating molecular ordering of the medium as the hydrophobic surfaces approach each other. The oscillation period, ∼0.5 nm, is larger than the ∼0.3 nm period observed with hydrophilic surfaces. Their range, ∼1.5 nm, is longer as well. These observations are consistent with a conspicuous change in the properties of the surrounding medium, taking place simultaneously with the onset of attraction as the two surfaces approach each other.

Autopilot for frequency-modulation atomic force microscopy.

One of the most challenging aspects of operating an atomic force microscope (AFM) is finding optimal feedback parameters. This statement applies particularly to frequency-modulation AFM (FM-AFM), which utilizes three feedback loops to control the cantilever excitation amplitude, cantilever excitation frequency, and z-piezo extension. These loops are regulated by a set of feedback parameters, tuned by the user to optimize stability, sensitivity, and noise in the imaging process. Optimization of these parameters is difficult due to the coupling between the frequency and z-piezo feedback loops by the non-linear tip-sample interaction. Four proportional-integral (PI) parameters and two lock-in parameters regulating these loops require simultaneous optimization in the presence of a varying unknown tip-sample coupling. Presently, this optimization is done manually in a tedious process of trial and error. Here, we report on the development and implementation of an algorithm that computes the control parameters automatically. The algorithm reads the unperturbed cantilever resonance frequency, its quality factor, and the z-piezo driving signal power spectral density. It analyzes the poles and zeros of the total closed loop transfer function, extracts the unknown tip-sample transfer function, and finds four PI parameters and two lock-in parameters for the frequency and z-piezo control loops that optimize the bandwidth and step response of the total system. Implementation of the algorithm in a home-built AFM shows that the calculated parameters are consistently excellent and rarely require further tweaking by the user. The new algorithm saves the precious time of experienced users, facilitates utilization of FM-AFM by casual users, and removes the main hurdle on the way to fully automated FM-AFM.