Trifluoromethylation of 2D Transition Metal Dichalcogenides: A Mild Functionalization and Tunable p-Type Doping Method.

Chemical modification is a powerful strategy for tuning the electronic properties of 2D semiconductors. Here we report the electrophilic trifluoromethylation of 2D WSe2 and MoS2 under mild conditions using the reagent trifluoromethyl thianthrenium triflate (TTT). Chemical characterization and density functional theory calculations reveal that the trifluoromethyl groups bind covalently to surface chalcogen atoms as well as oxygen substitution sites. Trifluoromethylation induces p-type doping in the underlying 2D material, enabling the modulation of charge transport and optical emission properties in WSe2. This work introduces a versatile and efficient method for tailoring the optical and electronic properties of 2D transition metal dichalcogenides.

Primary Photophysics of Nicotinamide Chromophores in Their Oxidized and Reduced Forms.

Nicotinamide adenine dinucleotide (NADH) is an important enzyme cofactor with emissive properties that allow it to be used in fluorescence microscopies to study cell metabolism. Its oxidized form NAD+, on the other hand, is considered to produce negligible fluorescence. In this contribution, we describe the photophysics of the isolated nicotinamidic system in both its reduced and oxidized states. This was achieved through the study of model molecules that do not carry the adenine nucleotide since its absorbance would overlap with the absorption spectrum of the nicotinamidic chromophores. We studied three model molecules: nicotinamide (niacinamide, an oxidized form without nitrogen substitution), the oxidized chromophore 1-benzyl-3-carbamoyl-pyridinium bromide (NBzOx), and its reduced form 1-benzyl-1,4-dihydronicotinamide (NBz). For a full understanding of the dynamics, we performed both femtosecond-resolved emission and transient absorption experiments. The oxidized systems, nicotinamide and NBzOx, have similar photophysics, where the originally excited bright state decays on an ultrafast timescale of less than 400 fs. The depopulation of this state is followed by excited-state positive absorption signals, which evolve in two timescales: the first one is from 1 to a few picoseconds and is followed by a second decaying component of 480 ps for nicotinamide in water and of 80-90 ps for nicotinamide in methanol and NBzOx in aqueous solution. The long decay times are assigned as the S1 lifetimes populated from the original higher-lying bright singlet, where this state is nonemissive but can be detected by transient absorption. While for NBzOx in aqueous solution and for nicotinamide in methanol, the S1 signal decays to the solvent-only level, for the aqueous solutions of nicotinamide, a small transient absorption signal remains after the 480 ps decay. This residual signal was assigned to a small population of triplet states formed during the slower S1 decay for nicotinamide in water. The experimental results were complemented by XMS-CASPT2 calculations, which reveal that in the oxidized forms, the rapid evolution of the initial π-π* state is due to a direct crossing with lower-energy dark n-π* singlet states. This coincides with the experimental observation of long-lived nonemissive states (80 to 480 ps depending on the system). On the other hand, the reduced model compound NBz has a long-lived emissive π-π* S1 state, which decays with a 510 ps time constant, similarly to the parent compound NADH. This is consistent with the XMS-CASPT2 calculations, which show that for the reduced chromophore, the dark states lie at higher energies than the bright π-π* S1 state.

Room-Temperature Phosphorescence from Pd(II) and Pt(II) Complexes as Supramolecular Luminophores: The Role of Self-Assembly, Metal-Metal Interactions, Spin-Orbit Coupling, and Ligand-Field Splitting.

The synthesis as well as the structural and photophysical characterization of two isoleptic bis-cyclometalated Pt(II) and Pd(II) complexes, namely [PtL] and [PdL], bearing a tailored dianionic tetradentate ligand (L2-) are reported. The isostructural character and intermolecular interactions of [PtL] and [PdL] were assessed by NMR spectroscopy and X-ray diffraction analysis. Both complexes show fully ligand-controlled aggregation, demonstrating that a judicious molecular design can tune the photophysical properties. In fact, by introduction of fluorine atoms on defined positions and methoxy groups on complementary sites, metal-metal interactions can be forced by a head-to-tail stacking. Hence, [PtL] shows luminescence from metal-perturbed ligand-centered or from metal-metal-to-ligand charge-transfer triplet states in diluted solutions, in frozen glasses and in crystals, with high photoluminescence quantum yields and long lifetimes in the microsecond range. At room temperature (RT) in concentrated fluid solutions, the palladium analogue [PdL] surprisingly emits luminescence from aggregated species involving supramolecular interactions. Time-resolved photoluminescence and transient absorption spectroscopies demonstrated that ultrafast intersystem crossing occurs for both metals, which outruns any competitive relaxation pathway from the photoexcited singlet state. Furthermore, we demonstrate that the radiationless deactivation can be suppressed in frozen glassy matrices at 77 K and by intermolecular interactions in fluid solutions at RT. In both cases and as indicated by density functional theory calculations, the lowest emissive state acts as an energy trap from which the thermal population of dissociative states with formal occupation of an antibonding Pd-centered 4dx2-y2 orbital is suppressed. This occurs as the energy gap between the emissive and the dark states surpasses kT.

Dynamic Tuning of the Bandgap of CdSe Quantum Dots through Redox-Active Exciton-Delocalizing N-Heterocyclic Carbene Ligands.

Ligands that enable the delocalization of excitons beyond the physical boundary of the inorganic core of semiconductor quantum dots (QDs), called "exciton-delocalizing ligands (EDLs)", offer the opportunity to design QD-based environmental sensors with dynamically responsive optical spectra, because the degree of exciton delocalization depends on the electronic structure of the EDL. This paper demonstrates dynamic, reversible tuning of the optical bandgap of a dispersion of CdSe QDs through the redox states of their 1,3-dimesitylnaphthoquinimidazolylidene N-heterocyclic carbene (nqNHC) ligands. Upon binding of the nqNHC ligands to the QD, the optical bandgap bathochromically shifts by up to 102 meV. Electrochemical reduction of the QD-bound nqNHC ligands shifts the bandgap further by up to 25 meV, a shift that is reversible upon reoxidation.

Quantum Dots Photocatalyze Intermolecular [2 + 2] Cycloadditions of Aromatic Alkenes Adsorbed to their Surfaces via van der Waals Interactions.

Triplet excited state-initiated photochemistry is a mild and selective route to cycloadditions, radical rearrangements, couplings, fragmentations, and isomerizations. Colloidal quantum dots are proven visible-light photosensitizers and structural scaffolds for triplet-initiated reactions of molecules that are functionalized (with carboxylates) to anchor on the QD surface. Here, with the aid of polyaromatic energy shuttles that act as noncovalent adsorption sites for substrates on the QD surface, the scope of QD-photocatalyzed intermolecular [2 + 2] cycloadditions is extended to freely diffusing substrates (no anchoring groups). QD-shuttle complexes photocatalyze homo- and heterointermolecular [2 + 2] photocycloadditions of benzalacetone, chalcone and its derivatives with up to 94% yield; the yields for all reactions are comparable to those achieved by Ir(ppy)3 but with the advantages of a factor of 2.5 lower catalyst loading, superior stability, and the ability to recover the catalyst by simple centrifugation and reuse it for multiple reaction cycles. Experiments imply a two-step triplet-triplet energy transfer mechanism, one energy transfer from the QD to the energy shuttle followed by a second energy transfer from the shuttle to the transiently adsorbed substrate.

Polariton Dynamics in Two-Dimensional Ruddlesden-Popper Perovskites Strongly Coupled with Plasmonic Lattices.

Strong coupling between light and matter can produce hybrid eigenstates known as exciton-polaritons. Although polariton dynamics are important photophysical properties, the relaxation pathways of polaritons in different coupling regimes have seen limited attention. This paper reports the dynamics of hybridized states from 2D Ruddlesden-Popper perovskites coupled to plasmonic nanoparticle lattices. The open cavity architecture of Al lattices enables the coupling strength to be modulated by varying either the lead halide perovskite film thickness or the superstrate refractive index. Both experiments and finite-difference time-domain simulations of the optical dispersion diagrams showed avoided crossings that are a signature of strong coupling. Our analytical model also elucidated the correlation between the exciton/plasmon mixing ratio and polariton coupling strength. Using fs-transient absorption spectroscopy, we found that both the upper and lower polaritons have shorter lifetimes than the excitons and that polaritons can show faster excited-state dynamics when they have access to additional energy transfer channels.

Chemo- and Stereoselective Intermolecular [2+2] Photocycloaddition of Conjugated Dienes using Colloidal Nanocrystal Photocatalysts.

The use of visible-light photosensitizers to power [2+2] photocycloadditions that produce complex tetrasubstituted cyclobutanes is a true success of photochemistry, but the scope of this reaction has been limited to activated α, ß-unsaturated carbonyls. This paper describes selective intermolecular homo- and hetero-[2+2] photocycloadditions of terminal and internal aryl conjugated dienes - substrates historically unsuited for this reaction because of their multiple possible reaction pathways and product configurations - through triplet-triplet energy transfer from CdSe nanocrystal photocatalysts, to generate valuable and elusive syn-trans aryl vinylcyclobutanes. The negligible singlet-triplet splitting of nanocrystals' excited states allows them to drive the [2+2] pathway over the competing [4+2] photoredox pathway, a chemoselectivity not achievable with any known molecular photosensitizer. Reversible tethering of the cyclobutane product to the nanocrystal surface results in near quantitative yield of the syn-trans product. Flat colloidal CdSe nanoplatelets produce cyclobutanes coupled at the terminal alkenes of component dienes with up to 89% regioselectivity.

Light-Triggered Switching of Quantum Dot Photoluminescence through Excited-State Electron Transfer to Surface-Bound Photochromic Molecules.

This paper describes reversible "on-off" switching of the photoluminescence (PL) intensity of CdSe quantum dots (QDs), mediated by photochromic furylfulgide carboxylate (FFC) molecules chemisorbed to the surfaces of the QDs. Repeated cycles of UV and visible illumination switch the FFC between "closed" and "open" isomers. Reversible switching of the QDs' PL intensity by >80% is enabled by different rates and yields of PL-quenching photoinduced electron transfer (PET) from the QDs to the respective isomers. This difference is consistent with cyclic voltammetry measurements and density functional calculations of the isomers' frontier orbital energies. This work demonstrates fatigue-resistant modulation of the PL of a QD-molecule complex through remote control of PET. Such control potentially enables applications, such as all-optical memory, sensing, and imaging, that benefit from a fast, tunable, and reversible response to light stimuli.

Tailoring the Optical Response of Pentacene Thin Films via Templated Growth on Hexagonal Boron Nitride.

The optoelectronic properties of organic thin films are strongly dependent on their molecular orientation and packing, which in turn is sensitive to the underlying substrate. Hexagonal boron nitride (hBN) and other van der Waals (vdW) materials are known to template different organic thin film growth modalities from conventional inorganic substrates such as SiO2. Here, the morphology and temperature-dependent optical properties of pentacene films grown on hBN are reported. Pentacene deposited on hBN forms large-grain films with a molecular π-face-on orientation unlike the dendritic edge-on thin-film phase on SiO2. Pentacene/hBN films exhibit a 40 meV lower free exciton emission than pentacene/SiO2 and an unconventional emission energy temperature dependence. Time-resolved photoluminescence (PL) decay measurements show a long-lived signal in the π-face-on phase related to delayed emission from triplet-triplet fusion. This work demonstrates that growth on vdW materials provides a pathway for controlling optoelectronic functionality in molecular thin films.

N-Heterocyclic Carbenes as Reversible Exciton-Delocalizing Ligands for Photoluminescent Quantum Dots.

Delocalization of excitons within semiconductor quantum dots (QDs) into states at the interface of the inorganic core and organic ligand shell by so-called "exciton-delocalizing ligands (EDLs)" is a promising strategy to enhance coupling of QD excitons with proximate molecules, ions, or other QDs. EDLs thereby enable enhanced rates of charge carrier extraction from, and transport among, QDs and dynamic colorimetric sensing. The application of reported EDLs-which bind to the QDs through thiolates or dithiocarbamates-is however limited by the irreversibility of their binding and their low oxidation potentials, which lead to a high yield of photoluminescence-quenching hole trapping on the EDL. This article describes a new class of EDLs for QDs, 1,3-dimethyl-4,5-disubstituted imidazolylidene N-heterocyclic carbenes (NHCs), where the 4,5-substituents are Me, H, or Cl. Postsynthetic ligand exchange of native oleate capping ligands for NHCs results in a bathochromic shift of the optical band gap of CdSe QDs (R = 1.17 nm) of up to 111 meV while the colloidal stability of the QDs is maintained. This shift is reversible for the MeNHC-capped and HNHC-capped QDs upon protonation of the NHC. The magnitude of exciton delocalization induced by the NHC (after scaling for surface coverage) increases with the increasing acidity of its π system, which depends on the substituent in the 4,5-positions of the imidazolylidene. The NHC-capped QDs maintain photoluminescence quantum yields of up to 4.2 ± 1.8% for shifts of the optical band gap as large as 106 meV.

Ultrafast Fluorescence Signals from ß-Dihydronicotinamide Adenine Dinucleotide: Resonant Energy Transfer in the Folded and Unfolded Forms.

ß-Dihydronicotinamide adenine dinucleotide (NADH) plays a critical role in biological redox processes. Inside the cell, NADH can be in a folded or an unfolded conformation, depending on the chemical environment that surrounds it. It is known that selective excitation of adenine in NADH can induce energy transfer events from this nucleotide to the reduced nicotinamide chromophore. From the anticipated time scales, this process must compete with adenine's internal conversion channel, which is known to occur in the sub-picosecond time scale. In this work, we studied the dynamics of the excited states of both chromophores in NADH through the time resolution of the spontaneous emission from both nucleotides. Through these experiments, we extend the knowledge about the competition between the different photophysical channels both in the folded and unfolded states. The study involved the folded and unfolded states of NADH by experiments in water and methanol solutions. Our femtosecond fluorescence results were complemented by the first nuclear magnetic resonance through space magnetization transfer measurements on NADH, which establish the solvent-dependent folded versus unfolded states. We determined the dynamics of the excited states by direct excitation of dihydronicotinamide at 380 nm and adenine at 266 nm. From this, we were able to measure for the folded state, a time constant of 90 fs for energy transfer. Additionally, we determined that even in what is referred to as an unfolded state in methanol, non-negligible excitation transfer events do take place. Spontaneous emission anisotropy measurements were used in order to confirm the presence of a minor energy transfer channel in the methanol solutions where the unfolded state dominates.

Unravelling the modus-operandi of chromenylium-cyanine fluorescent probes: a case study.

Small-molecule fluorescent probes having optimized optical properties, such as high photostability and brightness, local microenvironment sensitivity and specific subcellular localizations, are increasingly available. Although the basis for designing efficient fluorophores for bioimaging applications is well established, implementing an improvement in a given photophysical characteristic always tends to compromise another optical property. This problem has enormous consequences for in vivo imaging, where ensuring a specific localization and precise control of the probe response is challenging. Herein we discuss a fluorescent probe, CC334, as a case study of the chromenylium-cyanine family that commonly exhibits highly complex photophysical schemes and highly interfered bioanalytical responses. By an exhaustive and concise analysis of the CC334 optical responses including detailed spectroscopic calibrations, steady-state microenvironment effects, ultrafast photophysics analysis and computational studies, we elucidate a new strategy to apply the probe in the singlet oxygen reactive oxygen species (1O2-ROS) monitoring using in vitro and in vivo models. The probe provides a new avenue for designing fluorescent probes to understand the dynamic behavior of subcellular environments.

Nitrated Fluorophore Formation upon Two-Photon Excitation of an Azide with Extended Conjugation.

The transformation of an aromatic azide into a highly fluorescent species through a nonlinear optical process was studied. The azide system was designed to undergo N2 release and nitrene to nitro conversion upon two-photon electronic excitation. The formation of the nitro form of the compound through reactions with O2 and its high radiative quantum yield implies that the azide can be used as a biphotonic activatable fluorogen. The electronic state in which the azide to nitrene transformation takes place can be accessed nonlinearly with near-infrared light which allows for photoactivation with commonly available lasers. Furthermore, the system was built with a sulfonate functionality which allows for the molecule to be adsorbed at surfaces like that of cadmium sulfide nanocrystals which further improves the nonlinear optical absorption properties in the composite, through an energy transfer mechanism. The yield of the process as a function of the excitation photon energy together with computational studies indicate that the N2 release in this azide is due to a reactive channel in the second singlet excited state of the molecule. This feature implies that the system is intrinsically photostable for excitation below and above a certain wavelength and that the system can be phototriggered selectively by the nonlinear optical process.

The influence of push-pull states on the ultrafast intersystem crossing in nitroaromatics.

The photochemistry of nitro-substituted polyaromatic compounds is generally determined by the rapid decay of its S1 state and the rapid population of its triplet manifold. Previous studies have shown that such an efficient channel is due to a strong coupling of the fluorescent state with specific upper receiver states in the triplet manifold. Here we examine variations in this mechanism through the comparison of the photophysics of 2-nitrofluorene with that of 2-diethylamino-7-nitrofluorene. The only difference between these two molecules is the presence of a diethylamino group in a push-pull configuration for the latter compound. The femtosecond-resolved experiments presented herein indicate that 2-nitrofluorene shows ultrafast intersystem crossing which depopulates the S1 emissive state within less than a picosecond. On the other hand, the amino substituted nitrofluorene shows a marked shift in its S1 energy redounding in the loss of coupling with the receiver triplet state, and therefore a much longer lifetime of 100 ps in cyclohexane. In polar solvents, the diethylamino substituted compound actually shows double peaked fluorescence due to the formation of charge transfer states. Evaluation of the Stokes shifts in different solvents indicates that both bands correspond to intramolecular charge transfer states in equilibrium which are formed in an ultrafast time scale from the original locally excited (LE) state. The present study addresses the interplay between electron-donating and nitro substituents, showing that the addition of the electron-donating amino group is able to change the coupling with the triplet states due to a stabilization of the first excited singlet state and the rapid formation of charge transfer states in polar solvents. We include calculations at the TD-DFT level of theory with the PBE0 and B3LYP functionals which nicely predict the observed difference between the two compounds, showing how the specific S(π-π*)-T(n-π*) coupling normally prevalent in nitroaromatics is lost in the push-pull compound.

Dynamics of the higher lying excited states of cyanine dyes. An ultrafast fluorescence study.

The electronic relaxation dynamics of the second singlet excited states of several cyanine dyes was studied through the femtosecond fluorescence up-conversion technique. Our interest in these molecules comes from the potential applications of systems with upper excited singlet states with a long lifetime, which can include electron and energy transfer from the higher lying singlets after one- or two-photon absorption. We studied three series of cyanines with 4-quinolyl, 2-quinolyl, or benzothiazolyl type end groups, each with varying sp(2) carbon conjugation lengths in the methinic bridge. The dynamics after electronic excitation to singlet states above the fluorescent state vary significantly as a function of cyanine structure and conjugation length. In particular, for the 4-quinolyl series the cyanine with an intermediate conjugation length (three methinic carbons) has the slowest S2 decays with lifetimes of 5.4 ps in ethanol and 6.6 ps in ethylene glycol. On the other hand, we observed that the 2-quinolyl family has S2 decay times in the subpicosecond range independent of the conjugation length between the end groups. The slowest internal conversion was observed for the benzothiazolyl type cyanine with five methinic carbons, with an S2 lifetime of 17.3 ps in ethanol. For the planar cyanines of this study we observed for the first time a clear systematic trend in the S2 decay times which closely follow the energy gap law. It was also demonstrated that a slow S2 decay is as well observed upon excitation through degenerate two-photon absorption with near-IR pulses. The present study isolates the most important variables for the design of cyanines with long S2 lifetimes.