Spin-Vibronic Intersystem Crossing and Molecular Packing Effects in Heavy Atom Free Organic Phosphor.

We present a detailed investigation into the excited state properties of a planar D3h symmetric azatriangulenetrione, HTANGO, which has received significant interest due to its high solid-state phosphorescence quantum yield and therefore potential as an organic room temperature phosphorescent (ORTP) dye. Using a model linear vibronic coupling Hamiltonian in combination with quantum dynamics simulations, we observe that intersystem crossing (ISC) in HTANGO occurs with a rate of ∼1010 s-1, comparable to benzophenone, an archetypal molecule for fast ISC in heavy metal free molecules. Our simulations demonstrate that the mechanism for fast ISC is associated with the high density of excited triplet states which lie in close proximity to the lowest singlet states, offering multiple channels into the triplet manifold facilitating rapid population transfer. Finally, to rationalize the solid-state emission properties, we use quantum chemistry to investigate the excited state surfaces of the HTANGO dimer, highlighting the influence and importance of the rotational alignment between the two HTANGO molecules in the solid state and how this contributes to high phosphorescence quantum yield.

A Δ-learning strategy for interpretation of spectroscopic observables.

Accurate computations of experimental observables are essential for interpreting the high information content held within x-ray spectra. However, for complicated systems this can be difficult, a challenge compounded when dynamics becomes important owing to the large number of calculations required to capture the time-evolving observable. While machine learning architectures have been shown to represent a promising approach for rapidly predicting spectral lineshapes, achieving simultaneously accurate and sufficiently comprehensive training data is challenging. Herein, we introduce Δ-learning for x-ray spectroscopy. Instead of directly learning the structure-spectrum relationship, the Δ-model learns the structure dependent difference between a higher and lower level of theory. Consequently, once developed these models can be used to translate spectral shapes obtained from lower levels of theory to mimic those corresponding to higher levels of theory. Ultimately, this achieves accurate simulations with a much reduced computational burden as only the lower level of theory is computed, while the model can instantaneously transform this to a spectrum equivalent to a higher level of theory. Our present model, demonstrated herein, learns the difference between TDDFT(BLYP) and TDDFT(B3LYP) spectra. Its effectiveness is illustrated using simulations of Rh L3-edge spectra tracking the C-H activation of octane by a cyclopentadienyl rhodium carbonyl complex.

Disentangling the evolution of electrons and holes in photoexcited ZnO nanoparticles.

The evolution of charge carriers in photoexcited room temperature ZnO nanoparticles in solution is investigated using ultrafast ultraviolet photoluminescence spectroscopy, ultrafast Zn K-edge absorption spectroscopy, and ab initio molecular dynamics (MD) simulations. The photoluminescence is excited at 4.66 eV, well above the band edge, and shows that electron cooling in the conduction band and exciton formation occur in <500 fs, in excellent agreement with theoretical predictions. The x-ray absorption measurements, obtained upon excitation close to the band edge at 3.49 eV, are sensitive to the migration and trapping of holes. They reveal that the 2 ps transient largely reproduces the previously reported transient obtained at 100 ps time delay in synchrotron studies. In addition, the x-ray absorption signal is found to rise in ∼1.4 ps, which we attribute to the diffusion of holes through the lattice prior to their trapping at singly charged oxygen vacancies. Indeed, the MD simulations show that impulsive trapping of holes induces an ultrafast expansion of the cage of Zn atoms in <200 fs, followed by an oscillatory response at a frequency of ∼100 cm-1, which corresponds to a phonon mode of the system involving the Zn sub-lattice.

Circularly polarised luminescence in an RNA-based homochiral, self-repairing, coordination polymer hydrogel.

The aqueous equimolar reaction of Ag(i) ions with the thionucleoside enantiomer (-)6-thioguanosine, ((-)6tGH), yields a one-dimensional coordination polymer {Ag(-)tG} n , the self-assembly of which generates left-handed helical chains. The resulting helicity induces an enhanced chiro-optical response compared to the parent ligand. DFT calculations indicate that this enhancement is due to delocalisation of the excited state along the helical chains, with 7 units being required to converge the calculated CD spectra. At concentrations ≥15 mmol l-1 reactions form a sample-spanning hydrogel which shows self-repair capabilities with instantaneous recovery in which the dynamic reversibility of the coordination chains appears to play a role. The resulting gel exhibits circularly polarised luminescence (CPL) with a large dissymmetry factor of -0.07 ± 0.01 at 735 nm, a phenomenon not previously observed for this class of coordination polymer.

Tunable giant magnetoresistance in a single-molecule junction.

Controlling electronic transport through a single-molecule junction is crucial for molecular electronics or spintronics. In magnetic molecular devices, the spin degree-of-freedom can be used to this end since the magnetic properties of the magnetic ion centers fundamentally impact the transport through the molecules. Here we demonstrate that the electron pathway in a single-molecule device can be selected between two molecular orbitals by varying a magnetic field, giving rise to a tunable anisotropic magnetoresistance up to 93%. The unique tunability of the electron pathways is due to the magnetic reorientation of the transition metal center, resulting in a re-hybridization of molecular orbitals. We obtain the tunneling electron pathways by Kondo effect, which manifests either as a peak or a dip line shape. The energy changes of these spin-reorientations are remarkably low and less than one millielectronvolt. The large tunable anisotropic magnetoresistance could be used to control electronic transport in molecular spintronics.

Addressing the properties of "Metallo-DNA" with a Ag(i)-mediated supramolecular duplex.

The silver-nucleoside complex [Ag(i)-(N3-cytidine)2], 1, self-assembles to form a supramolecular metal-mediated base-pair array highly analogous to those seen in metallo-DNA. A combination of complementary hydrogen-bonding, hydrophobic and argentophilic interactions drive the formation of a double-helix with a continuous silver core. Electrical measurements on 1 show that despite having Ag···Ag distances within <5% of the metallic radii, the material is electrically insulating. This is due to the electronic structure which features a filled valence band, an empty conduction band dominated by the ligand, and a band gap of 2.5 eV. Hence, as-prepared, such Ag(i)-DNA systems should not be considered molecular nanowires but, at best, proto-wires. The structural features seen in 1 are essentially retained in the corresponding organogel which exhibits thixotropic self-healing that can be attributed to the reversible nature of the intermolecular interactions. Photo-reduced samples of the gel exhibit luminescence confirming that these poly-cytidine sequences appropriately pre-configure silver ions for the formation of quantum-confined metal clusters in line with contemporary views on DNA-templated clusters. Microscopy data reveals the resulting metal cluster/particles are approximately spherical and crystalline with lattice spacing (111) similar to bulk Ag.

Electronic effects and fundamental physics studied in molecular interfaces.

Scanning probe instruments in conjunction with a very low temperature environment have revolutionized the ability of building, functionalizing, and analysing two dimensional interfaces in the last twenty years. In addition, the availability of fast, reliable, and increasingly sophisticated methods to simulate the structure and dynamics of these interfaces allow us to capture even very small effects at the atomic and molecular level. In this review we shall focus largely on metal surfaces and organic molecular compounds and show that building systems from the bottom up and controlling the physical properties of such systems is no longer within the realm of the desirable, but has become day to day reality in our best laboratories.

Evidence for Ultralow-Energy Vibrations in Large Organic Molecules.

The quantum efficiency or the rate of conversion of incident photon to free electron in photosynthesis is known to be extremely high. It has long been thought that the origin of this efficiency are molecular vibrations leading to a very fast separation of electrons and holes within the involved molecules. However, molecular vibrations are commonly in the range above 100 meV, which is too high for excitations in an ambient environment. Here, we analyze experimental spectra of single organic molecules on metal surfaces at ∼4 K, which often exhibit a pronounced dip. We show that measurements on iron(II) [tetra-(pentafluorophenyl)]porphyrin resolve this single dip at 4 K into a series of step-shaped inelastic excitations at 0.4 K. Via extensive spectral maps under applied magnetic fields and corresponding theoretical analysis we find that the dip is due to ultralow-energy vibrations of the molecular frame, typically in the range below 20 meV. The result indicates that ultralow energy vibrations in organic molecules are much more common than currently thought and may be all-pervasive for molecules above a certain size.

Correlation of breaking forces, conductances and geometries of molecular junctions.

Electrical and mechanical properties of elongated gold-molecule-gold junctions formed by tolane-type molecules with different anchoring groups (pyridyl, thiol, amine, nitrile and dihydrobenzothiophene) were studied in current-sensing force spectroscopy experiments and density functional simulations. Correlations between forces, conductances and junction geometries demonstrate that aromatic tolanes bind between electrodes as single molecules or as weakly-conductive dimers held by mechanically-weak π - π stacking. In contrast with the other anchors that form only S-Au or N-Au bonds, the pyridyl ring also forms a highly-conductive cofacial link to the gold surface. Binding of multiple molecules creates junctions with higher conductances and mechanical strengths than the single-molecule ones.

Highly-effective gating of single-molecule junctions: an electrochemical approach.

We report an electrochemical gating approach with ∼100% efficiency to tune the conductance of single-molecule 4,4'-bipyridine junctions using scanning-tunnelling-microscopy break junction technique. Density functional theory calculation suggests that electrochemical gating aligns molecular frontier orbitals relative to the electrode Fermi-level, switching the molecule from an off resonance state to "partial" resonance.

Single-molecule conductance of functionalized oligoynes: length dependence and junction evolution.

We report a combined experimental and theoretical investigation of the length dependence and anchor group dependence of the electrical conductance of a series of oligoyne molecular wires in single-molecule junctions with gold contacts. Experimentally, we focus on the synthesis and properties of diaryloligoynes with n = 1, 2, and 4 triple bonds and the anchor dihydrobenzo[b]thiophene (BT). For comparison, we also explored the aurophilic anchor group cyano (CN), amino (NH2), thiol (SH), and 4-pyridyl (PY). Scanning tunneling microscopy break junction (STM-BJ) and mechanically controllable break junction (MCBJ) techniques are employed to investigate single-molecule conductance characteristics. The BT moiety is superior as compared to traditional anchoring groups investigated so far. BT-terminated oligoynes display a 100% probability of junction formation and possess conductance values which are the highest of the oligoynes studied and, moreover, are higher than other conjugated molecular wires of similar length. Density functional theory (DFT)-based calculations are reported for oligoynes with n = 1-4 triple bonds. Complete conductance traces and conductance distributions are computed for each family of molecules. The sliding of the anchor groups leads to oscillations in both the electrical conductance and the binding energies of the studied molecular wires. In agreement with experimental results, BT-terminated oligoynes are predicted to have a high electrical conductance. The experimental attenuation constants ßH range between 1.7 nm(-1) (CN) and 3.2 nm(-1) (SH) and show the following trend: ßH(CN) < ßH(NH2) < ßH(BT) < ßH(PY) ≈ ßH(SH). DFT-based calculations yield lower values, which range between 0.4 nm(-1) (CN) and 2.2 nm(-1) (PY).

Correlation of sonographic features of invasive ductal mammary carcinoma with age, tumor grade, and hormone-receptor status.

PURPOSE: To determine whether presenting sonographic features of invasive ductal carcinomas (IDC) are associated with patient age, tumor histologic grade, and hormonal receptor status. METHODS: Sonographic features of 101 consecutive cases of IDC seen at ultrasound were retrospectively assessed based on the BI-RADS criteria of posterior acoustic appearance, tumor margins, and echogenicity. Associations between sonographic features and tumor characteristics were statistically evaluated with attention to patient age. RESULTS: IDC with shadowing compared with unchanged posterior acoustic appearance were significantly more likely to be of low histologic grade (Odds Ratio [OR] = 5.00; p < 0.05) and estrogen receptor (ER) -positive (OR = 10.00; p < 0.05). Conversely, posterior enhancement was associated with ER-negative status (OR = 4.45; p < 0.01), particularly among patients younger than 60 years of age (OR = 5.36, p < 0.05). Circumscribed tumors were more often high grade, particularly among older women (p < 0.01), and hormone receptor--negative regardless of age group. Among older women, tumors with mixed echogenicity tended to be high grade and progesterone receptor--negative (p values < 0.05). Noncircumscribed borders were observed for all tumors with posterior shadowing, and 97% of such tumors were also ER positive. CONCLUSIONS: Sonographic features were significantly associated with tumor grade and hormone receptor status, with some differences based on patient age. Specifically, the presence of posterior shadowing was associated with lower histologic grade and ER-positive status, especially in older patients. In contrast, we found that posterior acoustic enhancement was more commonly associated with ER-negative status, especially in younger patients.

Rare breast lesions: correlation of imaging and histologic features with WHO classification.

Mammographers occasionally are surprised by the diagnosis of a rare lesion at breast biopsy. The imaging features of some breast lesions are unfamiliar because they are rarely seen in routine mammographic practice and they are not well described or well documented in the radiologic literature. Moreover, there may be wide variation in the appearances of rare breast lesions at mammography and ultrasonography (US). In addition, although a few rare breast lesions have a typical imaging appearance, most have mammographic and US features similar to those of breast carcinomas, and a needle biopsy is almost always necessary to obtain a diagnosis. However, even when a rare breast lesion is diagnosed on the basis of a needle biopsy, knowledge of the imaging features of such lesions may help the radiologist decide whether the results of pathologic analysis concur with the imaging findings and whether surgical excision is necessary. It is therefore important that radiologists be familiar with the broad spectrum of imaging features of rare breast lesions as well as with the correlation between their histopathologic features and their current classification according to the World Health Organization classification system.

Characterization of sonographic and mammographic features of granular cell tumors of the breast and estimation of their incidence.

OBJECTIVE: The purpose of this series was to review the spectrum of sonographic and mammographic features of granular cell tumors (GCTs) of the breast and to compare their frequency relative to breast carcinoma. METHODS: Ten cases of a breast GCT diagnosed during the last 13 years were analyzed for the imaging characteristics and clinical features. Sonographic images were reviewed for location, size, echogenicity, margins, height-width ratio, and sound transmission. Mammograms were reviewed for tumor size, location, margin characteristics, spiculations, and calcifications. The frequency of GCTs was compared with that of breast carcinoma during the same study period. RESULTS: Of 10 GCT cases, 9 tumors occurred in female patients, and 1 occurred in a male patient. The mean patient age was 51.8 years, and the mean lesion size was 1.57 cm. All 7 tumors visualized on sonography were hypoechoic masses. Posterior enhancement was noted in 3 of 7 cases, and posterior shadowing was noted in 2 of 7 cases. Two of 7 did not show any posterior enhancement or shadowing. Two of 7 masses were taller than wide. Of 8 tumors visualized on mammography, 5 were spiculated, and 3 were well circumscribed. Calcifications were not visualized in any tumor. During the same period, 1482 cases of breast carcinoma were diagnosed, making the frequency of GCTs of the breast about 6.7 per thousand breast carcinomas. CONCLUSIONS: Granular cell tumors of the breast are rare neoplasms with a relative frequency of 6.7 per thousand breast carcinomas in our series, which was higher than reported in literature. Spiculations are a common imaging feature and mimic carcinoma when present.

Mammographic surveillance after MammoSite breast brachytherapy: analysis of architectural patterns and additional interventions.

PURPOSE: To report the post-treatment mammographic findings of patients who received MammoSite brachytherapy at the Medical University of South Carolina (MUSC). MATERIALS AND METHODS: A total of 126 mammograms from a cohort of 38 patients who underwent MammoSite breast brachytherapy and post-treatment mammographic follow-up exclusively at MUSC are the subject of this analysis. Surveillance mammography commenced at 6 months after completion of brachytherapy and was repeated every 6 to 12 months thereafter, depending on Breast Imaging Reporting and Data System (BI-RADS) classification and further testing recommendations. Patients were clinically assessed at least every 3 months and all available data were reviewed for documentation of mammographic findings and subsequent interventions. The minimum and median follow-up was 6 and 28 months, respectively. RESULTS: Of the 126 mammograms analyzed, 22 (17%) were classified as BI-RADS category 2, 93 (74%) as category 3, 10 (8%) as category 4, and 1 (0.8%) as category 5. Further descriptions of the BI-RADS 3 studies were: 61 (65%) "surgical changes," 30 (32%) seromas, and 2 (2%) dystrophic calcifications. Additional interventions followed 10 (11%) of BI-RADS 3 studies, all revealing benign findings. All BI-RADS 4 or 5 studies led to needle aspiration (3) or breast biopsy (8). Two biopsies were positive for malignancy, and both were classified as elsewhere breast failures. CONCLUSION: Mammographic architectural patterns observed after partial breast irradiation and potential differences with respect to those traditionally seen following whole breast radiotherapy have yet to be well characterized. Our experience may be of clinical utility in the counseling of patients until data from randomized trials becomes available.

Gap-based estimation: choosing the smoothing parameters for probabilistic and general regression neural networks.

Probabilistic neural networks (PNN) and general regression neural networks (GRNN) represent knowledge by simple but interpretable models that approximate the optimal classifier or predictor in the sense of expected value of the accuracy. These models require the specification of an important smoothing parameter, which is usually chosen by cross-validation or clustering. In this article, we demonstrate the problems with the cross-validation and clustering approaches to specify the smoothing parameter, discuss the relationship between this parameter and some of the data statistics, and attempt to develop a fast approach to determine the optimal value of this parameter. Finally, through experimentation, we show that our approach, referred to as a gap-based estimation approach, is superior in speed to the compared approaches, including support vector machine, and yields good and stable accuracy.

Soft-tissue tumors: value of static and dynamic gadopentetate dimeglumine-enhanced MR imaging in prediction of malignancy.

PURPOSE: To prospectively evaluate static and dynamic gadopentetate dimeglumine-enhanced magnetic resonance (MR) imaging relative to nonenhanced MR imaging in differentiation of benign from malignant soft-tissue lesions and to evaluate which MR imaging parameters are most predictive of malignancy, with associated interobserver variability. MATERIALS AND METHODS: One hundred forty consecutive patients (78 male patients [median age, 51 years], 62 female patients [median age, 53 years]) with a soft-tissue mass underwent nonenhanced static and dynamic contrast material-enhanced MR imaging. Diagnosis was based on histologic findings in surgical specimens (86 of 140), findings at core-needle biopsy (43 of 140), or results of all imaging procedures with clinical follow-up (11 of 140). Multivariate logistic regression analysis was used to identify the best combination of MR imaging parameters that might be predictive of malignancy. Subjective overall performance of two observers was evaluated with receiver operating characteristic analysis. RESULTS: For subjective overall diagnosis, area under the receiver operating characteristic curve, a measure for diagnostic accuracy, was significantly larger for combined nonenhanced and contrast-enhanced MR imaging than it was for nonenhanced MR imaging alone, with no significant difference between observers. Multivariate analysis of all lesions revealed that combined nonenhanced static and dynamic contrast-enhanced MR imaging parameters were significantly superior to nonenhanced MR imaging parameters alone and to nonenhanced MR imaging parameters combined with static contrast-enhanced MR imaging parameters in prediction of malignancy. The most discriminating parameters were presence of liquefaction, start of dynamic enhancement (time interval between start of arterial and tumor enhancement), and lesion size (diameter). Results for extremity lesions were the same, with one exception: With dynamic contrast-enhanced MR imaging parameters, diagnostic performance of one observer did not improve. CONCLUSION: Static and dynamic contrast-enhanced MR imaging, when added to nonenhanced MR imaging, improved differentiation between benign and malignant soft-tissue lesions.

From the archives of the AFIP: benign musculoskeletal lipomatous lesions.

Benign lipomatous lesions involving soft tissue are common musculoskeletal masses that are classified into nine distinct diagnoses: lipoma, lipomatosis, lipomatosis of nerve, lipoblastoma or lipoblastomatosis, angiolipoma, myolipoma of soft tissue, chondroid lipoma, spindle cell lipoma and pleomorphic lipoma, and hibernoma. Soft-tissue lipoma accounts for almost 50% of all soft-tissue tumors. Radiologic evaluation is diagnostic in up to 71% of cases. These lesions are identical to subcutaneous fat on computed tomographic (CT) and magnetic resonance (MR) images and may contain thin septa. Lipomatosis represents a diffuse overgrowth of mature fat affecting either subcutaneous tissue, muscle or nerve, and imaging is needed to evaluate lesion extent. Lipoblastoma is a tumor of immature fat occurring in young children, and imaging features may reveal a mixture of fat and nonadipose tissue. Angiolipoma, myolipoma, and chondroid lipoma are rare lipomatous lesions that are infrequently imaged. Spindle cell and pleomorphic lipoma appear as a subcutaneous lipomatous mass in the posterior neck or shoulder, with frequent nonadipose components. Hibernoma appears as a lipomatous mass with serpentine vascular elements. Benign lipomatous lesions affecting bone, joint, or tendon sheath include intraosseous lipoma, parosteal lipoma, liposclerosing myxofibrous tumor, discrete lipoma of joint or tendon sheath, and lipoma arborescens. Intraosseous and parosteal lipoma have a pathognomonic CT or MR appearance, with fat in the marrow space or on the bone surface, respectively. Liposclerosing myxofibrous tumor is a rare intermixed histologic lesion commonly located in the medullary canal of the intertrochanteric femur. Benign lipomatous lesions may occur focally in a joint or tendon sheath or with diffuse villonodular proliferation in the synovium (lipoma arborescens) and are diagnosed based on location and identification of fat. Understanding the spectrum of appearances of the various benign musculoskeletal lipomatous lesions improves radiologic assessment and is vital for optimal patient management.

Pigmented villonodular synovitis: a disease in evolution.

Pigmented villonodular synovitis (PVNS) is a rare, benign, proliferative disease of the synovial membrane of joints, tendon sheaths, and bursas. Joint aspiration typically yields hemorrhagic or xanthochromic/serosanguinous (brown, murky) fluid. We describe a case of PVNS that presented as an acute, painless, nontraumatic right knee effusion with clear synovial fluid on arthrocentesis. Initial magnetic resonance imaging of the knee revealed no evidence for hemosiderin. A diagnostic arthroscopy and surgical arthrotomy revealed a unique case of PVNS evolving from local to diffuse involvement of the synovium.