Temporal probing of excitons in organic semiconductors

Photoinduced charge generation forms the physical basis for energy conversion in organic photovoltaic (OPV) technology. The fundamental initial steps involved are absorption of light by organic semiconductors (generally π-conjugated polymers) to generate photoexcited states (Frenkel excitons) followed by charge transfer and charge separation processes in presence of suitable acceptor. The absorbed photon energy must be utilized completely for achieving maximum device efficiency. However progressive relaxation losses of instantaneously generated high-energy or hot-excited states form major bottleneck for maximum derivable voltage. This efficiency limiting factor has been challenged recently by the role of hot-carriers in efficient generation of charges. Therefore tailoring the dissociation of hot-exciton to be temporally faster than all relaxation processes could minimize the energy loss pathways. Implementation of this concept of hot-carrier photovoltaics demands critical understanding of molecular parameters that circumvent all energy relaxation processes and favor hot-carrier generation. In my dissertation work, I have examined the fate of photo-generated excitons in the context of polymer backbone and morphology, and therefore obtain a fundamental structure-function correlation in organic semiconductors

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.This is the author's peer-reviewed final manuscript, as accepted by the publisher. The published article is copyrighted by the Royal Society of Chemistry and can be found at: http://pubs.rsc.org/en/journals/journalissues/cp

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

Transient Raman snapshots of the twisted intramolecular charge transfer state

Optically triggered twisted intramolecular charge transfer (TICT) states in donor-acceptor chromophores form the molecular basis for designing bio-imaging probes that sense polarity, microviscosity or pH in vivo.[1] However, a lack of predictive understanding of the ‘twist’ localization has limited rational structure-function correlations for TICT-based dyes. Time-resolved absorption and emission spectroscopy traditionally used to address this problem, could never unequivoally identify the twist location.[2] Here, using femtosecond stimulated Raman spectroscopy, we reveal distinct Raman signature of the TICT state for stilbazolium-class of mitochondrial staining dye for the first time.Published versio

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

Abstract 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

Trapping an Elusive Fe(IV)-Superoxo Intermediate Inside a Self-Assembled Nanocage at Room Temperature

Natural metalloenzymes stabilize reactive intermediates through specific metal-substrate interactions in protein confinement. Using the structural blueprint of enzyme pockets it is possible to trap elusive intermediates inside molecular cavities. Here we demonstrate room temperature trapping of a rare yet stable Fe(IV)-superoxo [FeIV(O2)-bTAML] intermediate subsequent to dioxygen binding at the Fe(III) site of a (Et4N)2[FeIII(Cl)(bTAML)] catalyst confined inside the hydrophobic interior of a water-soluble Pd6L412+ nanocage. <br /

Ultrafast Triplet Generation and its Sensitization Drives Efficient Photoisomerization of Tetra-<i>cis</i>-lycopene to All-<i>trans</i>-lycopene

Lycopene biosynthesis in photosynthetic organisms controls the metabolic flux of reaction center carotenoids like β-carotene and lutein through the geometric four-step isomerization of 7,9,9′,7′-tetra<i>-cis</i>-lycopene (prolycopene) to its all-<i>trans</i> form. In plants and cyanobacteria, a redox-controlled flavoenzyme carotenoid isomerase catalyzes the prolycopene isomerization although its functional loss inside the chloroplast can be rescued by light. In order to address the chloroplast-specificity and efficiency of the light-induced isomerization reaction, we need to critically understand the excited state dynamics of prolycopene and the nature of electronic states that lead to the isomerization. Using broadband femtosecond transient absorption spectroscopy, we observe ∼610 fs rise of the long-lived triplet state from the photoexcited S₂ with a quantum yield of ∼0.19. The triplet state eventually triggers the first CC bond isomerization at the symmetric 9 or 9′ position on the tetra-<i>cis</i> backbone to yield the tri-<i>cis</i> product with 15% quantum efficiency. However, direct sensitization of the photoreactive triplet state via <i>meso</i>-tetraphenyl porphyrin sensitizer under steady state illumination leads to an efficient production of all-<i>trans</i>-lycopene with 58% quantum yield. Our work implies that chlorophyll-enriched chloroplasts should form an optimized photoreaction vessel for prolycopene isomerization, and synthetic utilization of such <i>cis</i>-carotenoids can lead to efficient triplet harvesting photon-conversion devices

Photoactive Anthraquinone-Based Host-Guest Assembly for Long-Lived Charge Separation

Porous 2D-covalent organic frameworks (COF) that are assembled axially through weak π-stacking interactions can provide reticular charge transport channels while playing host to kinetically stabilized reactive molecular redox-states. Here we demonstrate the above paradigm by constructing a host-guest supramolecular charge transfer (CT) assembly using photoactive anthraquinone-based crystalline COF as an acceptor while incarcerating electron donor N,N-dimethylaniline (DMA) inside it. Employing femtosecond broadband transient absorption spectroscopy in combination with electron paramagnetic resonance (EPR) studies, we show that the CT occurs rapidly within <110 femtoseconds after photoexcitation, subsequently leading to long-lived charge separation with 13% quantum efficiency at room temperature. Photoinduced EPR signature of the long-lived confined DMA cation radical confirms the disparate regions of charge localization while 1H-13C correlation experiments using solid-state NMR spectroscopy enumerate the packing of the amines inside the host-guest COF assembly. Our work demonstrates the potency of rationally designed charge transport pathways in supramolecular assemblies for efficient charge separation which if optimally tuned should pave the way for COF-based photocatalytic reaction centres. </p