Wasielewski Research Lab

Enhanced Light Capture using Singlet Fission

Singlet fission (SF) is the spontaneous, spin-allowed decay of a singlet exciton to a pair of triplet excitons. Understanding the mechanism of SF is needed to design materials that harness this efficient method of generating excitons. The Wasielewski group studies how the interplay of singlet, correlated triplet, and charge transfer states drive rapid and efficient SF. By studying this process in model molecular dimers and trimers, we are able to tune the function of the charge transfer state(s) in SF (figure below), which also impacts the role of vibronic coupling. We also explore SF in designer molecular systems spanning the solution and multicrystalline solid states.

The Waz group studies:

• New, stable chromophores that undergo fast and efficient SF

• The effects of intermolecular geometry (e.g., slip stacking and core twists) on SF

• The role of intra- and intermolecular vibrations in driving SF

Representative Publications:

Singlet Fission in Terrylenediimide Single Crystals: Competition between Biexciton Annihilation and Free Triplet Exciton Formation. J. Phys. Chem. C 2021, 125(25), 13946-13953

Influence of Vibronic Coupling on Ultrafast Singlet Fission in a Linear Terrylenediimide Dimer. J. Am. Chem. Soc. 2021, 143(4), 2049–2058

Quintet-triplet mixing determines the fate of the multiexciton state produced by singlet fission in a terrylenediimide dimer at room temperature. Proc. Natl. Acad. Sci. U. S. A. 2019, 116, 8178-8183.

Singlet fission in covalent terrylenediimide dimers: Probing the nature of the multiexciton state using femtosecond mid-infrared spectroscopy. J. Am. Chem. Soc. 2018, 140, 9184-9192

Enabling singlet fission by controlling intramolecular charge transfer in π-stacked covalent terrylenediimide dimers. Nat. Chem. 2016, 8, 1120-1125.

Symmetry-breaking Charge Separation

Symmetry-breaking charge separation (SBCS) is important in natural and artificial light-harvesting. Because SBCS forms separated electron and hole products with minimal driving force, SBCS is very interesting for solid-state photovoltaic applications. However, SBCS is most often observed in polar solutions and unexpected in nonpolar systems. Furthermore, this striking phenomenon has been explained in terms of a disruption of symmetry in the excited state, likely driven by structural and/or solvent fluctuations, but much work remains to understand the fundamental mechanism(s) of SBCS.

The Waz group studies:

• The mechanism of SBCS and the function of intra- and inter-chromophoric interactions (e.g., Coulombic, charge-transfer, and vibronic couplings)

• New molecular systems that undergo ultrafast SBCS, especially in low dielectric environments.

Representative Publications:

Symmetry-Breaking Charge Separation in the Solid State: Tetra(Phenoxy)Perylenediimide Polycrystalline Films. J. Am. Chem. Soc. 2020, 142 (42), 18243–18250

Solvent Independent Symmetry-Breaking Charge Separation in Terrylenediimide Guanine-Quadruplex Nanoparticles. J. Chem. Phys. 2020, 153(20), 204302

Reversible symmetry-breaking charge separation in a series of perylenediimide cyclophanes. J. Phys. Chem. C 2020, 124, 10408-10419

Symmetry-breaking charge separation in a nanoscale terrylenediimide guanine-quadruplex assembly. J. Am. Chem. Soc. 2019, 141, 17512-17516

Coherence Phenomena in Light Capture and Charge Separation

Quantum coherence is suspected to influence numerous important photochemical processes, including energy and charge transfer, singlet fission, and phase transformations. Previous studies theorize that coherence may boost the efficiency, speed, and directionality of ultrafast photochemistry. The Wasielewski group uses advanced spectroscopic techniques to study coherence phenomena in rationally designed, multichromophoric systems in hopes of understanding how we can harness coherence for practical applications.

The Waz group studies:

• The role of vibronic coupling in ultrafast singlet fission (SF) and symmetry-breaking charge separation (SBCS)

• How coherence impacts the rate of electron transfer

• The evolution of photoinduced wavepackets

Representative Publications:

Coupling between Harmonic Vibrations Influences Quantum Beating Signatures in Two-Dimensional Electronic Spectra. J. Phys. Chem. C 2022, 126 (1), 120–131.

Influence of Vibronic Coupling on Ultrafast Singlet Fission in a Linear Terrylenediimide Dimer. J. Am. Chem. Soc. 2021, 143 (4), 2049–2058.

Quantum coherence enhances electron transfer rates to two equivalent electron acceptors. J. Am. Chem. Soc. 2019, 141, 12236-12239.

Quantum coherence in ultrafast photo-driven charge separation. Faraday Discuss. 2019, 216, 319-338

Integrating Charge Storage and Catalysis for Artificial Photosynthesis

Because solar radiation is the largest renewable energy source, the storage of solar energy in chemical bonds via generation of renewable fuel sources such as hydrogen or methanol is a critical research challenge. Furthermore, the use of solar energy to power other energy-demanding redox reactions such as nitrogen fixation and carbon–carbon bond formation is fast becoming a feasible goal as well. The Wasielewski group aims to use synthetic molecular frameworks to trigger and energize catalytic fuel-forming reactions, as well as understand mechanisms for efficient solar-to-chemical conversion. 

The Waz group studies:

• The use of photoexcited radical ions as superoxidants/superreductants to power solar fuels catalysis

• How catalytic processes for solar fuels generation can be made less energetically demanding

• The effects of structural and energetic factors on photoinduced electron transfer

Representative Publications:

Direct observation of the photoreduction products of Mn(NDI-bpy)(CO)3X CO2 reduction catalysts using femtosecond transient IR spectroscopy. J. Phys. Chem. C 2019, 123, 6416-6426.

Phenothiazine radical cation excited states as super-oxidants for energy-demanding reactions. J. Am. Chem. Soc. 2018, 140, 5290-5299.

Near-infrared excitation of the peri-xanthenoxanthene radical cation drives energy-demanding hole transfer reactions. J. Phys. Chem. C 2018, 122, 23364-23370

Electron transfer from photoexcited naphthalene diimide radical anion to electrocatalytically active Re(bpy)(CO)3Cl in a molecular triad. J. Phys. Chem. C 2018, 122, 2608-2617

Photoexcited radical anion super-reductants for solar fuels catalysis. Coord. Chem. Rev. 2018, 361, 98-119.