Photoexcited organic chromophores appended to stable radicals can serve as qubit and/or qudit candidates for quantum information applications. 1,6,7,12-Tetra-(4-tert-butylphenoxy)-perylene-3,4 : 9,10bis(dicarboximide) (tpPDI) linked to a partially deuterated α,γ-bisdiphenylene-β-phenylallyl radical (BDPAd 16 ) was synthesized and characterized by time-resolved optical and electron paramagnetic resonance (EPR) spectroscopies. Photoexcitation of tpPDI-BDPA-d 16 results in ultrafast radical-enhanced intersystem crossing to produce a quartet state (Q) followed by formation of a spin-polarized doublet ground state (D 0 ). Pulse-EPR experiments confirmed the spin multiplicity of Q and yielded coherence times of T m = 2.1 � 0.1 μs and 2.8 � 0.2 μs for Q and D 0 , respectively. BDPA-d 16 eliminates the dominant 1 H hyperfine couplings, resulting in a single narrow line for both the Q and D 0 states, which enhances the spectral resolution needed for good qubit addressability.
Perylenediimides (PDIs) are important molecular building blocks that are being investigated for their applicability in optoelectronic technologies. Covalently linking multiple PDI acceptors at the 2,5,8,11 (headland) positions adjacent to the PDI carbonyl groups is reported to yield higher power conversion efficiencies in photovoltaic cells relative to PDI acceptors linked at the 1,6,7,12 (bay) positions. While the photophysical properties of PDIs linked via the bay positions have been investigated extensively, those linked at the headland positions have received far less attention. We showed previously that symmetry-breaking charge separation (SB-CS) in PDIs hold promise as a strategy for increasing photovoltaic efficiency. Here we use transient absorption and emission spectroscopies to investigate the competition between SB-CS, fluorescence, and internal conversion in three related PDI dimers linked at the headland positions with o-, m-, and p-phenylene moieties: o-PDI 2 , m-PDI 2 , and p-PDI 2 , respectively. It is found that o-PDI 2 supports SB-CS yielding PDI •+ −PDI •− , which is in equilibrium with the o-PDI 2 first excited state in a polar solvent (CH 2 Cl 2 ) while m-PDI 2 and p-PDI 2 exhibit accelerated internal conversion due to the motion of the linker along with subnanosecond intersystem crossing (ISC). Electronic coupling and structural dynamics are shown to play a significant role, with o-PDI 2 being the only member of the series that exhibits significant through-bond interchromophore coupling. The pronounced o-PDI 2 steric congestion prevents the free internal rotation that leads to rapid deactivation of the excited state in the other dimers.
The rational design of photoacids requires accessible predictive models of the electronic effect of functional groups on chemical templates of interest. Here, the effect of substituents on the photoacidity and excited-state proton transfer (PT) pathways of prototype 2-naphthol (2OH) at the symmetric C7 position was investigated through photochemical and computational studies of 7-amino-2-naphthol (7N2OH) and 7-methoxy-2-naphthol (7OMe2OH). Time-resolved emission experiments of 7N2OH revealed that the presence of an electron-withdrawing versus electron-donating group (EWG vs EDG, NH3 + vs NH2) led to a drastic decline in photoacidity: pK a* = 1.1 ± 0.2 vs 9.6 ± 0.2. Time-dependent density functional theory calculations with explicit water molecules confirmed that the excited neutral state (x = NH2) is greatly stabilized by water, with equation-of-motion coupled cluster singles and doubles calculations supporting potential mixing between the La and Lb states. Similar suppression of photoacidity, however, was not observed for 7OMe2OH with EDG OCH3, pK a* = 2.7 ± 0.1. Hammett plots of the ground- and excited-state PT reactions of substituted 7-x-2OH compounds (x = CN, NH3 +, H, CH3, OCH3, OH, and NH2) vs Hammett parameters σp showed breaks in the linearity between the EDG and EWG regions: ρ ∼ 0 vs 1.14 and ρ* ∼ 0 vs 3.86. The divergent acidic behavior most likely arises from different mixing mechanisms of the lowest Lb state with the La and possible Bb states upon substitution of naphthalene in water.
The triplet state energy of bis(3′-aminopentyl)-perylene-(3,4:9,10)bis(dicarboximide) (C 5 PDI) in the solid state is 1.1 eV, so that achieving singlet fission (SF) in crystalline films of C 5 PDI can provide a potential means of delivering triplet excitons to silicon-based solar cells, whose band gap is also 1.1 eV, to enhance their performance by utilizing blue light in the solar spectrum. Here, we use transient absorption spectroscopy and microscopy to assess the effect of solid-state order on SF dynamics by comparing C 5 PDI single crystals and thin polycrystalline films. The X-ray single-crystal structure of C 5 PDI shows that it forms π-stacked dimers, wherein the PDIs are twisted ∼51°relative to one another. Formation of the correlated triplet pair state 1 (T 1 T 1 ) in the C 5 PDI single crystals occurs in τ = 56 ± 4 ps mediated by a mixed state having both excited singlet and charge-transfer character, while in a solvent-vapor-annealed C 5 PDI polycrystalline thin film, 1 (T 1 T 1 ) formation occurs in τ = 169 ± 6 ps. The quantum yield of the 1 (T 1 T 1 ) state formation in each case is nearly unified, yet the free triplet exciton quantum yield in the single crystals is 70%, while that in the annealed polycrystalline film is only 29%. Steady-state and time-resolved photoluminescence measurements indicate that the disorder in the polycrystalline film hinders free triplet excitons via long-lived excimer trap states at sites with suboptimal electronic coupling. The higher free triplet yield in the single crystal also clearly shows that the high degree of molecular order in the crystal enables competition between triplet annihilation and diffusional escape, which is critical for utilizing the triplet excitons to enhance solar cell performance.
Divergent multiple and solvent-dependent ESPT reactions of aminonaphthols
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