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1. The hole in the bucky: structure-property mapping of closed- vs. open-cage fullerene solar-cell blends via temperature/composition phase diagrams

   https://doi.org/10.1039/D1TC03082E  

   Stingelin, Natalie; Matrone, Giovanni Maria; Gutierrez-Meza, Elizabeth; Balzer, Alex H.; Khirbat, Aditi; Levitski, Artem; Sieval, Alexander B.; Frey, Gitti L.; Richter, Lee; Silva, Carlos (January 2021, Journal of Materials Chemistry C)

   null (Ed.)

   The morphology development of polymer-based blends, such as those used in organic photovoltaic (OPV) systems, typically arrests in a state away from equilibrium – how far from equilibrium this is will depend on the materials chemistry and the selected assembly parameters/environment. As a consequence, small changes during the blend assembly alters the solid-structure development from solution and, in turn, the final device performance. Comparing an open-cage ketolactam fullerene with the prototypical [6,6]-phenyl-C₆₁-butyric acid methyl ester in blends with poly[2,5-bis(3-hexadecylthiophen-2-yl)thieno[3,2-b]thiophene (PBTTT), we demonstrate that experimentally established, non-equilibrium temperature/composition phase diagrams can be useful beyond rationalization of optimum blend composition for OPV device performance. Indeed, they can be exploited as tools for rapid, qualitative structure-property mapping, providing insights into why apparent similar donor:acceptor blends display different optoelectronic processes resulting from changes in the phase-morphology formation induced by the different chemistries of the fullerenes. 

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2. State-specific solvation for restricted active space spin–flip (RAS-SF) wave functions based on the polarizable continuum formalism

   https://doi.org/10.1063/5.0091636  

   Alam, Bushra; Jiang, Hanjie; Zimmerman, Paul M.; Herbert, John M. (May 2022, The Journal of Chemical Physics)

   The restricted active space spin–flip (RAS-SF) formalism is a particular form of single-reference configuration interaction that can describe some forms of strong correlation at a relatively low cost and which has recently been formulated for the description of charge-transfer excited states. Here, we introduce both equilibrium and nonequilibrium versions of a state-specific solvation correction for vertical transition energies computed using RAS-SF wave functions, based on the framework of a polarizable continuum model (PCM). Ground-state polarization is described using the solvent’s static dielectric constant and in the nonequilibrium solvation approach that polarization is modified upon vertical excitation using the solvent’s optical dielectric constant. Benchmark calculations are reported for well-studied models of photo-induced charge transfer, including naphthalene dimer, C 2 H 4 ⋯C 2 F 4 , pentacene dimer, and perylene diimide (PDI) dimer, several of which are important in organic photovoltaic applications. For the PDI dimer, we demonstrate that the charge-transfer character of the excited states is enhanced in the presence of a low-dielectric medium (static dielectric constant ɛ 0 = 3) as compared to a gas-phase calculation ( ɛ 0 = 1). This stabilizes mechanistic traps for singlet fission and helps to explain experimental singlet fission rates. We also examine the effects of nonequilibrium solvation on charge-separated states in an intramolecular singlet fission chromophore, where we demonstrate that the energetic ordering of the states changes as a function of solvent polarity. The RAS-SF + PCM methodology that is reported here provides a framework to study charge-separated states in solution and in photovoltaic materials. 

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3. Development of Interface-Specific Two-Dimensional Vibrational–Electronic (i2D-VE) Spectroscopy for Vibronic Couplings at Interfaces

   https://doi.org/10.3390/spectroscj3010001  

   Qian, Yuqin; Huang-Fu, Zhi-Chao; Brown, Jesse B; Rao, Yi (March 2025, Spectroscopy Journal)

   Bulk 2D electronic–vibrational (2D-EV) and 2D vibrational–electronic spectroscopies (2D-VE) were previously developed to correlate the electronic and vibrational degrees of freedom simultaneously, which allow for the study of couplings between electronic and vibrational transitions in photo-chemical systems. Such bulk-dominated methods have been used to extensively study molecular systems, providing unique information such as coherence sensitivity, molecular configurations, enhanced resolution, and correlated states and their dynamics. However, the analogy of interfacial 2D spectroscopy has fallen behind. Our recent work presented interface-specific 2D-EV spectroscopy (i2D-EV). In this work, we develop interface-specific two-dimensional vibrational–electronic spectroscopy (i2D-VE). The fourth-order spectroscopy is based on a Mach–Zehnder IR interferometer that accurately controls the time delay of an IR pump pulse pair for vibrational transitions, followed by broadband interface second-harmonic generation to probe electronic transitions. We demonstrate step-by-step how a fourth-order i2D-VE spectrum of AP3 molecules at the air/water interface was collected and analyzed. The line shape and signatures of i2D-VE peaks reveal solvent correlations and the spectral nature of vibronic couplings. Together, i2D-VE and i2D-EV spectroscopy provide coupling of different behaviors of the vibrational ground state or excited states with electronic states of molecules at interfaces and surfaces. The methodology presented here could also probe dynamic couplings of electronic and vibrational motions at interfaces and surfaces, extending the usefulness of the rich data that are obtained. 

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4. Charge transfer dynamics and interlayer exciton formation in MoS2/VOPc mixed dimensional heterojunction

   https://doi.org/10.1063/5.0107791  

   Schwinn, Madison C.; Rafiq, Shahnawaz; Lee, Changmin; Bland, Matthew P.; Song, Thomas W.; Sangwan, Vinod K.; Hersam, Mark C.; Chen, Lin X. (November 2022, The Journal of Chemical Physics)

   Mixed-dimensional van der Waals heterojunctions involve interfacing materials with different dimensionalities, such as a 2D transition metal dichalcogenide and a 0D organic semiconductor. These heterojunctions have shown unique interfacial properties not found in either individual component. Here, we use femtosecond transient absorption to reveal photoinduced charge transfer and interlayer exciton formation in a mixed-dimensional type-II heterojunction between monolayer MoS2 and vanadyl phthalocyanine (VOPc). Selective excitation of the MoS2 exciton leads to hole transfer from the MoS2 valence band to VOPc highest occupied molecular orbit in ∼710 fs. On the contrary, selective photoexcitation of the VOPc layer leads to instantaneous electron transfer from its excited state to the conduction band of MoS2 in less than 100 fs. This light-initiated ultrafast separation of electrons and holes across the heterojunction interface leads to the formation of an interlayer exciton. These interlayer excitons formed across the interface lead to longer-lived charge-separated states of up to 2.5 ns, longer than in each individual layer of this heterojunction. Thus, the longer charge-separated state along with ultrafast charge transfer times provide promising results for photovoltaic and optoelectronic device applications. 

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5. Ultrafast Dynamics and Photoresponse of a Fungi‐Derived Pigment Xylindein from Solution to Thin Films

   https://doi.org/10.1002/chem.202005155  

   Krueger, Taylor D.; Giesbers, Gregory; Van Court, Ray C.; Zhu, Liangdong; Kim, Ryan; Beaudry, Christopher M.; Robinson, Seri C.; Ostroverkhova, Oksana; Fang, Chong (March 2021, Chemistry – A European Journal)

   Abstract Organic semiconductor materials have recently gained momentum due to their non‐toxicity, low cost, and sustainability. Xylindein is a remarkably photostable pigment secreted by fungi that grow on decaying wood, and its relatively strong electronic performance is enabled by π–π stacking and hydrogen‐bonding network that promote charge transport. Herein, femtosecond transient absorption spectroscopy with a near‐IR probe was used to unveil a rapid excited‐state intramolecular proton transfer reaction. Conformational motions potentially lead to a conical intersection that quenches fluorescence in the monomeric state. In concentrated solutions, nascent aggregates exhibit a faster excited state lifetime due to excimer formation, confirmed by the excimer→charge‐transfer excited‐state absorption band of the xylindein thin film, thus limiting its optoelectronic performance. Therefore, extending the xylindein sidechains with branched alkyl groups may hinder the excimer formation and improve optoelectronic properties of naturally derived materials. 

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