Understanding the influence of geometric and electronic structure on the excited state dynamical and photoredox properties of perinone chromophores.

In this work, a series of eight similarly structured perinone chromophores were synthesized and photophysically characterized to elucidate the electronic and structural tunability of their excited state properties, including excited state redox potentials and fluorescence lifetimes/quantum yields. Despite their similar structure, these chromophores exhibited a broad range of visible absorption properties, quantum yields, and excited state lifetimes. In conjunction with static and time-resolved spectroscopies from the ultrafast to nanosecond time regimes, time-dependent computational modeling was used to correlate this behavior to the relationship between non-radiative decay and the energy-gap law. Additionally, the ground and excited state redox potentials were calculated and found to be tunable over a range of 1 V depending on the diamine or anhydride used in their synthesis (Ered* = 0.45-1.55 V; Eox* = -0.88 to -1.67 V), which is difficult to achieve with typical photoredox-active transition metal complexes. These diverse chromophores can be easily prepared, and with their range of photophysical tunability, will be valuable for future use in photofunctional applications.

3d-d Excited States of Ni(II) Complexes Relevant to Photoredox Catalysis: Spectroscopic Identification and Mechanistic Implications.

Synthetic organic chemistry has seen major advances due to the merger of nickel and photoredox catalysis. A growing number of Ni-photoredox reactions are proposed to involve generation of excited nickel species, sometimes even in the absence of a photoredox catalyst. To gain insights about these excited states, two of our groups previously studied the photophysics of Ni(t-Bubpy)(o-Tol)Cl, which is representative of proposed intermediates in many Ni-photoredox reactions. This complex was found to have a long-lived excited state (τ = 4 ns), which was computationally assigned as a metal-to-ligand charge transfer (MLCT) state with an energy of 1.6 eV (38 kcal/mol). This work evaluates the computational assignment experimentally using a series of related complexes. Ultrafast UV-Vis and mid-IR transient absorption data suggest that a MLCT state is generated initially upon excitation but decays to a long-lived state that is 3d-d rather than 3MLCT in character. Dynamic cis,trans-isomerization of the square planar complexes was observed in the dark using 1H NMR techniques, supporting that this 3d-d state is tetrahedral and accessible at ambient temperature. Through a combination of transient absorption and NMR studies, the 3d-d state was determined to lie ∼0.5 eV (12 kcal/mol) above the ground state. Because the 3d-d state features a weak Ni-aryl bond, the excited Ni(II) complexes can undergo Ni homolysis to generate aryl radicals and Ni(I), both of which are supported experimentally. Thus, photoinduced Ni-aryl homolysis offers a novel mechanism of initiating catalysis by Ni(I).

Nanocrystals for Triplet Sensitization: Molecular Behavior from Quantum-Confined Materials.

 Inorganic semiconductor quantum dot-molecular constructs represent an emerging class of materials functioning as triplet photosensitizers. Fundamental investigations into the exciton transfer/transduction processes at the interface of these hybrid materials have revealed parallels in the operable mechanisms to those established in purely molecular systems. The similarities in the governing energy migration mechanisms in these quantum-confined materials permit conventional photophysical strategies to be implemented in future research endeavors. This Viewpoint provides a perspective on this emerging field of inorganic quantum dots as photosensitizers, in particular the transfer of triplet excitons at the molecule-nanomaterial interface. The current state-of-the-art will be explored while highlighting areas of potential growth toward exploiting these materials in photofunctional solar energy conversion schemes.

Enhancing the Visible-Light Absorption and Excited-State Properties of Cu(I) MLCT Excited States.

A computationally inspired Cu(I) metal-to-ligand charge transfer (MLCT) chromophore, [Cu(sbmpep)2]+ (sbmpep = 2,9-di(sec-butyl)-3,8-dimethyl-4,7-di(phenylethynyl)-1,10-phenanthroline), was synthesized in seven total steps, prepared from either dichloro- or dibromophenanthroline precursors. Complete synthesis, structural characterization, and electrochemistry, in addition to static and dynamic photophysical properties of [Cu(sbmpep)2]+, are reported on all relevant time scales. UV-Vis absorption spectroscopy revealed significant increases in oscillator strength along with a concomitant bathochromic shift in the MLCT absorption bands with respect to structurally related model complexes (ε = 16 500 M-1 cm-1 at 491 nm). Strong red photoluminescence (Φ = 2.7%, λmax = 687 nm) was observed from [Cu(sbmpep)2]+, which featured an average excited-state lifetime of 1.4 µs in deaerated dichloromethane. Cyclic and differential pulse voltammetry revealed ∼300 mV positive shifts in the measured one-electron reversible reduction and oxidation waves in relation to a Cu(I) model complex possessing identical structural elements without the π-conjugated 4,7-substituents. The excited-state redox potential of [Cu(sbmpep)2]+ was estimated to be -1.36 V, a notably powerful reductant for driving photoredox chemistry. The combination of conventional and ultrafast transient  absorption and luminescence spectroscopy successfully map the excited-state dynamics of [Cu(sbmpep)2]+ from initial photoexcitation to the formation of the lowest-energy MLCT excited state and ultimately its relaxation to the ground state. This newly conceived molecule appears poised for photosensitization reactions involving energy and electron-transfer processes relevant to photochemical upconversion, photoredox catalysis, and solar fuels photochemistry.

Diastereomerically Differentiated Excited State Behavior in Ruthenium(II) Hexafluoroacetylacetonate Complexes of Diphenyl Thioindigo Diimine.

Mono- and diruthenium hexafluoroacetylacetonate (hfac) complexes of the thioindigo-N,N'-diphenyldiimine chelating ligand 3 have been prepared. The thioindigo diimine ligand binds to ruthenium in a bidentate fashion in the mononuclear compound 2 and serves as a bidentate chelating bridging ligand in the diruthenium complexes 1a and 1b. Compound 2 was isolated as a racemic mixture while the diruthenium complexes were isolated as the meso (ΔΛ) 1a and rac (ΔΔ and ΛΛ) 1b diastereomers. In-depth structural characterization of the compounds was performed, including X-ray crystallography, 1H, 13C, and 19F nuclear magnetic resonance (NMR) spectroscopy, and 2D NMR correlation experiments. Electrochemical properties were evaluated utilizing cyclic voltammetry. Ground state optical properties of the complexes were examined using UV-visible spectroscopy and spectroelectrochemistry. The excited state dynamics of the series were investigated by ultrafast transient absorption spectroscopy. Variable temperature NMR experiments demonstrated that the rac diruthenium compound 1b undergoes conformational exchange with a rate constant of 8700 s-1 at 298 K, a behavior that is not observed in the meso diastereomer 1a. The series of complexes possess metal-to-ligand charge transfer (MLCT) absorption bands in the near-infrared (λmax 689-783 nm). The compounds do not display photoluminescence in room temperature solution-phase experiments or in experiments at 77 K. Transient absorption spectroscopy measurements revealed excited states with picosecond lifetimes for 1a, 1b, and 2, and spectroelectrochemical experiments confirmed assignment of the transient species as arising from MLCT transitions. Unexpectedly, the transient absorption measurements revealed disparate time constants for the excited state decay of diastereomers 1a and 1b.

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.

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.

Photoinduced structural distortions and singlet-triplet intersystem crossing in Cu(i) MLCT excited states monitored by optically gated fluorescence spectroscopy.

Copper(i) phenanthroline complexes represent viable earth-abundant alternatives to the ubiquitous Ru(ii) tris-bipyridine photosensitizers owing to their similar metal-to-ligand charge transfer (MLCT) properties. A well-established complication of Cu(i) phenanthroline complexes is that they can undergo significant photo-induced structural rearrangements, leading to excited states that are highly susceptible to exciplex formation and short-lived. In this work, a comprehensive analysis of the photo-induced structural distortions and singlet-triplet intersystem crossing dynamics of a series of four sterically encumbered Cu(i) phenanthroline chromophores has been conducted, namely, [Cu(dsbp)2]+ (dsbp = 2,9-di-sec-butyl-1,10-phenanthroline), [Cu(dsbtmp)2]+ (dsbtmp = 2,9-di-sec-butyl-3,4,7,8-tetramethyl-1,10-phenanthroline), [Cu(dipp)2]+ (dipp = 2,9-di-isopropyl-1,10-phenanthroline), and [Cu(diptmp)2]+ (diptmp = 2,9-di-isopropyl-3,4,7,8-tetramethyl-1,10-phenanthroline). Upconverted fluorescence decay kinetics were measured at wavelengths along the blue side of the photoluminescence spectrum. The experimental results displayed strong wavelength dependence of the singlet emission, with rapid sub-picosecond decay dominating at higher energies. At lower emission energies, increasing contribution of a longer decay component was revealed. This wavelength dependence is a signature of the excited state structural rearrangement of the phenanthroline ligands which concomitantly lower the excited state energy. The obtained time constants were in excellent agreement with those measured in the complementary ultrafast transient absorption experiments. The sub-picosecond component (prompt fluorescence) is associated with the photo-induced structural rearrangement that lowers the energy of the singlet excited state. The longer decay component represents the lifetime of the S1 excited state, and thus the time-scale of singlet-triplet intersystem crossing. Lastly, the observed dual emission was further characterized by constructing picosecond time-resolved emission spectra from the measured kinetic data. These qualitative luminescence spectra capture the resulting emission from both the S1 initial state and the S1 flattened state, providing further insight into the energy-lowering excited state distortion across the series.

Cuprous Phenanthroline MLCT Chromophore Featuring Synthetically Tailored Photophysics.

In the interest of expanding the inventory of available long lifetime, photochemically robust, and strongly reducing Cu(I) MLCT sensitizers, we present detailed structural, photophysical, and electrochemical characterization of [Cu(dipp)2]+, dipp = 2,9-diisopropyl-1,10-phenanthroline, and its sterically encumbered tetramethyl analogue [Cu(diptmp)2]+, diptmp = 2,9-diisopropyl-3,4,7,8-tetramethyl-1,10-phenanthroline. The achiral isopropyl substituents enable similar steric bulk effects to the previously investigated sec-butyl substituents while eliminating the complex NMR structural analyses associated with the presence of two chiral centers in the latter. The photophysical properties of [Cu(diptmp)2]+ are impressive, possessing a 2.3 µs lifetime in deaerated CH2Cl2 and a photoluminescence quantum yield of 4.7%, which were slightly attenuated in coordinating tetrahydrofuran (THF) solutions. Nanosecond transient absorption spectroscopy results matched the transient photoluminescence kinetics enabling complete characterization of MLCT excited-state decay in these molecules. The calculated excited-state potential for the Cu2+/Cu+* couple (E = -1.74 V vs Fc+/0) indicated that [Cu(diptmp)2]+* is a strong photoreductant potentially useful for myriad applications. Ultrafast transient absorption measurements performed in THF solutions are also reported, yielding the relative time scales for both the pseudo-Jahn-Teller distortion (0.4-0.8 ps in [Cu(dipp)2]+ and 0.12-0.5 ps in [Cu(diptmp)2]+) and singlet-triplet intersystem crossing (6.4-10.1 ps for [Cu(dipp)2]+ and 3.5-5.4 ps for [Cu(diptmp)2]+) within these molecules. The disparity in the time scales of pseudo-Jahn-Teller distortion and intersystem crossing between two complexes with different anticipated excited-state geometries suggests that strongly impeded structural distortion in the MLCT excited state (i.e., [Cu(diptmp)2]+) enables more rapid surface crossings in the initial deactivation dynamics.

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.

Accessing the triplet manifold of naphthalene benzimidazole-phenanthroline in rhenium(I) bichromophores.

The steady-state and ultrafast to supra-nanosecond excited state dynamics of fac-[Re(NBI-phen)(CO)3(L)](PF6) (NBI-phen = 16H-benzo[4',5']isoquinolino[2',1':1,2]imidazo[4,5-f][1,10]phenanthrolin-16-one) as well as their respective models of the general molecular formula [Re(phen)(CO)3(L)](PF6) (L = PPh3 and CH3CN) has been investigated using transient absorption and time-gated photoluminescence spectroscopy. The NBI-phen containing molecules exhibited enhanced visible light absorption with respect to their models and a rapid formation (<6 ns) of the triplet ligand-centred (LC) excited state of the organic ligand, NBI-phen. These triplet states exhibit an extended excited state lifetime that enable the energized molecules to readily engage in triplet-triplet annihilation photochemistry.

TIPS-pentacene triplet exciton generation on PbS quantum dots results from indirect sensitization.

Many fundamental questions remain in the elucidation of energy migration mechanisms across the interface between semiconductor nanomaterials and molecular chromophores. The present transient absorption study focuses on PbS quantum dots (QDs) of variable size and band-edge exciton energy (ranging from 1.15 to 1.54 eV) post-synthetically modified with a carboxylic acid-functionalized TIPS-pentacene derivative (TPn) serving as the molecular triplet acceptor. In all instances, selective excitation of the PbS NCs at 743 nm leads to QD size-dependent formation of an intermediate with time constants ranging from 2-13 ps, uncorrelated to the PbS QD valence band potential. However, the rate constant for the delayed formation of the TPn triplet excited state markedly increases with increasing PbS conduction band energy, featuring a parabolic Marcus free energy dependence in the normal region. These observations provide evidence of an indirect triplet sensitization process being inconsistent with a concerted Dexter-like energy transfer process. The collective data are consistent with the generation of an intermediate resulting from hole trapping of the initial PbS excited state by midgap states, followed by formation of the TPn triplet excited state whose rate constant and yield increases with decreasing quantum dot size.

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.

Direct observation of triplet energy transfer from semiconductor nanocrystals.

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.