Kathryn Corp

We demonstrate chemical tuning and laser-driven control of intermolecular H atom abstraction from protic solvent molecules. Using multipulse ultrafast pump-push-probe transient absorption (TA) spectroscopy, we monitor hydrogen abstraction by a functionalized heptazine (Hz) from substituted phenols in condensed-phase hydrogen-bonded complexes. Hz is the monomer unit of the ubiquitous organic polymeric photocatalyst graphitic carbon nitride (g-C3N4). Previously, we reported that the Hz derivative… Show more

We demonstrate chemical tuning and laser-driven control of intermolecular H atom abstraction from protic solvent molecules. Using multipulse ultrafast pump-push-probe transient absorption (TA) spectroscopy, we monitor hydrogen abstraction by a functionalized heptazine (Hz) from substituted phenols in condensed-phase hydrogen-bonded complexes. Hz is the monomer unit of the ubiquitous organic polymeric photocatalyst graphitic carbon nitride (g-C3N4). Previously, we reported that the Hz derivative TAHz can photochemically abstract H atoms from water, in addition to exhibiting photocatalytic activity for H2 evolution matching that of g-C3N4 in aqueous suspensions. In the present work, we combine ultrafast multipulse TA spectroscopy with predictive wave function-based ab initio electronic-structure calculations to explore the role of mixed nπ*/ππ* upper excited states in directing H atom abstraction from hydroxylic compounds. We use a 365 nm laser pulse to photoexcite TAHz to a bright upper excited state, and, after a relaxation period of 6 ps, we use a NIR (1150 nm) pulse to “push” the chromophore from the long-lived S1 state to a higher-lying excited state. When phenol is present, the NIR push induces a persistent decrease in the S1 TA signal magnitude, indicating an impulsively driven change in photochemical branching ratios. In the presence of substituted phenols with electron-donating moieties, the magnitude of ΔΔOD diminishes markedly due to the increased excited-state reactivity of these complexes that accompanies the cathodic shift in phenol oxidation potential. In the latter case, H atom abstraction proceeds unaided by additional energy from the push pulse. These results reveal new insight into branching mechanisms among unreactive locally excited states and reactive intermolecular charge-transfer states. They also suggest molecular design strategies for functionalizing aza-aromatics to drive important photoreactions, such as H atom abstraction from water. Show less

To inform prospective design rules for controlling aza-arene photochemistry, we studied hydrogen-bonded complexes of 2,5,8-tris(4-methoxyphenyl)-1,3,4,6,7,9,9b-heptaazaphenalene (TAHz), a molecular photocatalyst chemically related to graphitic carbon nitride, with a variety of phenol derivatives. We have focused on excited state proton-coupled electron transfer (ES-PCET) reactions of heptazines because the excited state properties governing this process remain conceptually opaque compared to… Show more

To inform prospective design rules for controlling aza-arene photochemistry, we studied hydrogen-bonded complexes of 2,5,8-tris(4-methoxyphenyl)-1,3,4,6,7,9,9b-heptaazaphenalene (TAHz), a molecular photocatalyst chemically related to graphitic carbon nitride, with a variety of phenol derivatives. We have focused on excited state proton-coupled electron transfer (ES-PCET) reactions of heptazines because the excited state properties governing this process remain conceptually opaque compared to proton reduction reactions for these materials. We used ground-state absorption, time-resolved photoluminescence, and ab initio quantum chemical calculations to analyze TAHz reactivity toward a series of six para-substituted phenol derivatives. We determined association constants (KA), excited-state quenching rate constants (kQ), kinetic isotope effects, and transition-state barriers (ΔE⧧). From this data, we provide a generalizable picture of hydrogen bond formation and excited state reactivity of heptazine-based materials with hydrogen-atom donating solvents. These results provide important insights into strategies to tune charge transfer state energies and increase ES-PCET rates. Show less

We demonstrate a completely heavy-atom-free red-to-yellow triplet–triplet annihilation (TTA) photon upconversion system using a thionated squaraine sensitizer, both in fluid solution and in a solid-state composite architecture. Previous works have shown that thionation introduces sulfur nonbonding (n) orbitals that invert the native energy ordering of the ππ*(S1) and nπ*(S2) singlet transitions on the squaraine core, opening a channel for efficient intersystem crossing in the thiosquaraine… Show more

We demonstrate a completely heavy-atom-free red-to-yellow triplet–triplet annihilation (TTA) photon upconversion system using a thionated squaraine sensitizer, both in fluid solution and in a solid-state composite architecture. Previous works have shown that thionation introduces sulfur nonbonding (n) orbitals that invert the native energy ordering of the ππ*(S1) and nπ*(S2) singlet transitions on the squaraine core, opening a channel for efficient intersystem crossing in the thiosquaraine without relying on the heavy-atom effect, as expected based on El-Sayed’s rule. Our thiosquaraine [2-(4-(dibutylamino)phenyl)-4-(4-(dibutyliminio)cyclohexa-2,5-dien-1-ylidene)-3-thioxocyclobut-1-enethiolate] exhibits an intense red absorption band, no measurable room-temperature fluorescence, and a native triplet lifetime on the order of 20 μs. This triplet is readily quenched (kQ = 1.4 × 109 M–1 s–1) upon sensitizing the triplet excited state of rubrene as a model upconversion emitter. We observe a 0.27 eV anti-Stokes shift, with selective 685 nm excitation of the thiosquaraine resulting in upconverted rubrene fluorescence centered at 570 nm. The system shows an upconversion quantum efficiency of ∼1.5% in deaerated toluene solution. This quantum efficiency is defined based on a maximum 50% quantum efficiency for TTA upconversion. This system exhibits upconversion under filtered (650 nm long-pass) simulated solar illumination and an intensity transition from quadratic to linear optical power dependence at ∼150 W/cm2 under 685 nm laser diode illumination. We also apply this thiosquaraine system to demonstrate red-to-yellow photon upconversion in a solid-state polymer composite, a prerequisite for light-harvesting device integration. We present an easily tunable squaraine dye that serves as a promising red-absorbing heavy-atom-free upconversion sensitizer for increased scalability and photostability. Show less

According to Hund’s rule, the lowest triplet state (T1) is lower in energy than the lowest excited singlet state (S1) in closed-shell molecules. The exchange integral lowers the energy of the triplet state and raises the energy of the singlet state of the same orbital character, leading to a positive singlet–triplet energy gap (ΔST). Exceptions are known for biradicals and charge-transfer excited states of large molecules in which the highest occupied molecular orbital (HOMO) and the lowest… Show more

According to Hund’s rule, the lowest triplet state (T1) is lower in energy than the lowest excited singlet state (S1) in closed-shell molecules. The exchange integral lowers the energy of the triplet state and raises the energy of the singlet state of the same orbital character, leading to a positive singlet–triplet energy gap (ΔST). Exceptions are known for biradicals and charge-transfer excited states of large molecules in which the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) are spatially separated, resulting in a small exchange integral. In the present work, we discovered with ADC(2), CC2, EOM-CCSD, and CASPT2 calculations that heptazine (1,3,4,6,7,9,9b-heptaazaphenalene or tri-s-triazine) exhibits an inverted S1/T1 energy gap (ΔST ≈ −0.25 eV). This appears to be the first example of a stable closed-shell organic molecule exhibiting S1/T1 inversion at its equilibrium geometry. The origins of this phenomenon are the nearly pure HOMO–LUMO excitation character of the S1 and T1 states and the lack of spatial overlap of HOMO and LUMO due to a unique structure of these orbitals of heptazine. The S1/T1 inversion is found to be extremely robust, being affected neither by substitution of heptazine nor by oligomerization of heptazine units. Using time-resolved photoluminescence and transient absorption spectroscopy, we investigated the excited-state dynamics of 2,5,8-tris(4-methoxyphenyl)-1,3,4,6,7,9,9b-heptaazaphenalene (TAHz), a chemically stable heptazine derivative, in the presence of external heavy atom sources as well as triplet-quenching oxygen. These spectroscopic data are consistent with TAHz singlet excited state decay in the absence of a low-energy triplet loss channel. The absence of intersystem crossing and an exceptionally low radiative rate result in unusually long S1 lifetimes (of the order of hundreds of nanoseconds in nonaqueous solvents). Show less

To gain mechanistic understanding of heptazine-based photochemistry, we synthesized and studied 2,5,8-tris(4-methoxyphenyl)-1,3,4,6,7,9,9b-heptaazaphenalene (TAHz), a model molecular photocatalyst chemically related to carbon nitride. On the basis of time-resolved photoluminescence (TR-PL) spectroscopy, we kinetically reveal a new feature that emerges in aqueous dispersions of TAHz. Using global target analysis, we spectrally and kinetically resolve the new emission feature to be blue shifted… Show more

To gain mechanistic understanding of heptazine-based photochemistry, we synthesized and studied 2,5,8-tris(4-methoxyphenyl)-1,3,4,6,7,9,9b-heptaazaphenalene (TAHz), a model molecular photocatalyst chemically related to carbon nitride. On the basis of time-resolved photoluminescence (TR-PL) spectroscopy, we kinetically reveal a new feature that emerges in aqueous dispersions of TAHz. Using global target analysis, we spectrally and kinetically resolve the new emission feature to be blue shifted from the steady-state luminescence, and observe a fast decay component exhibiting a kinetic isotope effect (KIE) of 2.9 in H2O versus D2O, not observed in the steady-state PL. From ab initio electronic-structure calculations, we attribute this new PL peak to the fluorescence of an upper excited state of mixed nπ*/ππ* character. In water, the KIE suggests the excited state is quenched by proton-coupled electron transfer, liberating hydroxyl radicals that we detect using terephthalic acid. Our findings are consistent with recent theoretical predictions that heptazine-based photocatalysts can participate in proton-coupled electron transfer with H2O. Show less

All-inorganic cesium lead bromide (CsPbBr3) perovskite quantum dots (QDs) have recently emerged as highly promising solution-processed materials for next-generation light-emitting applications. They combine the advantages of QD and perovskite materials, which makes them an attractive platform for achieving high optical gain with high stability. Here, we report an ultralow lasing threshold (0.39 μJ/cm2) from a hybrid vertical cavity surface emitting laser (VCSEL) structure consisting of a… Show more

All-inorganic cesium lead bromide (CsPbBr3) perovskite quantum dots (QDs) have recently emerged as highly promising solution-processed materials for next-generation light-emitting applications. They combine the advantages of QD and perovskite materials, which makes them an attractive platform for achieving high optical gain with high stability. Here, we report an ultralow lasing threshold (0.39 μJ/cm2) from a hybrid vertical cavity surface emitting laser (VCSEL) structure consisting of a CsPbBr3 QD thin film and two highly reflective distributed Bragg reflectors (DBRs). Temperature dependence of the lasing threshold and long-term stability of the device were also characterized. Notably, the CsPbBr3 QDs provide superior stability and enable stable device operation over 5 h/1.8 × 107 optical pulse excitations under ambient conditions. This work demonstrates the significant potential of CsPbBr3 perovskite QD VCSELs for highly reliable lasers, capable of operating in the short-pulse (femtosecond) and quasi-continuous-wave (nanosecond) regimes. Show less

Enhancement of the thermoelectric properties of bismuth due to size reduction has prompted a large body of studies and recent interest in the nanochemistry of this material. The size control at the nanometer scale is complicated because of bismuth's low melting point and little known surface chemistry. We report herein an efficient synthetic method producing bismuth nanoparticles (Bi NPs) of sizes below 10 nm, and of rhombohedral structure based on transmission electron microscopy (TEM) and… Show more

Enhancement of the thermoelectric properties of bismuth due to size reduction has prompted a large body of studies and recent interest in the nanochemistry of this material. The size control at the nanometer scale is complicated because of bismuth's low melting point and little known surface chemistry. We report herein an efficient synthetic method producing bismuth nanoparticles (Bi NPs) of sizes below 10 nm, and of rhombohedral structure based on transmission electron microscopy (TEM) and wide angle X-ray scattering (WAXS) analysis. The nature and dynamics of the ligands at their surface have been investigated by infrared spectroscopy (IR) and nuclear magnetic resonance (NMR) spectroscopy. We demonstrate that alkynyl ligands, seldom used in nanochemistry, efficiently coordinate at the surface and afford stable Bi NPs that can be easily purified, leading to powders with high bismuth payload which can be redispersed into stable colloidal solutions. Show less

Solar hydrogen generation from water represents a compelling component of a future sustainable energy portfolio. Recently, chemically robust heptazine-based polymers known as graphitic carbon nitrides (g-C3N4) have emerged as promising photocatalysts for hydrogen evolution using visible light while withstanding harsh chemical environments. However, since g-C3N4 electron-transfer dynamics are poorly understood, rational design rules for improving activity remain unclear. Here, we use visible and… Show more

Solar hydrogen generation from water represents a compelling component of a future sustainable energy portfolio. Recently, chemically robust heptazine-based polymers known as graphitic carbon nitrides (g-C3N4) have emerged as promising photocatalysts for hydrogen evolution using visible light while withstanding harsh chemical environments. However, since g-C3N4 electron-transfer dynamics are poorly understood, rational design rules for improving activity remain unclear. Here, we use visible and near-infrared femtosecond transient absorption (TA) spectroscopy to reveal an electron-transfer cascade that correlates with a near-doubling in photocatalytic activity from 2050 to 3810 μmol h–1 g–1 when we infuse a suspension of bulk g-C3N4 with 10% mass loading of chemically exfoliated carbon nitride. TA spectroscopy indicates that exfoliated carbon nitride quenches photogenerated electrons on g-C3N4 at rates approaching the molecular diffusion limit. The TA signal for photogenerated electrons on g-C3N4 decays with a time constant of 1/ke′ = 660 ps in the mixture versus 1/ke = 4.1 ns in g-C3N4 alone. Our TA measurements suggest that the charge generation efficiency in g-C3N4 is greater than 65%. Exfoliated carbon nitride, which liberates only trace hydrogen levels when photoexcited directly, does not appear to independently sustain appreciable long-lived charge generation. Thus, the activity enhancement in the two-component infusion evidently results from a cooperative effect in which charge is generated on g-C3N4, followed by electron transfer to exfoliated carbon nitride containing photocatalytic chain terminations. This correlation between electron transfer and photocatalytic activity provides new insight into structural modifications for controlling charge separation dynamics and activity of carbon-based photocatalysts. Show less