Electronic Structures and Spectra of Donor-Acceptor Conjugated Oligomers.

Narrow band gap donor-acceptor conjugated polymers present excellent paradigms in photonics and optoelectronics due to their chemical tunability, correlated electronic structures, and tunable open-shell electronic configurations. However, rational design for enhancing the properties of these molecular systems remains challenging. In this study, we employed density functional theory (DFT) calculations to investigate prototypical narrow band gap donor-acceptor conjugated oligomers, consisting of alternating cyclopentadithiophene (CPDT) donors paired with benzothiadiazole (BT), benzoselenadiazole (BSe), benzobisthiadiazole (BBT), and thiadiazoloquinoxaline (TQ) acceptors. Analyses of structures, singlet-triplet gaps, and absorption spectra of oligomers with up to ten repeat units have shown that when incorporating the BT, BSe, and TQ acceptors, the backbone curvature resulted in spiral structures that were energetically favored over their linear counterparts, causing differences in the calculated circular dichroism spectra. Oligomers with BBT-based acceptors preferred, however, a linear geometry, consistent with an open-shell electronic structure. Calculated singlet-triplet splittings demonstrated the importance of long chains and specific structures for consistency with the experiment, while effects of the solvent were also quantified. Based on the predicted low-energy conformations, one-photon absorption spectra for the considered oligomers have shown that using the Tamm-Dancoff approximation within time-dependent DFT for the large systems offers good agreement with the first absorption maxima in measured experimental spectra, thus validating the method for large donor-acceptor oligomers. Natural transition orbital analyses provided insights into the excited-state characteristics. Two-photon absorption maxima were accurately predicted, but the cross-sections were overestimated or underestimated, as dependent on the level of theory employed, to be addressed in future work.

Do The Twist: Efficient Heavy-Atom-Free Visible Light Polymerization Facilitated by Spin-Orbit Charge Transfer Inter-system Crossing.

The use of visible light to drive polymerizations with spatiotemporal control offers a mild alternative to contemporary UV-light-based production of soft materials. In this spectral region, photoredox catalysis represents the most efficient polymerization method, yet it relies on the use of heavy-atoms, such as precious metals or toxic halogens. Herein, spin-orbit charge transfer intersystem crossing from boron dipyrromethene (BODIPY) dyads bearing twisted aromatic groups is shown to enable efficient visible light polymerizations in the absence of heavy-atoms. A ≈5-15× increase in polymerization rate and improved photostability was achieved for twisted BODIPYs relative to controls. Furthermore, monomer polarity had a distinct effect on polymerization rate, which was attributed to charge transfer stabilization based on ultrafast transient absorption and phosphorescence spectroscopies. Finally, rapid and high-resolution 3D printing with a green LED was demonstrated using the present photosystem.

Synthetically Tunable White-, Green-, and Yellow-Green-Light Emission in Dual-Luminescent Gold(I) Complexes Bearing a Diphenylamino-2,7-fluorenyl Moiety.

The syntheses and photophysical characterization of five new gold(I) complexes bearing diphenylamine-substituted fluorenyl moieties are reported; four are characterized by X-ray diffraction crystallography. Ancillary ligation on gold(I) is provided by organophosphine and N-heterocyclic carbene ligands. Two complexes, Au-DPA0 and Au-DPA1, are σ-aryls, two, Au-ADPA0 and Au-ADPA1, are σ-alkynyls, and one, Au-TDPA1, is a σ-triazolyl bound through carbon. All complexes show vibronically structured absorption and luminescence bands that are assignable to π-π* transitions localized on the diphenylamine-substituted fluorenyl π system. The excited-state dynamics of all five chromophores are governed by selection of the ancillary ligand and σ attachment of the diphenylamine-substituted fluorenyl moiety. All of these chromophores are dual luminescent in a toluene solution at 298 K. The luminescence from the aryl derivatives, Au-ADPA0 and Au-DPA1, appears green. The alkynyl derivative containing a phosphine ancillary ligand, Au-ADPA0, is a white-light emitter, while the alkynyl derivative containing an N-heterocyclic carbene ancillary ligand, Au-ADPA1, is a yellow-light emitter. The luminescence from the triazolyl-linked chromophore, Au-TDPA1, appears as yellow-green. Spin-restricted density functional theory calculations support the assignments of ligand-centric optical transitions but with contributions of ligand-to-metal charge transfer involving the vacant Au 6p orbital.

Zn Coordination and the Identity of the Halide Ancillary Ligand Dramatically Influence the Excited-State Dynamics and Bimolecular Reactions of 2,3-Di(pyridin-2-yl)benzo[g]quinoxaline.

The optical properties of coordination complexes with ligands containing nitrogen heterocycles have been extensively studied for decades. One subclass of these materials, metal complexes utilizing substituted pyrazines and quinoxalines as ligands, has been employed in a variety of photochemical applications ranging from photodynamic therapy to organic light-emitting diodes. A vast majority of this work focuses on characterization of the metal-to-ligand charge-transfer states in these metal complexes; however, literature reports rarely investigate the photophysics of the parent pyrazine or quinoxaline ligand or perform control experiments utilizing metal complexes that lack low-lying charge-transfer (CT) states in order to determine how metal-atom coordination influences the photophysical properties of the ligand. With this in mind, we examined the steady-state and time-resolved photophysics of 2,3-di(pyridin-2-yl)benzo[g]quinoxaline (dpb) and explored how the coordination of ZnX2 (X = Cl-, Br-, I-) affects the photophysical properties of dpb. In dpb, we find that the dominant mode of deactivation from the singlet excited state is intersystem crossing (ISC). Coordination of ZnX2 perturbs the relative energies of the ππ* and nπ* excited states of dpb, leading to drastically different rates of ISC as well as radiative and nonradiative decay in the [Zn(dpb)X2] complexes compared to dpb. These differences in the rates change the dominant singlet-excited-state decay pathway from ISC in dpb to a mixture of ISC and fluorescence in [Zn(dpb)Cl2] and [Zn(dpb)Br2] and to nonradiative decay in [Zn(dpb)I2]. Coordination of ZnX2 and the choice of the halide ligand also have profound effects on the rate constants for excited-state bimolecular reactions, including triplet-triplet annihilation and oxygen quenching. These results demonstrate that metal coordination, even in complexes lacking low-lying CT states, and the choice of the ancillary ligand can dramatically alter the photophysical properties of chromophores containing nitrogen heterocycles.

Impact of iodine loading and substitution position on intersystem crossing efficiency in a series of ten methylated-meso-phenyl-BODIPY dyes.

Four core and six distyryl-extended methylated-meso-phenyl-BODIPY dyes with varying iodine content were synthesized. The influence of iodine loading and substitution position on the photophysical properties of these chromophores was evaluated. Selective iodine insertion at the 2- and 6-positions of the methylated-meso-phenyl-BODIPY core, rather than maximum iodine content, resulted in the highest intersystem crossing efficiency. Iodination of the distyryl-extended BODIPY core afforded intersystem crossing quantum yields comparable to 2,6-diiodo-BODIPY. Inclusion of an iodine at the para-meso-phenyl position generally enhanced non-radiative decay in the BODIPY excited-state, leading to lower fluorescence and intersystem crossing quantum yield values. Iodine substitution at the styryl-positions resulted in negligible changes to the excited-state dynamics. This study highlights: (1) the rate of radiative decay is similar in all ten derivatives (on the order of 1 × 108 s-1), (2) iodination of the 2,6-positions results in the greatest enhancement of intersystem crossing efficiency, (3) care must be taken when modifying the para-meso-phenyl position as it could have detrimental effects on the excited-state dynamics, (4) the excited-state is negligibly affected by iodination of the styryl groups, potentially enabling orthogonal functionalization without modifying the molecular photophysics, (5) distyryl extension of the chromophore core diminishes rates of non-radiative decay and intersystem crossing, resulting in higher fluorescence quantum yields and lower intersystem crossing yields in the π-extended derivatives compared to the core BDP derivatives, and (6) DFT calculations provide insight into the electronic and structural factors regulating intersystem crossing and vibrational relaxation in these molecules.

Two-photon absorption characterization and comparison of Au(I) fluorenyl benzothiazole complexes in the visible wavelength regime.

We use the two-photon excited fluorescence method to determine the two-photon absorption (2PA) cross sections of three series of (fluorenyl benzothiazole) gold(I) complexes in the visible wavelength range from 570 to 700 nm. We compare the effect of ancillary ligand substitutions on the 2PA magnitudes and find that the ancillary ligand does not drastically affect either the magnitude or the shape of 2PA. Even so, moderate 2PA cross sections were measured that ranged from 10 to 1000 s of GM (Göppert-Mayer, =10-50cm4s/photon), making these types of complexes nonlinear optical materials for two-photon absorbing applications.

From Dipyrrolonaphthyridinediones to Quinazolinoindolizinoindolizinoquinazolines.

The dipyrrolonaphthyridinedione (DPND) core can be readily converted into a series of acid-responsive quinazolinoindolizinoindolizinoquinazolines through a two-step route involving direct arylation followed by acid-catalyzed condensation. Unlike the majority of previously obtained DPNDs, these nonplanar dyes bearing eight fused rings are almost nonfluorescent, which is attributed to fast internal conversion relative to radiative decay and intersystem crossing.

Synthesis and photophysics of gold(i) alkynyls bearing a benzothiazole-2,7-fluorenyl moiety: a comparative study analyzing influence of ancillary ligand, bridging moiety, and number of metal centers on photophysical properties.

Three new gold(i) alkynyl complexes (Au-ABTF(0-2)) containing a benzothiazole fluorenyl moiety, with either an organic phosphine or N-heterocyclic carbene as ancillary ligand, have been synthesized and photophysically characterized. All three complexes display highly structured ground-state absorption and luminescence spectra. Dual-luminescence is observed in all three complexes at room temperature in toluene after three freeze-pump-thaw cycles. The phosphine complexes (Au-ABTF(0-1)) exhibit similar photophysics with fluorescent quantum yields ∼0.40, triplet-state quantum yields ∼0.50, and fluorescent lifetimes ∼300 ps. The carbene complex Au-ABTF2 displays different behavior; having a fluorescent quantum yield of 0.23, a triplet-state quantum yield of 0.61, and a fluorescent lifetime near 200 ps, demonstrating that the ancillary ligand alters excited-state dynamics. The compounds exhibit strong (on the order of 105 M-1 cm-1) and positive excited-state absorption in both their singlet and triplet excited states spanning the visible region. Delayed fluorescence resulting from triplet-triplet annihilation is also observed in freeze-pump-thaw deaerated samples of all the complexes in toluene. DFT calculations (both static and time-resolved) agree with the photophysical data where phosphine complexes have slightly larger S1-T2 energy gaps (0.28 eV and 0.26 eV) relative to the carbene complex (0.21 eV). Comparison of the photophysical properties of Au-ABTF(0-2) to previously published dinuclear gold(i) complexes and mononuclear gold(i) aryl complexes bearing the same benzothiazole-2,7-fluorenyl moiety are made. Structure-property relationships regarding ancillary ligand, bridging moiety, and number of metal centers are drawn.

Influence of Structural Isomerism on the Photophysical Properties of a Series of Donor-Acceptor 1-Naphthalenecarbonitrile Derivatives Possessing Amine Substituents.

In an effort to probe the influence of structural isomerism on the excited-state properties of a naphthalene-based donor-acceptor (D-A) system, four 1-naphthalenecarbonitrile compounds with amine substituents in the 2-, 3-, and 4-positions were synthesized and their photophysical properties were examined. Specifically, the molecules 2-dimethylamino-1-naphthalenecarbonitrile (2DA), 2-(1-piperidinyl)-1-naphthalenecarbonitrile (2P), 3-dimethylamino-1-naphthalenecarbonitrile (3DA), and 4-(1-piperidinyl)-1-naphthalenecarbonitrile (4P) were studied. The substitution position of the amine donor has a significant impact on both the ground-state absorption and excited-state properties of the complexes in toluene solution. The energy, band shape, and extinction coefficient of the ground-state absorption spectra are highly dependent on the substitution position of the amine donor. All of the derivatives exhibit fluorescence at room temperature. The fluorescence observed from 2DA, 2P, and 3DA demonstrates a vibronic structure with all three molecules possessing Stokes shifts on the order of 40 nm, whereas the fluorescence observed from 4P is broad and has a Stokes shift 2 times greater than the other derivatives. The fluorescence lifetimes, fluorescence quantum yields, and intersystem crossing quantum yields vary greatly with the substitution position of the amine donor. 2DA and 2P display intermediate fluorescence lifetimes (2.7 ns) and fluorescence quantum yields (0.20) while possessing the greatest intersystem quantum yield (0.80). 3DA has a much greater fluorescence lifetime (16.9 ns) and fluorescence quantum yield (0.82) at the expense of the intersystem crossing quantum yield (0.12). 4P has the shortest lifetime (0.53 ns), with the lowest fluorescence and intersystem crossing quantum yields (<0.05). The singlet-triplet energy gaps are nearly identical for 2DA, 2P, and 3DA with values on the order of 0.70 eV. This singlet-triplet gap is larger in 4P, with a calculated value of 0.94 eV. The triplet-triplet absorption spectra of 2DA, 2P, and 3DA are similar. Broad peaks in the UV and visible regions with maxima around 330 and 500 nm characterize all three spectra. The triplet excited-state extinction coefficient values for 3DA were found to be 1.5 times larger than those in 2DA and 2P. The triplet-triplet absorption spectrum of 4P is markedly different from the triplet-triplet absorption spectra of the other derivatives. The spectrum is broad, with the four local maxima observed at 374, 445, 624, and 774 nm. All four molecules display delayed fluorescence and laser-power-dependent triplet excited-state decay kinetics, indicating the involvement of triplet-triplet annihilation in the deactivation of the triplet excited states. Both the intrinsic triplet lifetimes and triplet-triplet annihilation rate constants were determined. These values are similar for all of the derivatives with triplet lifetimes on the order of 100 µs and diffusion-controlled rates of triplet-triplet annihilation.

Effects of intramolecular hydrogen bonding and sterically forced non-coplanarity on organic donor/acceptor two-photon-absorbing molecules.

Two photon absorption (2PA) is of great interest across many disciplines and there has been a large effort to increase the two-photon cross section (σ2) via synthetic modification, especially by enhancing intramolecular charge-transfer (ICT). This work takes the previously studied (7-benzothiazol-2-yl-9,9-diethylfluoren-2-yl)diphenylamine (AF240), an asymmetric D-π-A chromophore, and intentionally appends a functional group (-OH, AF240-OH or -OCH3, AF240-OMe) to the 6-position of the fluorenyl π-bridge of the new chromophores. Electrochemical results in both dichloromethane and acetonitrile support stabilization of the highest occupied molecular orbital in the derivatives due to inductive electron donating effects of the hydroxy and methoxy groups. The lowest unoccupied molecular orbital is stabilized via intramolecular hydrogen bonding to the benzothiazole moiety in the case of AF240-OH. As previously observed for AF240, the steady-state emission spectra show significant solvatochromism as they broaden and red shift with increasing solvent polarity. The fluorescence lifetimes and quantum yields show that the non-radiative rate constant is increased for AF240-OH in all solvents, especially in nonpolar media. The results suggest there is forced intramolecular hydrogen bonding to the benzothiazole in nonpolar solvents because the solvent poorly solubilizes the hydroxy group. This increases the non-radiative decay rate constant (knr) via additional vibrational decay pathways. While not as dramatic, the increase in knr in polar solvents supports some deactivation via hydrogen bonding to the solvent. Steric effects are also observed in the methoxy derivative, which inhibits planarization of the benzothiazole with the fluorene, increasing the energy of the excited state. Ultrafast transient absorption spectroscopy in tetrahydrofuran solution supports stabilization of the excited state in a few ps as solvent and structural reorganizations occur. In the case of AF240-OH, no evidence of proton transfer is observed. The decrease in emission energies in the case of AF240-OH support increased ICT driven by higher degree of coplanarity and the quinoidal structure in the excited state. However, a moderate increase in the intrinsic 2PA cross-section is resulted. It is likely because of the two possible and competing solvent-stabilized ICT processes (PICT and TICT) in AF240-OH. Nevertheless, the strategic presence of a hydroxide group capable of intramolecular hydrogen bonding in AF240-OH provides a much broader 2PA sensitivity window than AF240.

Manipulating triplet states: tuning energies, absorption, lifetimes, and annihilation rates in anthanthrene derivatives.

The photophysical properties of anthanthrene, four anthanthrene derivatives containing varying phenyl and p-tBu-phenyl substituents, and two anthanthrones with phenyl and p-tBu-phenyl substituents are examined. In general, as the anthanthrenes and anthanthrones become more substituted, red-shifts are observed in the peak maxima of the ground- and excited-state absorption and fluorescence spectra. The anthanthrones have large (>0.8) intersystem crossing (ISC) quantum yields (ΦT) likely caused by nπ* character in the ground or excited states. A bromo-substituted anthanthrene has a unity ISC yield due to an ISC rate constant of 2.5 × 1010 s-1 caused by heavy-atom induced, spin-orbit coupling. This leads to low fluorescence quantum yields (ΦF) in these three derivatives. The parent anthanthrene and remaining derivatives behave much differently. All have ΦF values from 0.58-0.84 with lower ΦT values as radiative decay outcompetes ISC. The anthanthrones have remarkable excited-state absorption with strong, broad transitions across the visible region with weaker transitions extending to nearly two µm. The anthanthrenes have very similar-shaped, broad transitions in the visible which can be shifted ∼60 nm by controlling the substituents. The triplet lifetimes range from 31-1200 µs and increase as the ISC yields decrease; the bromo-substituted anthanthrene is the shortest, followed by the anthanthrones then the other anthanthrenes. The rate of triplet-triplet annihilation is also affected by the presence of substituents; as the amount of steric bulk is increased, the rate of annihilation decreases.

Photodriven Oxygen Removal via Chromophore-Mediated Singlet Oxygen Sensitization and Chemical Capture.

We report a general, photochemical method for the rapid deoxygenation of organic solvents and aqueous solutions via visible light excitation of transition metal chromophores (TMCs) in the presence of singlet oxygen scavenging substrates. Either 2,5-dimethylfuran or an amino acid (histidine or tryptophan methyl ester) was used as the substrate in conjunction with an iridium or ruthenium TMC in toluene, acetonitrile, or water. This behavior is described for solutions with chromophore concentrations that are pertinent for both luminescence and transient absorption spectroscopies. These results consistently produce TMC lifetimes comparable to those measured using traditional inert gas sparging and freeze-pump-thaw techniques. This method has the added benefits of providing long-term stability (days to months); economical preparation due to use of inexpensive, commercially available oxygen scrubbing substrates; and negligible size and weight footprints compared to traditional methods. Furthermore, attainment of dissolved [O2] < 50 µM makes this method relevant to any solution application requiring low dissolved oxygen concentration in solution, provided that the oxygenated substrate does not interfere with the intended chemical process.

Switchable Optical Properties of Dyes and Nanoparticles in Electrowetting Devices.

The optical properties of light-absorbing materials in optical shutter devices are critical to the use of such platforms for optical applications. We demonstrate switchable optical properties of dyes and nanoparticles in liquid-based electrowetting-on-dielectric (EWOD) devices. Our work uses narrow-band-absorbing dyes and nanoparticles, which are appealing for spectral-filtering applications targeting specific wavelengths while maintaining device transparency at other wavelengths. Low-voltage actuation of boron dipyromethene (BODIPY) dyes and nanoparticles (Ag and CdSe) was demonstrated without degradation of the light-absorbing materials. Three BODIPY dyes were used, namely Abs 503 nm, 535 nm and 560 nm for dye 1 (BODIPY-core), 2 (I2BODIPY) and 3 (BODIPY-TMS), respectively. Reversible and low-voltage (≤20 V) switching of dye optical properties was observed as a function of device pixel dimensions (300 × 900, 200 × 600 and 150 × 450 µm). Low-voltage and reversible switching was also demonstrated for plasmonic and semiconductor nanoparticles, such as CdSe nanotetrapods (abs 508 nm), CdSe nanoplatelets (Abs 461 and 432 nm) and Ag nanoparticles (Abs 430 nm). Nanoparticle-based devices showed minimal hysteresis as well as faster relaxation times. The study presented can thus be extended to a variety of nanomaterials and dyes having the desired optical properties.

Optically Transparent Lead Halide Perovskite Polycrystalline Ceramics.

We utilize room-temperature uniaxial pressing at applied loads achievable with low-cost, laboratory-scale presses to fabricate freestanding CH3NH3PbX3 (X- = Br-, Cl-) polycrystalline ceramics with millimeter thicknesses and optical transparency up to ∼70% in the infrared. As-fabricated perovskite ceramics can be produced with desirable form factors (i.e., size, shape, and thickness) and high-quality surfaces without any postprocessing (e.g., cutting or polishing). This method should be broadly applicable to a large swath of metal halide perovskites, not just the compositions shown here. In addition to fabrication, we analyze microstructure-optical property relationships through detailed experiments (e.g., transmission measurements, electron microscopy, X-ray tomography, optical profilometry, etc.) as well as modeling based on Mie theory. The optical, electrical, and mechanical properties of perovskite polycrystalline ceramics are benchmarked against those of single-crystalline analogues through spectroscopic ellipsometry, Hall measurements, and nanoindentation. Finally, γ-ray scintillation from a transparent MAPbBr3 ceramic is demonstrated under irradiation from a 137Cs source. From a broader perspective, scalable methods to produce freestanding polycrystalline lead halide perovskites with comparable properties to their single-crystal counterparts could enable key advancements in the commercial production of perovskite-based technologies (e.g., direct X-ray/γ-ray detectors, scintillators, and nonlinear optics).

Engineered porosity for tissue-integrating, bioresorbable lifetime-based phosphorescent oxygen sensors.

Versatile silk protein-based material formats were studied to demonstrate bioresorbable, implantable optical oxygen sensors that can integrate with the surrounding tissues. The ability to continuously monitor tissue oxygenation in vivo is desired for a range of medical applications. Silk was chosen as the matrix material due to its excellent biocompatibility, its unique chemistry that facilitates interactions with chromophores, and the potential to tune degradation time without altering chemical composition. A phosphorescent Pd (II) benzoporphyrin chromophore was incorporated to impart oxygen sensitivity. Organic solvent-based processing methods using 1,1,1,3,3,3-hexafluoro-2-propanol were used to fabricate: 1) silk-chromophore films with varied thickness and 2) silk-chromophore sponges with interconnected porosity. All compositions were biocompatible and exhibited photophysical properties with oxygen sensitivities (i.e., Stern-Volmer quenching rate constants of 2.7-3.2 × 104 M-1) useful for monitoring physiological tissue oxygen levels and for detecting deviations from normal behavior (e.g., hyperoxia). The potential to tune degradation time without significantly impacting photophysical properties was successfully demonstrated. Furthermore, the ability to consistently monitor tissue oxygenation in vivo was established via a multi-week rodent study. Histological assessments indicated successful tissue integration for the sponges, and this material format responded more quickly to various oxygen challenges than the film samples.

pH control of intramolecular energy transfer and oxygen quenching in Ru(II) complexes having coupled electronic excited states.

This work illustrates the control of excited state energy transfer processes via variation of pH in transition metal complexes. In these systems a Ru(II) complex having two carboxylated bipyridyl ligands is covalently linked to pyrene via one of two different pyrene derivitized bipyridyl ligands. The energy of the Ru to carboxy-bipyridine (3)MLCT state is pH dependent while the pyrene triplet energy remains unchanged with solution acidity. At pH 0 the (3)MLCT state is the lowest energy state, and as the pH is raised and the carboxy-bipyridyl ligands are successively deprotonated, the energy of the (3)MLCT state rises above that of the pyrene triplet, resulting in a significant increase in the lifetime of the observed emission. Detailed analysis of ultrafast and microsecond time-resolved excited state decays result in a description of excited state decay that involves initial equilibration of the (3)MLCT and pyrene triplet states followed by relaxation to the ground state. The lifetime of excited state decay is defined by the position of the equilibrium, going from 2 µs at pH 0 to >10 µs at higher pH as the equilibrium favors the pyrene triplet. In addition, quenching of the excited state by dissolved oxygen exhibits a pH dependence that parallels that of the excited state lifetime. The results illustrate the utility of exploiting excited state equilibria of this type in the development of highly effective luminescent oxygen sensors.

Excitation Energy Dependence of Semiconductor Nanocrystal Emission Quantum Yields.

Accurate measurements of semiconductor nanocrystal (NC) emission quantum yields (QYs) are critical to condensed phase optical refrigeration. Of particular relevance to measuring NC QYs is a longstanding debate as to whether an excitation energy-dependent (EED) QY exists. Various reports indicate existence of NC EED QYs, suggesting that the phenomenon is linked to specific ensemble properties. We therefore investigate here the existence of EED QYs in two NC systems (CsPbBr3 and CdSe) that are possible candidates for use in optical refrigeration. The influence of NC size, size-distribution, surface ligand, and as-made emission QYs are investigated. Existence of EED QYs is assessed using two approaches (an absolute approach using an integrating sphere and a relative approach involving excitation spectroscopy). Altogether, our results show no evidence of EED QYs across samples. This suggests that parameters beyond those mentioned above are responsible for observations of NC EED QYs.

Interstitial Nature of Mn2+ Doping in 2D Perovskites.

Halide perovskites doped with magnetic impurities (such as the transition metals Mn2+, Co2+, Ni2+) are being explored for a wide range of applications beyond photovoltaics, such as spintronic devices, stable light-emitting diodes, single-photon emitters, and magneto-optical devices. However, despite several recent studies, there is no consensus on whether the doped magnetic ions will predominantly replace the octahedral B-site metal via substitution or reside at interstitial defect sites. Here, by performing correlated nanoscale X-ray microscopy, spatially and temporally resolved photoluminescence measurements, and magnetic force microscopy on the inorganic 2D perovskite Cs2PbI2Cl2, we show that doping Mn2+ into the structure results in a lattice expansion. The observed lattice expansion contrasts with the predicted contraction expected to arise from the B-site metal substitution, thus implying that Mn2+ does not replace the Pb2+ sites. Photoluminescence and electron paramagnetic resonance measurements confirm the presence of Mn2+ in the lattice, while correlated nano-XRD and X-ray fluorescence track the local strain and chemical composition. Density functional theory calculations predict that Mn2+ atoms reside at the interstitial sites between two octahedra in the triangle formed by one Cl- and two I- atoms, which results in a locally expanded structure. These measurements show the fate of the transition metal dopants, the local structure, and optical emission when they are doped at dilute concentrations into a wide band gap semiconductor.

Solvent effects on isomerization in a ruthenium sulfoxide complex.

We report the structure, electrochemistry, and isomerization kinetics for [Ru(bpy)(biq)(OSO)](PF(6)), where bpy is 2,2'-bipyridine, biq is 2,2'-biquinoline, and OSO is 2-methylsulfinylbenzoate. UV-visible and infrared data are suggestive of intramolecular S-->O and O-->S isomerization of the sulfoxide. Cyclic voltammetry reveals evidence for isomerization triggered by oxidation and reduction. Of particular note is the variation of the S-->O isomerization rate constant in different solvents. The rates were found to be 3.2 (+/-0.4) s(-1) in propylene carbonate, 0.80 (+/-0.03) s(-1) in acetonitrile, and 0.26 (+/-0.01) s(-1) in dichloromethane.

Photophysical properties of organogold(i) complexes bearing a benzothiazole-2,7-fluorenyl moiety: selection of ancillary ligand influences white light emission.

Herein we report three new gold(i) complexes with a benzothiazole-2,7-fluorenyl moiety bound through a gold-carbon σ-bond and either an N-heterocyclic carbene or organophosphine as ancillary ligands. The complexes have been characterized by NMR spectroscopy, X-ray crystallography, high resolution mass spectrometry, elemental analysis, and static and time-resolved optical spectroscopy. These compounds absorb almost strictly in the ultraviolet region and exhibit dual-luminescence following three freeze-pump-thaw cycles in toluene. The selection of the ancillary ligand significantly influences the excited-state dynamics of the complexes. The two phosphine containing complexes have similar fluorescence and phosphorescence quantum yields leading to generation of white light emission. The carbene containing complex exhibits a higher fluorescence quantum yield compared to its phosphorescence quantum yield resulting in a violet emission. Extensive photophysical characterization of these compounds suggests that the phosphine complexes undergo intersystem crossing more efficiently than the carbene complex. This is supported by a three-fold increase in luminescence lifetime, a halving in fluorescence quantum yield, and an increase in intersystem crossing efficiency by 25 percent for the phosphine complexes. Density-functional theory calculations support these observations where the energy gap between the S1 and T2 states for the carbene is roughly twice that of the phosphine complexes. To our knowledge this is the first example of single-component mononuclear gold(i) complexes exhibiting non-excimeric state white light emission, although a similar phenomenon has been realized for gold(iii) aryl compounds. Further, the triplet lifetimes of all three complexes are on the order of one ms in freeze-pump-thaw degassed toluene. These molecules also exhibit delayed fluorescence; all of the complexes display diffusion-controlled rate constants for triplet-triplet annihilation. Strong excited-state absorption is observed from the singlet and triplet excited-states in these molecules as well. The singlet states have excited-state extinction coefficients on the order of 1.5 × 105 M-1 cm-1 and the triplet states have excited-state extinction coefficients on the order of 1.0 × 105 M-1 cm-1.