Ramkrishna Adhikary

Organic−Inorganic Nanocomposites by Placing Conjugated Polymers in Intimate Contact with Quantum Rods

Recent advances in the synthesis [ 1 ] and assembly [ 2 ] of nanocrystals (NCs) provide unique opportunities to exploit NCs for the development of next generation organic/inorganic hybrid solar cells as one of the most promising alternatives to Si solar cells to deliver effi cient energy conversion with inexpensive fabrication. [ 1 , 2 ] These conjugated polymer-based photovoltaic devices capitalize on the advantages peculiar to conjugated polymers (CPs), such as light weight, fl exibility, processability, roll-to-roll production, low cost, and large area, in conjunction with the high electron mobility and tunable optical properties of inorganic NCs. Of the organic/inorganic hybrids, poly(3-hexylthiopene) (P3HT) is one of the most extensively utilized CPs due to its excellent solution processability, environmental stability, high charge carrier mobility, and tailorable electrochemical properties. [ 3 , 4 ] CdSe quantum dots (QDs) are the most commonly investigated NCs because of their quantum-confi ned nature and well-matched energy level with P3HT. [ 5–11 ]

Semiconductor Anisotropic Nanocomposites Obtained by Directly Coupling Conjugated Polymers with Quantum Rods

Conjugated polymers (CPs) have received considerable attention as promising materials for use in organic photovoltaics, light-emitting diodes (LEDs), thin film transistors, and biosensors. Among various types of CPs, poly(3hexylthiopene) (P3HT) is one of the most widely studied organic semiconductors. P3HT possesses excellent solution processability, environmental stability, high charge-carrier mobility, and tailorable electrochemical properties. Owing to their quantum-confined nature, for quantum dots (QDs) such as cadmium selenide (CdSe), variation of the nanocrystal size provides continuous and predictable changes in fluorescence emission, thus rendering them useful for a wide range of applications in photovoltaic cells, 7] LEDs, biosensors, and bio-imaging. CP-based organic/inorganic hybrid solar cells (e.g., CP/QD composites) are favorable alternatives to inorganic solar cells as they have many advantages peculiar to CPs, such as light weight, flexibility, processability, roll-to-roll production, low cost, and large area. However, the CP/QD composites are most often prepared by simply physically mixing the CPs and QDs. This procedure, however, suffers from several severe problems, including microscopic phase separation and the existence of insulating interfacial layers, thereby reducing the interfacial area between CPs and QDs and thus limiting the performance of the resulting devices. Recently, various methods have been utilized to overcome these problems, such as the use of cosolvent mixtures or binary solvent mixtures and surface modification of QDs. The most elegant approach is to chemically tether CPs on the QD surface (i.e., preparing CP–QD nanocomposites), hence enabling direct electronic coupling between CPs and QDs. Notably, this strategy has only recently been developed and has been primarily implemented by ligand exchange, which permits the derivatization of the composite with a broad range of functional groups. However, ligandexchange chemistry suffers from incomplete surface coverage. In this context, recently P3HT–CdSe-QD nanocomposites have been synthesized by directly grafting vinyl-terminated P3HT onto a [(4-bromophenyl)methyl]dioctylphosphine oxide(DOPO–Br)-functionalized CdSe QD surface by a mild palladium-catalyzed Heck coupling without the need for ligand exchange. The ability to manipulate the shape of nanocrystals has led to quantum rods (hereafter referred to as nanorods; NRs) with diameters that range from 2 to 10 nm and lengths ranging from 5 to 100 nm. Owing to their intrinsic structural anisotropy, NRs possess many unique properties that make them potentially better nanocrystals than QDs for photovoltaics and biomedical applications. Photovoltaic cells made of NRs and CPs show an improved optical absorption in the red and near-infrared ranges that originates from the NRs. Moreover, the long axis of the NRs provides continuous paths for the transport of electrons, an advantage over QDs, in which electron hopping between QDs is required. The performance of photovoltaic cells can be further improved if NRs are vertically aligned between two electrodes to minimize the carrier transport pathways. It is worth noting that although CP–NR nanocomposites were recently produced by ligand exchange of CPs with insulating ligands that were initially attached to the NR surface, direct grafting of CPs onto anisotropic nanocrystals has not yet been explored. Herein, we report one simple yet robust route to CP–NR nanocomposites that displaces the need for ligand-exchange chemistry. In this strategy, the catalyst-free alkyne–azide cycloaddition, which belongs to the emerging field of click chemistry, was utilized in the preparation of P3HT–CdSeNR nanocomposites. As shown in Scheme 1, CdSe NRs were passivated with bromobenzylphosphonic acid (BBPA), which not only induced elongated growth but also functionalized the CdSe NR surface and led to the formation of BBPA–CdSeNRs. Subsequently, the aryl bromide groups of BBPA were converted into azide groups, thus forming N3–BPA–CdSe NRs. Finally, catalyst-free Huisgen 1,3-dipolar cycloaddition between ethynyl-terminated P3HT and N3–BPA–CdSe NRs successfully gave P3HT–CdSe NR nanocomposites without the introduction of any deleterious metallic impurity. Click reactions possess several attractive features, including an extremely versatile bond-formation process, no need for protecting groups, good selectivity, nearly complete conversion, and generally no need for purification. As such, it stands out as a promising method to simplify the synthetic procedure and opens opportunities to increase the grafting density for large-scale synthesis. The charge transfer occurred at the P3HT/CdSe NR interface and was confirmed by [*] L. Zhao, Dr. X. Pang, Prof. Z. Lin Department of Materials Science and Engineering Iowa State University, Ames, IA 50011 (USA) and School of Materials Science and Engineering Georgia Institute of Technology, Atlanta GA 30332 (USA) Fax: (+1)515-294-7202 E-mail: zqlin@iastate.edu

Excited-State Intramolecular Hydrogen Atom Transfer and Solvation Dynamics of the Medicinal Pigment Curcumin

The potential use of the naturally occurring yellow-orange pigment curcumin as a photodynamic therapy agent is one of the most exciting applications of this medicinal compound. Although subnanosecond spectroscopy has been used to investigate the photophysical processes of curcumin, the time resolution is insufficient to detect important and faster photoinduced processes, including solvation and excited-state intramolecular hydrogen atom transfer (ESIHT). In this study, the excited-state photophysics of curcumin is studied by means of ultrafast fluorescence upconversion spectroscopy. The results show two decay components in the excited-state kinetics with time scales of 12-20 ps and approximately 100 ps in methanol and ethylene glycol. The resulting prominent isotope effect in the long component upon deuteration indicates that curcumin undergoes ESIHT in these solvents. The short component (12-20 ps) is insensitive to deuteration, and multiwavelength fluorescence upconversion results show that this decay component is due to solvation of excited-state curcumin.

Excited-State Intramolecular Hydrogen Atom Transfer of Curcumin in Surfactant Micelles

Femtosecond fluorescence upconversion experiments were performed on the naturally occurring medicinal pigment, curcumin, in anionic, cationic, and neutral micelles. In our studies, the micelles are composed of sodium dodecyl sulfate (SDS), dodecyl trimethyl ammonium bromide (DTAB), and triton X-100 (TX-100). We demonstrate that the excited-state kinetics of curcumin in micelles have a fast (3-8 ps) and slow (50-80 ps) component. While deuteration of curcumin has a negligible effect on the fast component, the slow component exhibits a pronounced isotope effect of approximately 1.6, indicating that micelle-captured curcumin undergoes excited-state intramolecular hydrogen atom transfer. Studies of solvation dynamics of curcumin in a 10 ps time window reveal a fast component (< or = 300 fs) followed by a 8, 6, and 3 ps component in the solvation correlation function for the TX-100, DTAB, and SDS micelles, respectively.

Solvation dynamics of the fluorescent probe PRODAN in heterogeneous environments: contributions from the locally excited and charge-transferred states.

The coexistence of different excited states with different properties of the same chromophores could have significant consequences for the accurate characterization of solvation dynamics in a heterogeneous environment, such as a protein. The purpose of this work is to study the contributions of the locally excited (LE) and charge-transferred (CT) states of the fluorescent probe molecule 6-propionyl-2-(N,N-dimethylamino)naphthalene (PRODAN) to its solvation dynamics in the heterogeneous environment provided by reverse micelles formed by sodium 1,4-bis-(2-ethylhexyl) sulfosuccinate (AOT)/n-heptane/water. We have found that the LE and CT states of PRODAN solvate on different time scales in reverse micelles (2 and approximately 0.4 ns, respectively), consistent with results suggested in the literature, and have concluded that PRODANs use as a probe of heterogeneous environments must be used with caution and that, more importantly, the same caution must be exercised with any chromophore capable of emitting from different excited states.

Considerations for the Construction of the Solvation Correlation Function and Implications for the Interpretation of Dielectric Relaxation in Proteins

The dielectric response of proteins is conveniently measured by monitoring the time-dependent Stokes shift of an associated chromophore. The interpretation of these experiments depends critically upon the construction of the solvation correlation function, C(t), which describes the time-dependence of the Stokes shift and hence the dielectric response of the medium to a change in charge distribution. We provide an analysis of various methods of constructing this function and review selected examples from the literature. The naturally occurring amino acid, tryptophan, has been frequently used as a probe of solvation dynamics in proteins. Its nonexponential fluorescence decay has stimulated the generation of an alternative method of constructing C(t). In order to evaluate this method, we have studied a system mimicking tryptophan. The system is comprised of two coumarins (C153 and C152) having different fluorescence lifetimes but similar solvation times. The coumarins are combined in different proportions in methanol to make binary probe mixtures. We use fluorescence upconversion spectroscopy to obtain wavelength-resolved kinetics of the individual coumarins in methanol as well as the binary mixtures of 75:25, 50:50, and 25:75 of C153:C152. The solvation correlation functions are constructed for these systems using different methods and are compared.

IR probes of protein microenvironments: utility and potential for perturbation.

A variety of IR-active moieties with absorptions that are distinct from those of proteins have been developed as probes of local protein environments, including carbon-deuterium bonds (CD), cyano groups (CN), and azides (N3 ); however, no systematic analysis of their utility in a protein has been published. Previously, we characterized the N-terminal Src homology 3 domain of the murine adapter protein Crk-II (nSH3) with CD bonds site-selectively incorporated throughout, and showed that it is relatively rigid and electrostatically heterogeneous and that it thermally unfolds under equilibrium conditions via a simple two-state mechanism. We now report the synthesis and characterization of eight variants of nSH3 with CN and/or N3 probes at five of the same positions. In agreement with previous studies, the position-dependent spectra suggest that both probes are predominantly sensitive to hydration, and not to their local electrostatic environments. Importantly, both probes also tend to significantly perturb the protein if they are not incorporated at surface-exposed positions. Thus, unlike CD labels, which are both sensitive to their environment and non-perturbative, CN and N3 probes should be used with caution.

Accumulation and Interaction of Hypericin in Low‐density Lipoprotein— A Photophysical Study

The accumulation and interaction of hypericin with the biologically important macromolecule, low‐density lipoprotein (LDL), is investigated using various steady‐state and time‐resolved fluorescence measurements. It is concluded that multiple hypericins can penetrate considerably deeply into the LDL molecule. Up to ∼20 nonaggregated hypericin molecules can enter LDL; but upon increasing the hypericin concentration, the fluorescence lifetime of hypericin decreases drastically, suggesting most likely the self‐quenching of aggregated hypericin. There is also evidence of energy transfer from tryptophans of the constituent protein, apoB‐100, to hypericin in LDL. The results demonstrate the ability of LDL to solubilize hypericin (a known photosensitizer) in nonaggregated form, which has implications for the construction of drug delivery systems.

Evidence of an unusual N-H···N hydrogen bond in proteins.

Many residues within proteins adopt conformations that appear to be stabilized by interactions between an amide N-H and the amide N of the previous residue. To explore whether these interactions constitute hydrogen bonds, we characterized the IR stretching frequencies of deuterated variants of proline and the corresponding carbamate, as well as the four proline residues of an Src homology 3 domain protein. The CδD2 stretching frequencies are shifted to lower energies due to hyperconjugation with Ni electron density, and engaging this density via protonation or the formation of the Ni+1-H···Ni interaction ablates this hyperconjugation and thus induces an otherwise difficult to explain blue shift in the C-D absorptions. Along with density functional theory calculations, the data reveal that the Ni+1-H···Ni interactions constitute H-bonds and suggest that they may play an important and previously underappreciated role in protein folding, structure, and function.

Comparison of the Dielectric Response Obtained from Fluorescence Upconversion Measurements and Molecular Dynamics Simulations for Coumarin 153−Apomyoglobin Complexes and Structural Analysis of the Complexes by NMR and Fluorescence Methods

We present a comparison of the dielectric response obtained from fluorescence upconversion experiments and from molecular dynamics simulations of the complexes of coumarin 153 with five apomyoglobins (apoMbs): wild-type horse heart (HH-WT) and those of wild-type sperm whale (SW-WT); its two triple mutants, L29F/H64Q/V68F and H64L/V68F/P88A; and its double mutant, L29F/V68L. Comparisons between experimental and simulated solvation relaxation functions, C(t)s, for the wild-type proteins range from very good to excellent. For the three mutants we investigated, however, agreement between experiment and simulation was considerably inferior. Thus, an NMR study of the complex of the HH-WT complex apoMb, and fluorescence energy transfer and anisotropy studies of the five complexes, were performed to investigate the structures upon which the simulations were based. The NMR measurements confirm our earlier conclusions that the C153 lies in the heme pocket of the HH-WT apoMb. For the wild-type complexes, fluorescence energy transfer measurements provide two rise times, suggesting a definite spatial relationship between the two Trp donors and the C153 acceptor. These results confirm the structural integrity of the wild-type complexes and validate the initial structures used for the molecular dynamics simulations. On the other hand, the three mutants provided single exponential rise times for energy transfer, suggesting that the position of the C153 used in the simulations may have been in error or that the C153 is mobile on the time scale of the energy transfer experiment. Fluorescence anisotropy studies also suggest that the double mutant was not structurally intact. Furthermore, examination of these systems demonstrates the sensitivity of C153 to its environment and permits the observation of differences in the heme pockets. These results point to the importance of structural characterization of modified proteins used in studies of the dielectric response and suggest strategies for performing molecular dynamics simulations of modified proteins.