Picosecond Control of Photogenerated Radical Pair Lifetimes Using a Stable Third Radical

Photoinduced electron transfer reactions in organic donor–acceptor systems leading to long-lived radical ion pairs (RPs) have attracted broad interest for their potential applications in fields as diverse as solar energy conversion and spintronics. We present the photophysics and spin dynamics of an electron donor − electron acceptor − stable radical system consisting of a <i>meta</i>-phenylenediamine (mPD) donor covalently linked to a 4-aminonaphthalene-1,8-dicarboximide (ANI) electron-accepting chromophore as well as an α,γ-bisdiphenylene-β-phenylallyl (BDPA) stable radical. Selective photoexcitation of ANI produces the BDPA–mPD^+•–ANI^–• triradical in which the mPD^+•–ANI^–• RP spins are strongly exchange coupled. The presence of BDPA is found to greatly increase the RP intersystem crossing rate from the initially photogenerated BDPA–¹(mPD^+•–ANI^–•) to BDPA–³(mPD^+•–ANI^–•), resulting in accelerated RP recombination via the triplet channel to produce BDPA–mPD–^3*ANI as compared to a reference molecule lacking the BDPA radical. The RP recombination rates observed are much faster than those previously reported for weakly coupled triradical systems. Time-resolved EPR spectroscopy shows that this process is also associated with strong spin polarization of the stable radical. Overall, these results show that RP intersystem crossing rates can be strongly influenced by stable radicals nearby strongly coupled RP systems, making it possible to use a third spin to control RP lifetimes down to a picosecond time scale

Effect of Perylene Photosensitizer Attachment to [Pd(triphosphine)L]²⁺ on CO₂ Electrocatalysis

Two new covalently linked chromophore–CO₂ reduction catalyst systems were prepared using a perylene chromophore and a bis­[(dicyclohexylphosphino)­ethyl]­phenylphosphinopalladium­(II) catalyst. The primary goal of this study is to probe the influence of photosensitizer attachment on the electrocatalytic performance. The position either para or meta to the phosphorus on the phenyl group of the palladium complex was linked via a 2,5-xylyl group to the 3 position of perylene. The electrocatalytic CO₂ reduction activity of the palladium complex is maintained in the meta-linked system, but is lost in the para-linked system, possibly because of unfavorable interactions of the perylene chromophore with the glassy carbon electrode used. Following selective photoexcitation of the perylene, an enhanced perylene excited-state decay rate was observed in the palladium complexes compared to perylene attached to the free ligands. This decrease is accompanied by formation of the perylene cation radical, showing that electron transfer from perylene to the palladium catalyst occurs. Electron transfer and charge recombination were both found to be faster in the para-linked system than in the meta-linked one, which is attributed to stronger electronic coupling in the former. These results illustrate the need to carefully tune the electronic coupling between a photosensitizer chromophore and the catalyst to promote photodriven electron transfer yet inhibit adverse electronic effects of the chromophore on electrocatalysis

Carrier Diffusion Lengths of over 500 nm in Lead-Free Perovskite CH₃NH₃SnI₃ Films

The dynamics of photoexcited lead-free perovskite films, CH₃NH₃SnI₃, were studied using broadband transient absorption and time-resolved fluorescence spectroscopy. Similar to its lead analogue CH₃NH₃PbI₃, we show that free carrier (electrons and holes) recombination is also the dominant relaxation pathway in CH₃NH₃SnI₃ films. The slow hot carrier relaxation time is 0.5 ps. Long carrier diffusion lengths for electrons (279 ± 88 nm) and holes (193 ± 46 nm) were obtained from fluorescence quenching measurements. We also show that SnF₂ doping in the CH₃NH₃SnI₃ film effectively increases the fluorescence lifetime up to 10 times and gives diffusion lengths exceeding 500 nm. These results suggest that the photophysics of CH₃NH₃SnI₃ perovskite are as favorable as those of CH₃NH₃PbI₃, demonstrating that it is a promising nontoxic lead-free replacement for lead iodide perovskite-based solar cells

Phenothiazine Radical Cation Excited States as Super-oxidants for Energy-Demanding Reactions

We demonstrate that the 10-phenyl-10<i>H</i>-phenothiazine radical cation (PTZ^+•) has a manifold of excited doublet states accessible using visible and near-infrared light that can serve as super-photooxidants with excited-state potentials is excess of +2.1 V vs SCE to power energy demanding oxidation reactions. Photoexcitation of PTZ^+• in CH₃CN with a 517 nm laser pulse populates a D_n electronically excited doublet state that decays first to the unrelaxed lowest electronic excited state, D₁′ (τ < 0.3 ps), followed by relaxation to D₁ (τ = 10.9 ± 0.4 ps), which finally decays to D₀ (τ = 32.3 ± 0.8 ps). D₁′ can also be populated directly using a lower energy 900 nm laser pulse, which results in a longer D₁′→D₁ relaxation time (τ = 19 ± 2 ps). To probe the oxidative power of PTZ^+• photoexcited doublet states, PTZ^+• was covalently linked to each of three hole acceptors, perylene (Per), 9,10-diphenylanthracene (DPA), and 10-phenyl-9-anthracenecarbonitrile (ACN), which have oxidation potentials of 1.04, 1.27, and 1.6 V vs SCE, respectively. In all three cases, photoexcitation wavelength dependent ultrafast hole transfer occurs from D_n, D₁′, or D₁ of PTZ^+• to Per, DPA, and ACN. The ability to take advantage of the additional oxidative power provided by the upper excited doublet states of PTZ^+• will enable applications using this chromophore as a super-oxidant for energy-demanding reactions

Excimer Formation and Symmetry-Breaking Charge Transfer in Cofacial Perylene Dimers

The use of multiple chromophores as photosensitizers for catalysts involved in energy-demanding redox reactions is often complicated by electronic interactions between the chromophores. These interchromophore interactions can lead to processes, such as excimer formation and symmetry-breaking charge separation (SB-CS), that compete with efficient electron transfer to or from the catalyst. Here, two dimers of perylene bound either directly or through a xylyl spacer to a xanthene backbone were synthesized to probe the effects of interchromophore electronic coupling on excimer formation and SB-CS using ultrafast transient absorption spectroscopy. Two time constants for excimer formation in the 1–25 ps range were observed in each dimer due to the presence of rotational isomers having different degrees of interchromophore coupling. In highly polar acetonitrile, SB-CS competes with excimer formation in the more weakly coupled isomers followed by charge recombination with τ_CR = 72–85 ps to yield the excimer. The results of this study of perylene molecular dimers can inform the design of chromophore–catalyst systems for solar fuel production that utilize multiple perylene chromophores

Probing Distance Dependent Charge-Transfer Character in Excimers of Extended Viologen Cyclophanes Using Femtosecond Vibrational Spectroscopy

Facile exciton transport within ordered assemblies of π-stacked chromophores is essential for developing molecular photonic and electronic materials. Excimer states having variable charge transfer (CT) character are frequently implicated as promoting or inhibiting exciton mobility in such systems. However, determining the degree of CT character in excimers as a function of their structure has proven challenging. Herein, we report on a series of cyclophanes in which the interplanar distance between two phenyl-extended viologen (<b>ExV</b>^<b>2+</b>) chromophores is varied systematically using a pair of <i>o</i>-, <i>m</i>-, or <i>p</i>-xylylene (<i><b>o</b></i>-, <i><b>m</b></i>-, or <i><b>p</b></i><b>-Xy</b>) covalent linkers to produce <i><b>o</b></i><b>-ExBox</b>^<b>4+</b> (3.5 Å), <i><b>m</b></i><b>-ExBox</b>^<b>4+</b> (5.6 Å), and <i><b>p</b></i><b>-ExBox</b>^<b>4+</b> (7.0 Å), respectively. The cyclophane structures are characterized using NMR spectroscopy in solution and single-crystal X-ray diffraction in the solid state. Femtosecond transient mid-IR and stimulated Raman spectroscopies show that the CT contribution to the excimer states formed in <i><b>o</b></i><b>-ExBox</b>^<b>4+</b> and <i><b>m</b></i><b>-ExBox</b>^<b>4+</b> depends on the distance between the chromophores within the cyclophanes, while in the weak interaction limit, as represented by <i><b>p</b></i><b>-ExBox</b>^<b>4+</b> (7.0 Å), the lowest excited singlet state of <b>ExV</b>^<b>2+</b> exclusively photo-oxidizes the <i><b>p</b></i><b>-Xy</b> spacer to give the <i><b>p</b></i><b>-Xy</b>^<b>+•</b>-<b>ExV</b>^<b>+•</b> ion pair. Moreover, the vibrational spectra of the excimer state show that it assumes a geometry that is intermediate between that of the locally excited and CT states, approximately reflecting the degree of CT character

Quantitative Determination of the Differential Raman Scattering Cross Sections of Glucose by Femtosecond Stimulated Raman Scattering

Femtosecond stimulated Raman spectroscopy (FSRS) is a vibrational spectroscopy technique that has been used in a wide variety of applications: from transient vibrational signature tracking to amplifying weak normal Raman scattering signals. Presented here is an application of FSRS to quantify the differential Raman scattering cross sections (DRSCs) of glucose. In using FSRS to determine the DRSCs of multiple glucose vibrational modes, we demonstrate the applicability of both stimulated Raman loss (SRL) spectroscopy and stimulated Raman gain (SRG) FSRS. Using the two analogous FSRS techniques, SRG and SRL, we determine that the DRSCs of glucose excited at 514.5 nm range from a low of 5.0 ± 1.1 × 10^–30 to a high of 8.9 ± 0.9 × 10^–30 cm² molecule^–1 sr^–1. This work establishes both the compatibility of SRL for measuring DRSCs and values for the DRSC of multiple vibrational modes of glucose

Solvent-Templated Folding of Perylene Bisimide Macrocycles into Coiled Double-String Ropes with Solvent-Sensitive Optical Signatures

A series of semirigid perylene bisimide (PBI) macrocycles with varied ring size containing two to nine PBI chromophores were synthesized in a one-pot reaction and their photophysical properties characterized by fluorescence, steady-state, and transient absorption spectroscopy as well as femtosecond stimulated Raman spectroscopy. These macrocycles show solvent-dependent conformational equilibria and excited-state properties. In dichloromethane, the macrocycles prevail in wide-stretched conformations and upon photoexcitation exhibit symmetry-breaking charge separation followed by charge recombination to triplet states, which photosensitize singlet oxygen formation. In contrast, in aromatic solvents folding of the macrocycles with a distinct odd–even effect regarding the number of PBI chromophore units was observed in steady-state and time-resolved absorption and fluorescence spectroscopy as well as femtosecond stimulated Raman spectroscopy. These distinctive optical properties are attributable to the folding of the even-membered macrocycles into exciton-vibrational coupled dimer pairs in aromatic solvents. Studies in a variety of aromatic solvents indicate that these solvents embed between PBI dimer pairs and accordingly template the folding of even-membered PBI macrocycles into ropelike folded conformations that give rise to solvent-specific exciton-vibrational couplings in UV–vis absorption spectra. As a consequence of the embedding of solvent molecules in the coiled double-string rope architecture, highly solvent specific intensity ratios are observed for the two lowest-energy exciton-vibrational bands, enabling assignment of the respective solvent simply based on the absorption spectra measured for the tetramer macrocycle

Harnessing Intermolecular Interactions to Promote Long-Lived Photoinduced Charge Separation from Copper Phenanthroline Chromophores

Facilitating photoinduced electron transfer (PET) while minimizing rapid charge-recombination processes to produce a long-lived charge-separated (CS) state represents a primary challenge associated with achieving efficient solar fuel production. Natural photosynthetic systems employ intermolecular interactions to arrange the electron-transfer relay in reaction centers and promote a directional flow of electrons. This work explores a similar tactic through the synthesis and ground- and excited-state characterization of two Cu(I)bis(phenanthroline) chromophores with homoleptic and heteroleptic coordination geometries and which are functionalized with negatively charged sulfonate groups. The addition of sulfonate groups enables solubility in pure water, and it also induces assembly with the dicationic electron acceptor methyl viologen (MV2+) via bimolecular, dynamic electrostatic interactions. The effect of the sulfonate groups on the ground- and excited-state properties was evaluated by comparison with the unsulfonated analogues in 1:1 acetonitrile/water. The excited-state lifetimes for all sulfonated complexes are similar to what we expect from previous literature, with the exception of the sulfonated heteroleptic complex whose metal-to-ligand charge-transfer (MLCT) lifetime in water has two components that are fit to 10 and 77 ns. For the sulfonated complexes, we detected reduced MV+• in both solvent environments following MLCT excitation, but control measurements in 1:1 acetonitrile/water with the unsulfonated analogues showed no PET to MV2+, indicating that electrostatically driven supramolecular assemblies of the sulfonated complexes with MV2+ facilitate the observed PET. Additionally, the strength of the intermolecular interactions driving the formation of these assemblies changes drastically with the solvent environment. In 1:1 acetonitrile/water, PET occurred from both sulfonated complexes with quantum yields (ΦET) of 2–3% but increased to a remarkable 98% for the sulfonated heteroleptic complex with a 3 μs CS-state lifetime in water

Spin-Selective Photoreduction of a Stable Radical within a Covalent Donor–Acceptor–Radical Triad

Controlling spin–spin interactions in multispin molecular assemblies is important for developing new approaches to quantum information processing. In this work, a covalent electron donor–acceptor–radical triad is used to probe spin-selective reduction of the stable radical to its diamagnetic anion. The molecule consists of a perylene electron donor chromophore (D) bound to a pyromellitimide acceptor (A), which is, in turn, linked to a stable α,γ-bisdiphenylene-β-phenylallyl radical (R^•) to produce D-A-R^•. Selective photoexcitation of D within D-A-R^• results in ultrafast electron transfer to form the D^+•-A^–•-R^• triradical, where D^+•-A^–• is a singlet spin-correlated radical pair (SCRP), in which both SCRP spins are uncorrelated relative to the R^• spin. Subsequent ultrafast electron transfer within the triradical forms D^+•-A-R^–, but its yield is controlled by spin statistics of the uncorrelated A^–•-R^• radical pair, where the initial charge separation yields a 3:1 statistical mixture of D^+•-³(A^–•-R^•) and D^+•-¹(A^–•-R^•), and subsequent reduction of R^• only occurs in D^+•-¹(A^–•-R^•). These findings inform the design of multispin systems to transfer spin coherence between molecules targeting quantum information processing using the agency of SCRPs