Sofia Garakyaraghi

Direct observation of triplet energy transfer from semiconductor nanocrystals.

A different way to put triplets in play Most molecules adopt a singlet spin configuration: All their electrons are arranged in pairs. Unpaired triplet states engage in a variety of useful reactions but are hard to produce. Quantum mechanics dictates that photo-excitation from singlet to triplet states is inefficient. Instead, chemists rely on sensitizers, which populate the triplet states of their neighbors through energy transfer after absorbing light themselves. Mongin et al. now show that certain nanoparticles can act as triplet sensitizers. Science, this issue p. 369 Nanoparticles can channel photoexcitation to surface molecules to instigate spin-state–dependent chemistry in solution. Triplet excitons are pervasive in both organic and inorganic semiconductors but generally remain confined to the material in which they originate. We demonstrated by transient absorption spectroscopy that cadmium selenide semiconductor nanoparticles, selectively excited by green light, engage in interfacial Dexter-like triplet-triplet energy transfer with surface-anchored polyaromatic carboxylic acid acceptors, extending the excited-state lifetime by six orders of magnitude. Net triplet energy transfer also occurs from surface acceptors to freely diffusing molecular solutes, further extending the lifetime while sensitizing singlet oxygen in an aerated solution. The successful translation of triplet excitons from semiconductor nanoparticles to the bulk solution implies that such materials are generally effective surrogates for molecular triplets. The nanoparticles could thereby potentially sensitize a range of chemical transformations that are relevant for fields as diverse as optoelectronics, solar energy conversion, and photobiology.

Transient absorption dynamics of sterically congested Cu(I) MLCT excited states.

Subpicosecond through supra-nanosecond transient absorption dynamics of the homoleptic Cu(I) metal-to-ligand charge transfer (MLCT) photosensitizers including the benchmark [Cu(dmp)2](+) (dmp =2,9-dimethyl-1,10-phenanthroline) chromophore, as well as [Cu(dsbp)2](+) (dsbp =2,9-di(sec-butyl)-1,10-phenanthroline and [Cu(dsbtmp)2](+) (dsbtmp =2,9-di(sec-butyl)-3,4,7,8-tetramethyl-1,10-phenanthroline) were investigated in dichloromethane and tetrahydrofuran solutions. Visible and near-IR spectroelectrochemical measurements of the singly reduced [Cu(dsbp)2](+) and [Cu(dsbtmp)2](+) species were determined in tetrahydrofuran, allowing for the identification of redox-specific phenanthroline-based radical anion spectroscopic signatures prevalent in the respective transient absorption experiments. This study utilized four different excitation wavelengths (418, 470, 500, and 530 nm) to elucidate dynamics on ultrafast times scales spanning probe wavelengths ranging from the UV to the near-IR (350 to 1450 nm). With the current time resolution of ∼150 fs, initial excited state decay in all three compounds was found to be independent of excitation wavelength. Not surprisingly, there was little to no observed influence of solvent in the initial stages of excited state decay in any of these molecules including [Cu(dmp)2](+), consistent with results from previous investigators. The combined experimental data revealed two ranges of time constants observed on short time scales in all three MLCT chromophores and both components lengthen as a function of structure in the following manner: [Cu(dsbtmp)2](+) < [Cu(dsbp)2](+) < [Cu(dmp)2](+). The molecule with the most inhibited potential for distortion, [Cu(dsbtmp)2](+), possessed the fastest ultrafast dynamics as well as the longest excited state lifetimes in both solvents. These results are consistent with a small degree of excited state distortion, rapid intersystem crossing, and weak vibronic coupling to the ground state. The concomitant systematic variation in both initial time constants, assigned to pseudo-Jahn-Teller distortion and intersystem crossing, suggest that both processes are intimately coupled in all molecules in the series. The variability in these time scales illustrate that strongly impeded structural distortion in Cu(I) MLCT excited state enables more rapid surface crossings in the initial deactivation dynamics.

Delayed Molecular Triplet Generation from Energized Lead Sulfide Quantum Dots

The generation and transfer of triplet excitons across the molecular-semiconductor interface represents an important technological breakthrough featuring numerous fundamental scientific questions. This contribution demonstrates curious delayed formation of TIPS-pentacene molecular triplet excitons bound on the surface of PbS nanocrystals mediated through the initial production of a proposed charge transfer intermediate following selective excitation of the PbS quantum dots. Ultrafast UV-vis and near-IR transient absorption spectroscopy was used to track the dynamics of the initial PbS exciton quenching as well as time scale of the formation of molecular triplet excited states that persisted for 10 μs on the PbS surface, enabling subsequent energy and electron transfer reactivity. These results provide the pivotal proof-of-concept that PbS nanocrystals absorbing near-IR radiation can ultimately generate molecular triplets on their surfaces through processes distinct from direct Dexter triplet energy transfer. More broadly, this work establishes that small metal chalcogenide semiconductor nanocrystals interfaced with molecular chromophores exhibit behavior reminiscent of supramolecular chemical systems, a potentially impactful concept for nanoscience.

Charge Localization after Ultrafast Photoexcitation of a Rigid Perylene Perylenediimide Dyad Visualized by Transient Stark Effect

The intramolecular charge-transfer (CT) dynamics of a rigid and strongly conjugated perylenediimide-bridge-perylene dyad (PDIPe) has been investigated in dichloromethane using ultrafast transient electronic absorption spectroscopy and quantum chemical calculations. The strong electronic coupling between the dyad units gives rise to a CT band. Its photoexcitation forms a delocalized CT state with well-preserved ion bands despite the strong coupling. In the dyad, the electronic transition dipole moment of the electron donor perylene is aligned along the axis of the electric field vector with respect to the CT species. This alignment makes the donor sensitive to the Stark effect and thus charge density fluctuations in the CT state. Charge localization on the picosecond time scale manifests as a time-dependent Stark shift in the visible region. Quantum chemical calculations reveal a twist around the acetylene bridging unit to be the responsible mechanism generating a partial to an almost complete CT state. An estimate of the electric field strength in the CT state yields approximately 25 MV/cm, which increases to around 31 MV/cm during charge localization. Furthermore, the calculations illustrate the complexity of electronic structure in this strongly delocalized superchromophore and reflect the complications in the interpretation of transient absorption results when compared to steady-state approaches such as spectroelectrochemistry and model chromophore experiments such as photoinduced bimolecular charge transfer.

Energy Transfer Dynamics in Triplet–Triplet Annihilation Upconversion Using a Bichromophoric Heavy-Atom-Free Sensitizer

A heavy-atom-free triplet sensitizer suitable for triplet-triplet annihilation-based photon upconversion was developed from the thermally activated delayed fluorescence (TADF) molecule 4CzPN by covalently tethering a pyrene derivative (DBP) as a triplet acceptor. The triplet exciton produced by 4CzPN is captured by the intramolecular pyrenyl acceptor and subsequently transferred via intermolecular triplet-triplet energy transfer (TTET) to freely diffusing pyrenyl acceptors in toluene. Transient absorption and time-resolved photoluminescence spectroscopy were employed to examine the dynamics of both the intra- and intermolecular TTET processes, and the results indicate that the intramolecular energy transfer from 4CzPN to DBP is swift, quantitative, and nearly irreversible. The reverse intersystem crossing is suppressed while intersystem crossing remains efficient, achieving high triplet yield and long triplet lifetime simultaneously. The ultralong excited state lifetime characteristic of the DBP triplet was shown to be crucial for enhancing the intermolecular TTET efficiency and the subsequent triplet-triplet annihilation photochemistry. It was also demonstrated that with the long triplet lifetime of the tethered DBP, TTET was enabled under low free acceptor concentrations and/or with sluggish molecular diffusion in polymer matrixes.

Quantum dot photosensitizers as a new paradigm for photochemical activation

Photosensitizers are an essential component of solar energy conversion processes, in which they are used to generate the highly reactive excited states that enable energy conversion (e.g., photochemical upconversion).1, 2 Typically, molecular triplet photosensitizers are used for such applications, but to improve the solar energy conversion process, the identification and preparation of next-generation triplet photosensitizers is required. However, the design of such photosensitizers—suitable for solar energy conversion and photocatalytic applications— remains a challenge.3 Semiconductor nanocrystals are stable light-emitting materials that can be systematically tuned to produce intense absorptions and photoluminescence. Futhermore, semiconductor nanocrystals offer several advantages over molecular photosensitizers, e.g., simple preparative synthesis, photochemical stability, size-tunable electronic and photophysical properties, high molar extinction coefficients, and trivial postsynthetic functionalization. Moreover, the inherently large, energy-consuming singlet-triplet gaps that are characteristic of molecular sensitizers can be avoided with the use of semiconductor nanocrystals that feature closely spaced excited-state energy levels.4 The characteristic broadband light absorption properties of these materials can be extended into the near-IR and can thus potentially be exploited for numerous triplet excited-state reactions, such as photoredox catalysis, singlet oxygen generation, photochemical upconversion, and excited state electron transfer. In this work,5 we have investigated the possibility of using quantum dots as effective alternatives to molecular triplet photosensitizers. With our experiments, we show definitively Figure 1. Artistic illustration of quantum dot-to-molecule triplet energy transfer and example subsequent reactions.