Photocatalytic Terminal C-C Coupling Reaction Inside Water Soluble Nanocages.

Developing methods that activate C-H bonds directly with high selectivity for C-C bond formation in complex organic synthesis has been a major chemistry challenge. Recently it has been shown that photoactivation of weakly polarized C-H bonds can be carried out inside a cationic water-soluble nanocage with visible light-mediated host-guest charge transfer (CT) chemistry. Using this novel photoredox activation paradigm, here we demonstrate C-C bond formation to photo-generate 1,3-diynes at room temperature in water from terminal aromatic alkynes for the first time. The formation of cavity-confined alkyne radical cation and the proton-removed neutral radical species highlight the unique C-C coupling step driven by supramolecular preorganization.

Excited state dynamics of a spontaneously generated TTF radical cation inside a water-soluble nanocage.

Supramolecular cavities have been traditionally used to stabilize reactive redox intermediates. Recently with the success of multiple new photoredox catalytic strategies that use supramolecular cages, there is a growing demand for photogeneration strategies of diverse reactive intermediates inside confined spaces, which will drive enzyme-like catalysis in real time. Here we report the excited state dynamics of a redox-active TTF radical cation and its corresponding dimethyl-derivative DiMeTTF inside a confined supramolecular cavity. We prepare the radical cation by spontaneous oxidation of neutral TTF upon incarceration inside a water-soluble nanocage Pd6L412+, and characterize it with a combination of resonance Raman and electron paramagnetic resonance spectroscopy. Using broadband transient absorption spectroscopy, we demonstrate that the confined native TTF radical cation and its dimethyl derivative upon photoexcitation rapidly de-excite to form the hot ground state, thereby inhibiting further oxidation to a TTF+2 dication. We discuss our results in the context of excited state crossings of the radical cation potentials as well as modifying the cage energetics to generate a stable dication. Our work has important implications for the usage of such radical cations for photoactivated catalysis.

Visible-Light-Driven Photocatalytic CO2 Reduction to CO/CH4 Using a Metal-Organic "Soft" Coordination Polymer Gel.

The self-assembly of a well-defined and astutely designed, low-molecular weight gelator (LMWG) based linker with a suitable metal ion is a promising method for preparing photocatalytically active coordination polymer gels. Here, we report the design, synthesis, and gelation behaviour of a tetrapodal LMWG based on a porphyrin core connected to four terpyridine units (TPY-POR) through amide linkages. The self-assembly of TPY-POR LMWG with RuII ions results in a Ru-TPY-POR coordination polymer gel (CPG), with a nanoscroll morphology. Ru-TPY-POR CPG exhibits efficient CO2 photoreduction to CO (3.5 mmol g-1 h-1 ) with >99 % selectivity in the presence of triethylamine (TEA) as a sacrificial electron donor. Interestingly, in the presence of 1-benzyl-1,4-dihydronicotinamide (BNAH) with TEA as the sacrificial electron donor, the 8e- /8H+ photoreduction of CO2 to CH4 is realized with >95 % selectivity (6.7 mmol g-1 h-1 ). In CPG, porphyrin acts as a photosensitizer and covalently attached [Ru(TPY)2 ]2+ acts as a catalytic center as demonstrated by femtosecond transient absorption (TA) spectroscopy. Further, combining information from the in situ DRIFT spectroscopy and DFT calculation, a possible reaction mechanism for CO2 reduction to CO and CH4 was outlined.

Photochemistry using a host-guest charge transfer paradigm: DMABN as a dynamical probe of ground and excited states.

Photoexciting charge transfer (CT) transitions arising from host-guest interactions in a confined environment can efficiently yield kinetically trapped radicals. In order to predispose these photogenerated radicals for diffusion limited reactions it becomes imperative to understand the nature of the host-guest CT interactions in the ground and excited states. Here we probe the heterogeneity of guest orientations and the ensuing excited state charge transfer dynamics of an electron-rich molecular probe N,N-dimethylaminobenzonitrile (DMABN) incarcerated inside an electron deficient water-soluble cationic Pd6L412+ nanohost. Using a combination of 1H-NMR, resonance Raman spectrosocopy, and pump-probe spectroscopy we highlight the necessary challenges that need to be addressed in order to use molecular cages as photocatalytic reaction vessels.

Conformational switching via an intramolecular H-bond modulates the fluorescence lifetime in a novel coumarin-imidazole conjugate.

Achieving synthetic control over light-driven molecular dynamics is essential for designing complex molecule-based devices. Here we design a novel coumarin-imidazole conjugate (1) whose excited state structural dynamics are primarily controlled by a distant intramolecular H-bonding interaction within the backbone. The coumarin conjugate is based on a 1,2,4,5-aryl substituted imidazole framework (aryl = -Ph and -PhOH) covalently connected to the coumarin moiety via a C-N bond. A carefully positioned OH group in the aryl part of the imidazole fragment resulted in achieving two dissimilar O-HN and O-HO distal intramolecular hydrogen bonding interactions. NMR studies in conjunction with density functional theory (DFT) at the B3LYP/6-311G(d,p) level of theory show the existence of two ground state conformers with a rotational barrier of 6.12 kcal mol-1. Due to the presence of conformational isomers of 1, the local excited state dynamics of the parent coumarin get biased towards a long-lived fluorescence state with diminished non-radiative decay channels. Time-resolved emission studies show an ∼4-5 times increase in the excited state lifetime in 1 when compared to coumarin-imidazole conjugates, 2 and 3, without the OH group. Solvent dependent studies show that solvent polarity, the H-bond donating ability and viscosity dictate the conformational distribution in the ground state and the dynamical evolution to the final emissive state. Our studies highlight the importance of rotamerism around the C1-C4 single bond, which leads to rigidification along the coumarin-imidazole backbone through a combination of distal H-bonding and solvent interactions. The concept of new emission signaling pathways caused by conformational switching between two states offers a new paradigm to introduce functional allostery in macromolecular backbones.

Facile Synthesis and Chain-Length Dependence of the Optical and Structural Properties of Diketopyrrolopyrrole-Based Oligomers.

Here, we report the synthesis, optical properties, and solid-state packing of monodisperse oligomers of diketopyrrolopyrrole (DPP) up to five repeating units. The optical properties of DPP oligomers in solution and the solid state were investigated by a combination of steady-state and transient spectroscopy. Transient absorption spectroscopy and time-correlated single photon counting (TCSPC) measurements show that the fluorescence lifetime decreases with an increase in the oligomer size from monomer to trimer, thereby reaching saturation for pentameric DPP oligomers. The solid-state packing and crystallinity were probed by using advanced techniques, which included grazing incidence small-angle X-ray scattering (GISAXS) and X-ray diffraction (XRD) to elucidate the structure-property trend. Collectively, our chain-length dependent studies establish the fundamental correlation between the structure and property and provide a comprehensive understanding of the solid-state properties in DPP-DPP based conjugated systems.

Non-Bonded Interactions Drive the Sub-Picosecond Bilin Photoisomerization in the P(fr) State of Phytochrome Cph1.

Phytochromes are protein-based photoreceptors harboring a bilin-based photoswitch in the active site. The timescale of photosignaling via C15 =C16 E-to-Z photoisomerization has been ambiguous in the far-red-absorbing Pfr state. Here we present a unified view of the structural events in phytochrome Cph1 post excitation with femtosecond precision, obtained via stimulated Raman and polarization-resolved transient IR spectroscopy. We demonstrate that photoproduct formation occurs within 700 fs, determined by a two-step partitioning process initiated by a planarization on the electronic excited state with a 300 fs time scale. The ultrafast isomerization timescale for Pfr -to-Pr conversion highlights the active role of the nonbonding methyl-methyl clash initiating the reaction in the excited state. We envision that our results will motivate the synthesis of new artificial photoswitches with precisely tuned non-bonded interactions for ultrafast response.

Light-induced proton-coupled electron transfer inside a nanocage.

Triggering proton-coupled electron-transfer (PCET) reactions with light in a nanoconfined host environment would bring about temporal control on the reactive pathways via kinetic stabilization of intermediates. Using a water-soluble octahedral Pd6L4 molecular cage as a host, we show that optical pumping of host-guest charge transfer (CT) states lead to generation of kinetically stable phenoxyl radical of the incarcerated 4-hydroxy-diphenylamine (1-OH). Femtosecond broadband transient absorption studies reveal that CT excitation initiates the proton movement from the 1-OH radical cation to a solvent water molecule in ~890 fs, faster than the time scale for bulk solvation. Our work illustrates that optical host-guest CT excitations can drive solvent-coupled ultrafast PCET reactions inside nanocages and if optimally tuned should provide a novel paradigm for visible-light photocatalysis.

Tuning light-driven oxidation of styrene inside water-soluble nanocages.

Selective functionalization of innate sp2 C-H bonds under ambient conditions is a grand synthetic challenge in organic chemistry. Here we combine host-guest charge transfer-based photoredox chemistry with supramolecular nano-confinement to achieve selective carbonylation of styrene by tuning the dioxygen concentration. We observe exclusive photocatalytic formation of benzaldehyde under excess O2 (>1 atm) while Markovnikov addition of water produced acetophenone in deoxygenated condition upon photoexcitation of confined styrene molecules inside a water-soluble cationic nanocage. Further by careful tuning of the nanocage size, electronics, and guest preorganization, we demonstrate rate enhancement of benzaldehyde formation and a complete switchover to the anti-Markovnikov product, 2-phenylethan-1-ol, in the absence of O2. Raman spectroscopy, 2D 1H-1H NMR correlation experiments, and transient absorption spectroscopy establish that the site-selective control on the confined photoredox chemistry originates from an optimal preorganization of styrene molecules inside the cavity. We envision that the demonstrated host-guest charge transfer photoredox paradigm in combination with green atom-transfer reagents will enable a broad range of sp2 carbon-site functionalization.

Evolutionary origins of the photosynthetic water oxidation cluster: bicarbonate permits Mn(2+) photo-oxidation by anoxygenic bacterial reaction centers.

The enzyme that catalyzes water oxidation in oxygenic photosynthesis contains an inorganic cluster (Mn4 CaO5 ) that is universally conserved in all photosystem II (PSII) protein complexes. Its hypothesized precursor is an anoxygenic photobacterium containing a type 2 reaction center as photo-oxidant (bRC2, iron-quinone type). Here we provide the first experimental evidence that a native bRC2 complex can catalyze the photo-oxidation of Mn(2+) to Mn(3+) , but only in the presence of bicarbonate concentrations that allows the formation of (bRC2)Mn(2+) (bicarbonate)1-2 complexes. Parallel-mode EPR spectroscopy was used to characterize the photoproduct, (bRC2)Mn(3+) (CO3 (2-) ), based on the g tensor and (55) Mn hyperfine splitting. (Bi)carbonate coordination extends the lifetime of the Mn(3+) photoproduct by slowing charge recombination. Prior electrochemical measurements show that carbonate complexation thermodynamically stabilizes the Mn(3+) product by 0.9-1 V relative to water ligands. A model for the origin of the water oxidation catalyst is presented that proposes chemically feasible steps in the evolution of oxygenic PSIIs, and is supported by literature results on the photoassembly of contemporary PSIIs.

Visible light-mediated C (sp3)-H bond functionalization inside an all-organic nanocavity.

Ultrafast C-H bond activation and functionalization in confinement using visible light will enable engineering chemical reactions with extraordinary speed and selectivity. To provide a transition metal-free route, here we demonstrate C-H bond activation reactions on poly-aromatic hydrocarbons (PAH) in all-organic cationic nanocage ExBox4+ for the first time. Visible light excitation in the host-guest charge transfer (CT) state allows the formation of oxidized photoproducts with high selectivity. Mechanistic understanding of this CT-mediated photoreaction using femtosecond broadband transient absorption revealed a few ∼100 ps timescale for C-H bond breaking on the attached -CH3 group via sequential electron transfer and proton transfer steps. We envision that our photosensitizer-free method will open up new avenues to pursue organic reactions using cavities that could serve both as photoredox catalysts and hosts for reactive reaction intermediates.

Raman Snapshots of Side-Chain Dependent Polaron Dynamics in PolyThiophene Films.

Photogenerated polarons in π-conjugated polymers are the precursors to free charges at donor-acceptor interfaces. Unraveling the relationship between film morphology and polaron formation is conjectured to enable efficient charge generation in organic photovoltaic devices. However, it has been challenging to track the ultrafast dynamics of polarons selectively and thus evaluate the molecular coordinates that drive charge generation in films. Using a combination of broadband femtosecond transient absorption and resonance-selective femtosecond stimulated Raman spectroscopy, here, we investigate the polaron generation dynamics exclusively in traditional crystalline poly(3-hexylthiophene) (P3HT) and its amorphous side-chain variant poly(3-(2-ethylhexyl)thiophene-2,5-diyl) (P3EHT) films. The transient Raman data unequivocally provides evidence for an initial delocalization of the polaronic states via thiophene backbone planarization in ∼100 fs while capturing the subsequent morphology-dependent cooling dynamics in a few picoseconds. Our work highlights the structural significance of crystalline morphology in generating hot-charges and thereby emphasizes the importance of side-chain engineering in designing highly efficient conjugated polymer films for hot-carrier photovoltaic devices.

Biomimetic Approach toward Visible Light-Driven Hydrogen Generation Based on a Porphyrin-Based Coordination Polymer Gel.

There has been a widespread interest in developing self-assembled porphyrin nanostructures to mimic nature's light-harvesting processes. Herein, porphyrin-based coordination polymer gel (CPG) has been developed as a "soft" photocatalyst material for hydrogen (H2) production from water under visible light. The CPG offers a hierarchical nanofibrous network structure obtained through self-assembly of a terpyridine alkyl-amide appended porphyrin (TPY-POR)-based low molecular weight gelator with ruthenium ions (RuII) and produces H2 with a rate of 5.7 mmol g-1 h-1 in the presence of triethylamine (TEA) as a sacrificial electron donor. Further, the [Fe2(bdt)(CO)6] (dbt = 1,2-benzenedithiol) cocatalyst, which can mimic the activity of iron hydrogenase, is coassembled in the CPG and shows remarkable improvement in H2 evolution (catalytic activity; rate ∼10.6 mmol g-1 h-1 and turnover number ∼1287). The significant enhancement in catalytic activity was supported by several controlled experiments, including femtosecond transient absorption (TA) spectroscopy and also DFT calculation. The TA study supported the cascade electron transfer process from porphyrin core to [Ru(TPY)2]2+ center, and subsequently, the electron transfers to the cocatalyst [Fe2(bdt)(CO)6] for H2 production.

Conformational homogeneity and excited-state isomerization dynamics of the bilin chromophore in phytochrome Cph1 from resonance Raman intensities.

The ground-state structure and excited-state isomerization dynamics of the P(r) and P(fr) forms of phytochrome Cph1 are investigated using resonance Raman intensity analysis. Electronic absorption and stimulated resonance Raman spectra of P(r) and P(fr) are presented; vibronic analysis of the Raman intensities and absorption spectra reveals that both conformers exist as a single, homogeneous population of molecules in the ground state. The homogeneous and inhomogeneous contributions to the overall electronic broadening are determined, and it is found that the broadening is largely homogeneous in nature, pointing to fast excited-state decay. Franck-Condon displacements derived from the Raman intensity analysis reveal the initial atomic motions in the excited state, including the highly displaced, nontotally symmetric torsional and C(15)-H HOOP modes that appear because of symmetry-reducing distortions about the C(14)-C(15) and C(15)=C(16) bonds. P(fr) is especially well primed for ultrafast isomerization and torsional Franck-Condon analysis predicts a <200 fs P(fr) → P(r) isomerization. This time is significantly faster than the observed 700 fs reaction time, indicating that the P(fr) S(1) surface has a D-ring rotational barrier caused by steric interactions with the protein.

Probing structural evolution along multidimensional reaction coordinates with femtosecond stimulated Raman spectroscopy.

Mapping out multidimensional potential energy surfaces has been a goal of physical chemistry for decades in the quest to both predict and control chemical reactivity. Recently a new spectroscopic approach called Femtosecond Stimulated Raman Spectroscopy or FSRS was introduced that can structurally interrogate multiple dimensions of a reactive potential energy surface. FSRS is an ultrafast laser technique which provides complete time-resolved, background-free Raman spectra in a few laser shots. The FSRS technique provides simultaneous ultrafast time (~50 fs) and spectral (~8 cm(-1)) resolution, thus enabling one to follow reactive structural evolutions as they occur. In this perspective we summarize how FSRS has been used to follow structural dynamics and provide mechanistic detail on three classical chemical reactions: a structural isomerization, an electron transfer reaction, and a proton transfer reaction.

Ultrafast excited-state isomerization in phytochrome revealed by femtosecond stimulated Raman spectroscopy.

Photochemical interconversion between the red-absorbing (P(r)) and the far-red-absorbing (P(fr)) forms of the photosensory protein phytochrome initiates signal transduction in bacteria and higher plants. The P(r)-to-P(fr) transition commences with a rapid Z-to-E photoisomerization at the C(15)=C(16) methine bridge of the bilin prosthetic group. Here, we use femtosecond stimulated Raman spectroscopy to probe the structural changes of the phycocyanobilin chromophore within phytochrome Cph1 on the ultrafast time scale. The enhanced intensity of the C(15)-H hydrogen out-of-plane (HOOP) mode, together with the appearance of red-shifted C=C stretch and N-H in-plane rocking modes within 500 fs, reveal that initial distortion of the C(15)=C(16) bond occurs in the electronically excited I* intermediate. From I*, 85% of the excited population relaxes back to P(r) in 3 ps, whereas the rest goes on to the Lumi-R photoproduct consistent with the 15% photochemical quantum yield. The C(15)-H HOOP and skeletal modes evolve to a Lumi-R-like pattern after 3 ps, thereby indicating that the C(15)=C(16) Z-to-E isomerization occurs on the excited-state surface.

Long-Range Charge Delocalization Mediates the Ultrafast Ligand-to-Metal Charge Transfer Dynamics at the Cu2+-Active Site in Azurin.

The blue color in metalloprotein azurin has traditionally been attributed to the intense cysteine-to-Cu2+ ligand-to-metal charge transfer transition centered at 628 nm. Although resonance Raman measurements of the Cu2+ active site have implied that the LMCT transition electronically couples to the protein scaffold well beyond its primary metal-ligand coordination shell, the structural extent of this electronic coupling and visualization of the protein-mediated charge transfer dynamics have remained elusive. Here, using femtosecond broadband transient absorption and impulsive Raman spectroscopy, we provide direct evidence for a rapid relaxation between two distinct charge transfer states, having different spatial delocalization, within ∼300 fs followed by recombination of charges in subpicosecond time scales. We invoke the formation of a protein-centered radical cation, possibly Trp48 or a Phe residue, within 100 fs substantiating the long-range electronic coupling for the first time beyond the traditional copper active site. The Raman spectra of the excited CT state show the presence of protein-centric vibrations along with the vibrational modes assigned to the copper active site. Our results demonstrate a large delocalization length scale of the initially populated CT state, thereby highlighting the possibility of exploiting azurin photochemistry for energy conversion techniques.

A highly contorted push-pull naphthalenediimide dimer and evidence of intramolecular singlet exciton fission.

Singlet fission is a process by which two molecular triplet excitons are generated subsequent to the absorption of one photon. Molecules that enable singlet fission have triplet state energy at least half of the bright singlet state energy. This stringent energy criteria have challenged chemists to device new molecular and supramolecular design principles to modulate the singlet-triplet energy gap and build singlet fission systems from a wide range of organic chromophores. Herein, we report for the first time intramolecular singlet fission in the seminal naphthalenediimide (NDI) scaffold constrained in a push-pull cyclophane architecture, while individually the NDI chromophore does not satisfy the energy criterion. The challenging synthesis of this highly contorted push-pull cyclophane is possible from the preorganized pincer-like precursor. The special architecture establishes the shortest co-facial NDI⋯NDI contacts (3.084 Å) realized to date. Using broadband femtosecond transient absorption, we find that the correlated T-T pair forms rapidly within 380 fs of photoexcitation. Electronic structure calculations at the level of state-averaged CASSCF (ne,mo)/XMCQDPT2 support the existence of the multi-excitonic T-T pair state, thereby confirming the first example of singlet exciton fission in a NDI scaffold.

Parallel triplet formation pathways in a singlet fission material.

Harvesting long-lived free triplets in high yields by utilizing organic singlet fission materials can be the cornerstone for increasing photovoltaic efficiencies potentially. However, except for polyacenes, which are the most studied systems in the singlet fission field, spin-entangled correlated triplet pairs and free triplets born through singlet fission are relatively poorly characterized. By utilizing transient absorption and photoluminescence spectroscopy in supramolecular aggregate thin films consisting of Hamilton-receptor-substituted diketopyrrolopyrrole derivatives, we show that photoexcitation gives rise to the formation of spin-0 correlated triplet pair 1(TT) from the lower Frenkel exciton state. The existence of 1(TT) is proved through faint Herzberg-Teller emission that is enabled by vibronic coupling and correlated with an artifact-free triplet-state photoinduced absorption in the near-infrared. Surprisingly, transient electron paramagnetic resonance reveals that long-lived triplets are produced through classical intersystem crossing instead of 1(TT) dissociation, with the two pathways in competition. Moreover, comparison of the triplet-formation dynamics in J-like and H-like thin films with the same energetics reveals that spin-orbit coupling mediated intersystem crossing persists in both. However, 1(TT) only forms in the J-like film, pinpointing the huge impact of intermolecular coupling geometry on singlet fission dynamics.

Torsionally broken symmetry assists infrared excitation of biomimetic charge-coupled nuclear motions in the electronic ground state.

The concerted interplay between reactive nuclear and electronic motions in molecules actuates chemistry. Here, we demonstrate that out-of-plane torsional deformation and vibrational excitation of stretching motions in the electronic ground state modulate the charge-density distribution in a donor-bridge-acceptor molecule in solution. The vibrationally-induced change, visualised by transient absorption spectroscopy with a mid-infrared pump and a visible probe, is mechanistically resolved by ab initio molecular dynamics simulations. Mapping the potential energy landscape attributes the observed charge-coupled coherent nuclear motions to the population of the initial segment of a double-bond isomerization channel, also seen in biological molecules. Our results illustrate the pivotal role of pre-twisted molecular geometries in enhancing the transfer of vibrational energy to specific molecular modes, prior to thermal redistribution. This motivates the search for synthetic strategies towards achieving potentially new infrared-mediated chemistry.