Brian T. Phelan

Unified model for singlet fission within a non-conjugated covalent pentacene dimer

When molecular dimers, crystalline films or molecular aggregates absorb a photon to produce a singlet exciton, spin-allowed singlet fission may produce two triplet excitons that can be used to generate two electron–hole pairs, leading to a predicted ∼50% enhancement in maximum solar cell performance. The singlet fission mechanism is still not well understood. Here we report on the use of time-resolved optical and electron paramagnetic resonance spectroscopy to probe singlet fission in a pentacene dimer linked by a non-conjugated spacer. We observe the key intermediates in the singlet fission process, including the formation and decay of a quintet state that precedes formation of the pentacene triplet excitons. Using these combined data, we develop a single kinetic model that describes the data over seven temporal orders of magnitude both at room and cryogenic temperatures.

Strongly oxidizing perylene-3,4-dicarboximides for use in water oxidation photoelectrochemical cells

Perylene-3,4-dicarboximide (PMI) based chromophores have demonstrated the ability to inject electrons into TiO2 for dye-sensitized solar cell applications and to accept electrons from metal complexes relevant to water oxidation, but they are nearly unexplored for use in photoelectrochemical cells (PECs) for solar fuels generation. A series of related PMIs with high oxidation potentials and carboxylate binding groups was synthesized and investigated for this purpose. Charge injection and recombination dynamics were measured using transient absorption (TA) spectroscopy on the picosecond to second timescales. The dynamics and electron injection yields were correlated with the PMI energetics and structures. Injection began in less than 1 ps for the dye with the best performance and a significant charge-separated state yield remained at long times. Finally, this chromophore was used to oxidize a covalently bound water oxidation precatalyst following electron injection into TiO2 to demonstrate the utility of the dyes for use in PECs.

Solvent-Templated Folding of Perylene Bisimide Macrocycles into Coiled Double-String Ropes with Solvent-Sensitive Optical Signatures

A series of semirigid perylene bisimide (PBI) macrocycles with varied ring size containing two to nine PBI chromophores were synthesized in a one-pot reaction and their photophysical properties characterized by fluorescence, steady-state, and transient absorption spectroscopy as well as femtosecond stimulated Raman spectroscopy. These macrocycles show solvent-dependent conformational equilibria and excited-state properties. In dichloromethane, the macrocycles prevail in wide-stretched conformations and upon photoexcitation exhibit symmetry-breaking charge separation followed by charge recombination to triplet states, which photosensitize singlet oxygen formation. In contrast, in aromatic solvents folding of the macrocycles with a distinct odd-even effect regarding the number of PBI chromophore units was observed in steady-state and time-resolved absorption and fluorescence spectroscopy as well as femtosecond stimulated Raman spectroscopy. These distinctive optical properties are attributable to the folding of the even-membered macrocycles into exciton-vibrational coupled dimer pairs in aromatic solvents. Studies in a variety of aromatic solvents indicate that these solvents embed between PBI dimer pairs and accordingly template the folding of even-membered PBI macrocycles into ropelike folded conformations that give rise to solvent-specific exciton-vibrational couplings in UV-vis absorption spectra. As a consequence of the embedding of solvent molecules in the coiled double-string rope architecture, highly solvent specific intensity ratios are observed for the two lowest-energy exciton-vibrational bands, enabling assignment of the respective solvent simply based on the absorption spectra measured for the tetramer macrocycle.

Excimer Formation and Symmetry-Breaking Charge Transfer in Cofacial Perylene Dimers

The use of multiple chromophores as photosensitizers for catalysts involved in energy-demanding redox reactions is often complicated by electronic interactions between the chromophores. These interchromophore interactions can lead to processes, such as excimer formation and symmetry-breaking charge separation (SB-CS), that compete with efficient electron transfer to or from the catalyst. Here, two dimers of perylene bound either directly or through a xylyl spacer to a xanthene backbone were synthesized to probe the effects of interchromophore electronic coupling on excimer formation and SB-CS using ultrafast transient absorption spectroscopy. Two time constants for excimer formation in the 1-25 ps range were observed in each dimer due to the presence of rotational isomers having different degrees of interchromophore coupling. In highly polar acetonitrile, SB-CS competes with excimer formation in the more weakly coupled isomers followed by charge recombination with τCR = 72-85 ps to yield the excimer. The results of this study of perylene molecular dimers can inform the design of chromophore-catalyst systems for solar fuel production that utilize multiple perylene chromophores.

Picosecond Control of Photogenerated Radical Pair Lifetimes Using a Stable Third Radical

Photoinduced electron transfer reactions in organic donor-acceptor systems leading to long-lived radical ion pairs (RPs) have attracted broad interest for their potential applications in fields as diverse as solar energy conversion and spintronics. We present the photophysics and spin dynamics of an electron donor - electron acceptor - stable radical system consisting of a meta-phenylenediamine (mPD) donor covalently linked to a 4-aminonaphthalene-1,8-dicarboximide (ANI) electron-accepting chromophore as well as an α,γ-bisdiphenylene-β-phenylallyl (BDPA) stable radical. Selective photoexcitation of ANI produces the BDPA-mPD(+•)-ANI(-•) triradical in which the mPD(+•)-ANI(-•) RP spins are strongly exchange coupled. The presence of BDPA is found to greatly increase the RP intersystem crossing rate from the initially photogenerated BDPA-(1)(mPD(+•)-ANI(-•)) to BDPA-(3)(mPD(+•)-ANI(-•)), resulting in accelerated RP recombination via the triplet channel to produce BDPA-mPD-(3*)ANI as compared to a reference molecule lacking the BDPA radical. The RP recombination rates observed are much faster than those previously reported for weakly coupled triradical systems. Time-resolved EPR spectroscopy shows that this process is also associated with strong spin polarization of the stable radical. Overall, these results show that RP intersystem crossing rates can be strongly influenced by stable radicals nearby strongly coupled RP systems, making it possible to use a third spin to control RP lifetimes down to a picosecond time scale.

Crystal-Phase Transitions and Photocatalysis in Supramolecular Scaffolds

The energy landscape of a supramolecular material can include different molecular packing configurations that differ in stability and function. We report here on a thermally driven crystalline order transition in the landscape of supramolecular nanostructures formed by charged chromophore amphiphiles in salt-containing aqueous solutions. An irreversible transition was observed from a metastable to a stable crystal phase within the nanostructures. In the stable crystalline phase, the molecules end up organized in a short scroll morphology at high ionic strengths and as long helical ribbons at lower salt content. This is interpreted as the result of the competition between electrostatic repulsive forces and attractive molecular interactions. Only the stable phase forms charge-transfer excitons upon exposure to visible light as indicated by absorbance and fluorescence features, second-order harmonic generation microscopy, and femtosecond transient absorbance spectroscopy. Interestingly, the supramolecular reconfiguration to the stable crystalline phase nanostructures enhances photosensitization of a proton reduction catalyst for hydrogen production.

Photoinduced electron transfer from rylenediimide radical anions and dianions to Re(bpy)(CO) 3 using red and near-infrared light

Photoinduced electron transfer dynamics are described for a set of dyads comprising rylenediimide anion chromophores and a Re(bpy)(CO)3 metal center.

Quantitative Determination of the Differential Raman Scattering Cross Sections of Glucose by Femtosecond Stimulated Raman Scattering

Femtosecond stimulated Raman spectroscopy (FSRS) is a vibrational spectroscopy technique that has been used in a wide variety of applications: from transient vibrational signature tracking to amplifying weak normal Raman scattering signals. Presented here is an application of FSRS to quantify the differential Raman scattering cross sections (DRSCs) of glucose. In using FSRS to determine the DRSCs of multiple glucose vibrational modes, we demonstrate the applicability of both stimulated Raman loss (SRL) spectroscopy and stimulated Raman gain (SRG) FSRS. Using the two analogous FSRS techniques, SRG and SRL, we determine that the DRSCs of glucose excited at 514.5 nm range from a low of 5.0 ± 1.1 × 10-30 to a high of 8.9 ± 0.9 × 10-30 cm2 molecule-1 sr-1. This work establishes both the compatibility of SRL for measuring DRSCs and values for the DRSC of multiple vibrational modes of glucose.

Fast photo-driven electron spin coherence transfer: The effect of electron-nuclear hyperfine coupling on coherence dephasing

Selective photoexcitation of the donor in an electron donor–acceptor1–acceptor2 (D–A1–A2) molecule, in which D = perylene and both A1 and A2 = naphthalene-1,8:4,5-bis(dicarboximide), results in sub-nanosecond formation of a spin-correlated singlet radical pair 1(D+˙–A1−˙–A2) having a large electron spin–spin exchange interaction, 2J, which precludes its observation by transient EPR spectroscopy. Subsequent selective photoexcitation of A1−˙ rapidly produces 1(D+˙–A1–A2−˙), resulting in a dramatic decrease in 2J, which allows coherent spin evolution to mix the singlet (S) radical pair state 1(D+˙–A1–A2−˙) with the T0 triplet sublevel of 3(D+˙–A1–A2−˙) in an applied magnetic field, where B ≫ 2J. A spin-polarized transient EPR spectrum characteristic of the spin-correlated radical pair D+˙–A1–A2−˙ is then observed. The time delay between the two laser pulses was incremented to measure the rate of decoherence in 1(D+˙–A1−˙–A2) in toluene at 295 K, which was found to be 8.1 × 107 s−1. Deuteration of the perylene donor or the toluene solvent decreases the decoherence rate constant of 1(D+˙–A1−˙–A2) to 4.3 × 107 s−1 and 4.6 × 107 s−1, respectively, while deuteration of both the perylene donor and the toluene solvent reduced the decoherence rate constant by more than half to 3.4 × 107 s−1. The data show that decreasing electron-nuclear hyperfine interactions significantly increases the zero quantum coherence lifetime of the spin-correlated radical pair.

Probing Distance Dependent Charge-Transfer Character in Excimers of Extended Viologen Cyclophanes Using Femtosecond Vibrational Spectroscopy

Facile exciton transport within ordered assemblies of π-stacked chromophores is essential for developing molecular photonic and electronic materials. Excimer states having variable charge transfer (CT) character are frequently implicated as promoting or inhibiting exciton mobility in such systems. However, determining the degree of CT character in excimers as a function of their structure has proven challenging. Herein, we report on a series of cyclophanes in which the interplanar distance between two phenyl-extended viologen (ExV2+) chromophores is varied systematically using a pair of o-, m-, or p-xylylene (o-, m-, or p-Xy) covalent linkers to produce o-ExBox4+ (3.5 Å), m-ExBox4+ (5.6 Å), and p-ExBox4+ (7.0 Å), respectively. The cyclophane structures are characterized using NMR spectroscopy in solution and single-crystal X-ray diffraction in the solid state. Femtosecond transient mid-IR and stimulated Raman spectroscopies show that the CT contribution to the excimer states formed in o-ExBox4+ and m-ExBox4+ depends on the distance between the chromophores within the cyclophanes, while in the weak interaction limit, as represented by p-ExBox4+ (7.0 Å), the lowest excited singlet state of ExV2+ exclusively photo-oxidizes the p-Xy spacer to give the p-Xy+•-ExV+• ion pair. Moreover, the vibrational spectra of the excimer state show that it assumes a geometry that is intermediate between that of the locally excited and CT states, approximately reflecting the degree of CT character.