Singlet and Triplet Excitation Management in a Bichromophoric Near-Infrared-Phosphorescent BODIPY-Benzoporphyrin Platinum Complex

Multichromophoric arrays provide one strategy for assembling molecules with intense absorptions across the visible spectrum but are generally focused on systems that efficiently produce and manipulate singlet excitations and therefore are burdened by the restrictions of (a) unidirectional energy transfer and (b) limited tunability of the lowest molecular excited state. In contrast, we present here a multichromophoric array based on four boron dipyrrins (BODIPY) bound to a platinum benzoporphyrin scaffold that exhibits intense panchromatic absorption and efficiently generates triplets. The spectral complementarity of the BODIPY and porphryin units allows the direct observation of fast bidirectional singlet and triplet energy transfer processes (k_(ST)(^1BDP→^1Por) = 7.8 × 10^(11) s^(−1), k_(TT)(^3Por→^3BDP) = 1.0 × 10^(10) s^(−1), k_(TT)(^3BDP→^3Por) = 1.6 × 10^(10) s^(−1)), leading to a long-lived equilibrated [^3BDP][Por]⇔[BDP][^3Por] state. This equilibrated state contains approximately isoenergetic porphyrin and BODIPY triplets and exhibits efficient near-infrared phosphorescence (λ_(em) = 772 nm, Φ = 0.26). Taken together, these studies show that appropriately designed triplet-utilizing arrays may overcome fundamental limitations typically associated with core−shell chromophores by tunable redistribution of energy from the core back onto the antennae

Ultrafast Spectroscopy Reveals Electron-Transfer Cascade That Improves Hydrogen Evolution with Carbon Nitride Photocatalysts

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/<i>k</i>_e′ = 660 ps in the mixture versus 1/<i>k</i>_e = 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

Kinetic Competition between Charge Separation and Triplet Formation in Small-Molecule Photovoltaic Blends

Using transient absorption, time-resolved photoluminescence, and device measurements, we show that fullerene aggregation in small-molecule organic photovoltaic blends correlates with photocurrent enhancement due to kinetically avoided recombination to thermodynamically favored triplet states. We evaluate the electron donor chloroboron subphthalocyanine (SubPc) blended with a C₆₀ fullerene electron acceptor. We show that photocurrent generation nearly doubles for SubPc:C₆₀ blends with a higher C₆₀ ratio (1:2 versus 1:1) and enhanced fullerene aggregation. Our spectroscopic results suggest that aggregation at the higher C₆₀ loading ratio aids in sustaining the free charge population by inhibiting recombination to form SubPc triplets. By also examining blended SubPc:C₆₀ films in which aggregation and charge transfer are disrupted by an inert matrix, we further highlight an additional energy transfer pathway for SubPc triplet formation facilitated by intersystem crossing centered on C₆₀. This energy transfer pathway is kinetically outcompeted by charge transfer in the condensed films employed in devices. Our results provide new insight into the role that aggregation plays in promoting charge separation and photocurrent collection in small-molecule organic photovoltaics. Our findings suggest new avenues for improving device performance by kinetically avoiding recombination to triplet states, despite the presence of multiple thermodynamically accessible pathways for triplet formation in these blended films

Local Hydrogen Bonding Determines Branching Pathways in Intermolecular Heptazine Photochemistry

Heptazine is the molecular core of the widely studied photocatalyst carbon nitride. By analyzing the excited-state intermolecular proton-coupled electron-transfer (PCET) reaction between a heptazine derivative and a hydrogen-atom donor substrate, we are able to spectroscopically identify the resultant heptazinyl reactive radical species on a picosecond time scale. We provide detailed spectroscopic characterization of the tri-anisole heptazine:4-methoxyphenol hydrogen-bonded intermolecular complex (TAHz:MeOPhOH), using femtosecond transient absorption spectroscopy and global analysis, to reveal distinct product absorption signatures at ∼520, 1250, and 1600 nm. We assign these product peaks to the hydrogenated TAHz radical (TAHzH•) based on control experiments utilizing 1,4-dimethoxybenzene (DMB), which initiates electron transfer without concomitant proton transfer, i.e., no excited-state PCET. Additional control experiments with radical quenchers, protonation agents, and UV–vis–NIR spectroelectrochemistry also corroborate our product peak assignments. These spectral assignments allowed us to monitor the influence of the local hydrogen-bonding environment on the resulting evolution of photochemical products from excited-state PCET of heptazines. We observe that the preassociation of heptazine with the substrate in solution is extremely sensitive to the hydrogen-bond-accepting character of the solvent. This sensitivity directly influences which product signatures we detect with time-resolved spectroscopy. The spectral signature of the TAHzH• radical assigned in this work will facilitate future in-depth analysis of heptazine and carbon nitride photochemistry. Our results may also be utilized for designing improved PCET-based photochemical systems that will require precise control over local molecular environments. Examples include applications such as preparative synthesis involving organic photoredox catalysis, on-site solar water purification, as well as photocatalytic water splitting and artificial photosynthesis

Size-Dependent Charge Transfer Yields in Conjugated Polymer/Quantum Dot Blends

We investigate the effect of quantum dot size on photocurrent and photoinduced charge transfer yields in blends of the conjugated polymer, poly­((4,8-bis­(octyloxy)­benzo­(1,2-<i>b</i>:4,5-<i>b</i>′)­dithiophene-2,6-diyl)­(2-((dodecyloxy)­carbonyl)­thieno­(3,4-<i>b</i>)­thiophenediyl)) (PTB1), with PbS nanocrystal quantum dots (QDs). These hybrid solar cells exhibit external quantum efficiencies of over 70% and power conversion efficiencies of up to 2.8%. We use photoinduced absorption (PIA) spectroscopy and device EQE measurements to probe long-lived charge transfer at the polymer/QD interface as a function of QD size. We observe that both the PIA signal associated with charge formation on the polymer, as well as the external quantum efficiency of the hybrid photovoltaic devices decrease in magnitude with increasing quantum dot size, despite the broader absorption spectrum of the larger dots. We interpret these results as evidence that PTB1/PbS blends behave at least partially as bulk heterojunction (BHJ) solar cells, and conclude that the long-lived charge transfer yield is diminished at larger dot sizes as the energy level offset at the polymer/quantum dot interface is changed through decreasing quantum confinement