Vandana Tiwari

All Publications

• Excitonic energy transfer in red algal Photosystem I reveals an evolutionary bridge between cyanobacteria and plants PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA Cui, M., Liu, Z., Izzo, M., Zhou, J., He, E., Tiwari, V., Lambrev, P. H., Miller, R., Kargul, J., Zheng, F., Jha, A., Duan, H. 2026; 123 (30): e2530661123

  Abstract

  Photosystem I (PSI) converts light into chemical energy with near-unity quantum efficiency, yet its energy-transfer and charge-separation mechanisms remain debated. Evolution has diversified PSI architectures: Cyanobacterial PSI trimers confine red-shifted pigments to the core, whereas plant PSI-Light Harvesting Complex Isupercomplexes incorporate extensive peripheral red and charge-transfer states that reshape trapping. The unicellular red alga Cyanidioschyzon merolae exemplifies functional diversification across distinct evolutionary branches, combining a photosystem II and plant-like monomeric PSI core associated with a varying number of light harvesting antenna subunits, Light Harvesting Complexes from Red Lineage (LHCR). This hybrid organization functionally bridges mechanistic models across different lineages. We applied two-dimensional electronic spectroscopy at ultralow temperatures (8 and 80 K) to disentangle overlapping excitation pathways in C. merolae PSI. Cryogenic measurements suppressed thermal broadening, resolving five dynamical components: subpicosecond equilibration (0.3 to 0.8 ps) across the core-LHCR interface, subsequent population transfer (2.6 to 4 ps) into progressively lower-energy manifolds, and slower feeding (18 to 53 ps) into red pools distributed across both core and antenna. On the longest timescales (hundreds of ps), a persistent ground-state bleach signifies excitons stabilized in terminal sinks. Notably, comparison of 8 K and 80 K spectra reveals that excitations are heterogeneously partitioned among multiple sinks at low disorder, whereas modest thermal activation (kT [Formula: see text] 55 cm-1) promotes selective convergence into core-associated red chlorophylls. Atomistic excitonic modeling with time-nonlocal master equations supports these observations, revealing temperature-dependent energy redistribution. Overall, C. merolae PSI expands the kinetic funnel by distributing trapping sites, enhancing spectral coverage while maintaining high efficiency, which is an important functional diversification during evolution.

  View details for DOI 10.1073/pnas.2530661123

  View details for Web of Science ID 001826780500008

  View details for PubMedID 42479839

• Unraveling Exciton-Carrier Correlations in Orthorhombic Lead Halide Perovskite JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Tiwari, V., Zheng, F., Liu, Z., Nayak, P. K., Thorwart, M., Miller, R., Duan, H., Jha, A. 2025

  Abstract

  Exciton correlations to charge carriers in condensed matter systems represent a rich field of study with significant implications for both fundamental physics and technological applications. Hybrid organic-inorganic perovskites represent an exceptional material platform for such explorations. In contrast to the tetragonal phase at room temperature, the orthorhombic phase of methylammonium lead halide perovskites exhibits a pronounced excitonic absorption, enabling simultaneous generation and temporal tracking of excitons and charge carriers using broadband laser pulses. In this study, we employed two-dimensional electronic spectroscopy (2DES) to examine the temporal evolution and correlations of these photoinduced excitons and carriers at 15 K. The low-temperature conditions enhance spectral resolution and allow for the identification of spectral features associated with exciton-carrier interactions. Notably, the 2DES spectra exhibit prominent exciton-related signals and cross-peaks between excitons and carriers, while the free-carrier diagonal features are absent. This absence is interpreted as arising from many-body effects, specifically excitation-induced dephasing and excitation-induced shifts, mediated by interactions between excitons and a bath of incoherent free carriers. To get insight into exciton-carrier correlations, we utilized fully quantum mechanical dynamics simulations based on the hierarchy equation of motion method. Our simulations incorporated vibrational modes using a linear vibronic coupling model and charge carriers modeled as a Fermion bath. The results demonstrate that low- and high-frequency vibrational modes substantially influence population dynamics, while variations in vibronic coupling strength have minimal impact on coherence lifetimes. Overall, our findings reveal that many-body interactions strongly influence exciton-carrier dynamics in orthorhombic perovskites, contributing to a deeper understanding of exciton-carrier interactions.

  View details for DOI 10.1021/jacs.5c15799

  View details for Web of Science ID 001612287200001

  View details for PubMedID 41217774

• Intermolecular Interactions in Crystals Modulate Intramolecular Excited State Proton Transfer Reactions. The journal of physical chemistry. B Hwang, H., Mackenzie, A., Kochman, M. A., Gallagher-Jones, M., Tiwari, V., Bittmann, S. F., Tellkamp, F., Botchway, S., Kirkland, A. I., Duan, H., Miller, R. J., Jha, A. 2025

  Abstract

  Proton transfer is a fundamental process underlying chemical and biological phenomena, and its dynamics are significantly influenced by the surrounding environment. This paper studies the excited state intramolecular proton transfer (ESIPT) process, which is crucial to the photostability of hydroxyanthraquinone-based pigments through efficient energy dissipation, by investigating how crystalline packing influences photoinduced proton transfer dynamics in single crystals of dihydroxyanthraquinone (DHAQ) constitutional isomers. Comparing the proton transfer dynamics in crystalline and solution phases, we show substantial differences due to the crystalline environment, particularly in the 1,4- and 1,5-DHAQ isomers. These isomers show intermolecular hydrogen bonding within the crystal lattice, resulting in larger excitonic couplings, which significantly alters their reaction pathways compared to their behavior in solution. We show that 1,8-DHAQ, which does not form intermolecular hydrogen bonds in the crystal, shows minimal changes in dynamics between the phases. In contrast, in the case of 1,4-DHAQ, intermolecular interactions within the molecular crystal phase open up an ESIPT relaxation channel, which is not observed in the solution phase. These findings highlight the critical role of crystal packing in modulating proton transfer dynamics and offer insights into how molecular packing can be strategically manipulated to control and optimize reaction pathways in solid-state environments.

  View details for DOI 10.1021/acs.jpcb.5c03855

  View details for PubMedID 40717365

• Capturing Ultrafast Spin Dynamics in Single-Molecule Magnets Using Femtosecond X-ray Emission Spectroscopy JOURNAL OF PHYSICAL CHEMISTRY LETTERS Barlow, K., Phelps, R., Eng, J., Ingle, R. A., Khakhulin, D., Biednov, M., Dutta, S., Jiang, Y., Lima, F. A., Tiwari, V., Milne, C., Katayama, T., Coletta, M., Brechin, E. K., Penfold, T. J., Johansson, J. 2025; 16 (17): 4148-4154

  Abstract

  Achieving ultrafast photomagnetic switching of single-molecule magnets (SMMs) could lead to simultaneous fast and dense data storage devices. To facilitate this, a thorough understanding of the ultrafast dynamics emerging after ultrashort laser pulse excitation is essential. However, the complex nature of these materials means there is a lack of established experimental techniques that can probe the spin dynamics in SMMs. Herein, we perform femtosecond time-resolved Mn K-edge X-ray emission spectroscopy on a Mn(III)-based trinuclear SMM (Mn3) and the model system Mn(acac)3. The spectral changes of Mn(acac)3 are consistent with switching between Jahn-Teller distorted structures expected after photoexcitation. A similar result is observed for Mn3; however, the Kβ signal also reveals insight into the distribution of spin states populated within 100 fs. The importance of using probes across the electromagnetic spectrum to gain a thorough understanding of the dynamics of exchange-coupled complexes is highlighted.

  View details for DOI 10.1021/acs.jpclett.5c00383

  View details for Web of Science ID 001469180800001

  View details for PubMedID 40245360

  View details for PubMedCentralID PMC12051188

• Crystal Lattice-Induced Stress modulates Photoinduced Jahn-Teller Distortion Dynamics ACS PHYSICAL CHEMISTRY AU Tiwari, V., Gallagher-Jones, M., Hwang, H., Duan, H., Kirkland, A. I., Miller, R., Jha, A. 2024

  View details for DOI 10.1021/acsphyschemau.4c00047

  View details for Web of Science ID 001327117700001

• Unraveling Electronic and Vibrational Coherences Following a Charge Transfer Process in a Photosystem II Reaction Center PHOTONICS Zhou, J., Zhang, X., Tiwari, V., Mei, C., Jha, A., Zhang, P., Duan, H. 2024; 11 (6)

  View details for DOI 10.3390/photonics11060519

  View details for Web of Science ID 001257747400001

• Multitype Electronic Interactions in Precursor Solutions of Molecular Doped P3HT Polymer JOURNAL OF PHYSICAL CHEMISTRY B Tiwari, V., Li, X., Li, Z., Jacobs, I. E., Duan, H., Sirringhaus, H., Miller, R., Jha, A. 2024; 128 (13): 3249-3257

  Abstract

  Spin-casting of molecularly doped polymer solution mixtures is one of the commonly used methods to obtain conductive organic semiconductor films. In spin-casted films, electronic interaction between the dopant and polymer is one of the crucial factors that dictates the doping efficiency. Here, we investigate excitonic couplings using ultrafast two-dimensional electronic spectroscopy to examine the different types of electronic interactions in ion pairs of the prototype F4TCNQ-doped P3HT polymer system in a precursor solution mixture for spin-casting. Off-diagonal peaks in the 2D spectra clearly establish the excitonic coupling between P3HT+ and F4TCNQ- ions in solution. The observed excitonic coupling is the direct manifestation of a Coulombic interaction between the ion pair. The excited-state lifetime of F4TCNQ- in ion pairs shows biexponential decay at 30 and 200 fs, which hints toward the presence of a heterogeneous population with different interaction strengths. To examine the nature of these different types of interactions in solution mixtures, we study the system using molecular dynamics simulations on a fully solvated model employing the generalized Amber force field. We retrieve three dominant interaction modes of F4TCNQ anions with P3HT: side chain, π-stack, and slipped stack. To quantify these interactions, we complement our studies with electronic structure calculations, which reveal the excitonic coupling strengths of ∼ 75 cm-1 for side chain, ∼ 150 cm-1 for π-π-stack, and ∼69 cm-1 for slipped stack. These various interaction modes provide information about the key geometries of the seed structures in precursor solution mixtures, which may determine the final structures in spin-casted films. The insights gained from our study may guide new strategies to control and ultimately tune Coulomb interactions in polymer-dopant solutions.

  View details for DOI 10.1021/acs.jpcb.4c00584

  View details for Web of Science ID 001189957500001

  View details for PubMedID 38507573

• Unraveling quantum coherences mediating primary charge transfer processes in photosystem II reaction center SCIENCE ADVANCES Jha, A., Zhang, P., Tiwari, V., Chen, L., Thorwart, M., Miller, R., Duan, H. 2024; 10 (10): eadk1312

  Abstract

  Photosystem II (PSII) reaction center (RC) is a unique complex that is capable of efficiently separating electronic charges across the membrane. The primary energy- and charge-transfer (CT) processes occur on comparable ultrafast timescales, which makes it extremely challenging to understand the fundamental mechanism responsible for the near-unity quantum efficiency of the transfer. Here, we elucidate the role of quantum coherences in the ultrafast energy and CT in the PSII RC by performing two-dimensional (2D) electronic spectroscopy at the cryogenic temperature of 20 kelvin, which captures the distinct underlying quantum coherences. Specifically, we uncover the electronic and vibrational coherences along with their lifetimes during the primary ultrafast processes of energy and CT. We construct an excitonic model that provides evidence for coherent energy and CT at low temperature in the 2D electronic spectra. The principles could provide valuable guidelines for creating artificial photosystems with exploitation of system-bath coupling and control of coherences to optimize the photon conversion efficiency to specific functions.

  View details for DOI 10.1126/sciadv.adk1312

  View details for Web of Science ID 001187009700014

  View details for PubMedID 38446882

  View details for PubMedCentralID PMC10917350

• Quantum coherent energy transport in the Fenna-Matthews-Olson complex at low temperature PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA Duan, H., Jha, A., Chen, L., Tiwari, V., Cogdell, R. J., Ashraf, K., Prokhorenko, V. I., Thorwart, M., Miller, R. 2022; 119 (49): e2212630119

  Abstract

  In the primary step of natural light harvesting, the solar photon energy is captured in a photoexcited electron-hole pair, or an exciton, in chlorophyll. Its conversion to chemical potential occurs in the special pair reaction center, which is reached by downhill ultrafast excited-state energy transport through a network of chromophores. Being inherently quantum, transport could in principle occur via a matter wave, with vast implications for efficiency. How long a matter wave remains coherent is determined by the intensity by which the exciton is disturbed by the noisy biological environment. The stronger this is, the stronger the electronic coupling between chromophores must be to overcome the fluctuations and phase shifts. The current consensus is that under physiological conditions, quantum coherence vanishes on the 10-fs time scale, rendering it irrelevant for the observed picosecond transfer. Yet, at low-enough temperature, quantum coherence should in principle be present. Here, we reveal the onset of longer-lived electronic coherence at extremely low temperatures of ∼20 K. Using two-dimensional electronic spectroscopy, we determine the exciton coherence times in the Fenna-Matthew-Olson complex over an extensive temperature range. At 20 K, coherence persists out to 200 fs (close to the antenna) and marginally up to 500 fs at the reaction center. It decays markedly faster with modest increases in temperature to become irrelevant above 150 K. At low temperature, the fragile electronic coherence can be separated from the robust vibrational coherence, using a rigorous theoretical analysis. We believe that by this generic principle, light harvesting becomes robust against otherwise fragile quantum effects.

  View details for DOI 10.1073/pnas.2107266119

  View details for Web of Science ID 001016362900003

  View details for PubMedID 36442134

  View details for PubMedCentralID PMC9894199

• Two-dimensional confinement for generating thin single crystals for applications in time-resolved electron diffraction and spectroscopy: an intramolecular proton transfer study CHEMICAL COMMUNICATIONS Hwang, H., Tiwari, V., Duan, H., Bittmann, S. F., Tellkamp, F., Jha, A., Miller, R. 2022; 58 (70): 9774-9777

  Abstract

  Thin single organic crystals (≤1 μm) with large area (≥100 × 100 μm2) are desirable to explore photoinduced processes using ultrafast spectroscopy and electron-diffraction. Here, we present a general method based on spatial confinement to grow such crystals using the prototypical proton transfer system, 1,5-dihydroxyanthraquinone, as an example, and provide the protocol for optically characterizing structural dynamics to enable proper assignments using diffraction methods.

  View details for DOI 10.1039/d2cc02468c

  View details for Web of Science ID 000840434500001

  View details for PubMedID 35968881

• Torsionally broken symmetry assists infrared excitation of biomimetic charge-coupled nuclear motions in the electronic ground state CHEMICAL SCIENCE Chatterjee, G., Jha, A., Blanco-Gonzalez, A., Tiwari, V., Manathunga, M., Duan, H., Tellkamp, F., Prokhorenko, V., Ferre, N., Dasgupta, J., Olivucci, M., Miller, R. 2022; 13 (32): 9392-9400

  Abstract

  The concerted interplay between reactive nuclear and electronic motions in molecules actuates chemistry. Here, we demonstrate that out-of-plane torsional deformation and vibrational excitation of stretching motions in the electronic ground state modulate the charge-density distribution in a donor-bridge-acceptor molecule in solution. The vibrationally-induced change, visualised by transient absorption spectroscopy with a mid-infrared pump and a visible probe, is mechanistically resolved by ab initio molecular dynamics simulations. Mapping the potential energy landscape attributes the observed charge-coupled coherent nuclear motions to the population of the initial segment of a double-bond isomerization channel, also seen in biological molecules. Our results illustrate the pivotal role of pre-twisted molecular geometries in enhancing the transfer of vibrational energy to specific molecular modes, prior to thermal redistribution. This motivates the search for synthetic strategies towards achieving potentially new infrared-mediated chemistry.

  View details for DOI 10.1039/d2sc02133a

  View details for Web of Science ID 000837386200001

  View details for PubMedID 36093002

  View details for PubMedCentralID PMC9384489

• Intermolecular vibrations mediate ultrafast singlet fission. Science advances Duan, H. G., Jha, A. n., Li, X. n., Tiwari, V. n., Ye, H. n., Nayak, P. K., Zhu, X. L., Li, Z. n., Martinez, T. J., Thorwart, M. n., Miller, R. J. 2020; 6 (38)

  Abstract

  Singlet fission is a spin-allowed exciton multiplication process in organic semiconductors that converts one spin-singlet exciton to two triplet excitons. It offers the potential to enhance solar energy conversion by circumventing the Shockley-Queisser limit on efficiency. We study the primary steps of singlet fission in a pentacene film by using a combination of TG and 2D electronic spectroscopy complemented by quantum chemical and nonadiabatic dynamics calculations. We show that the coherent vibrational dynamics induces the ultrafast transition from the singlet excited electronic state to the triplet-pair state via a degeneracy of potential energy surfaces, i.e., a multidimensional conical intersection. Significant vibronic coupling of the electronic wave packet to a few key intermolecular rocking modes in the low-frequency region connect the excited singlet and triplet-pair states. Along with high-frequency local vibrations acting as tuning modes, they open a new channel for the ultrafast exciton transfer through the resulting conical intersection.

  View details for DOI 10.1126/sciadv.abb0052

  View details for PubMedID 32948583

• Origin of poor doping efficiency in solution processed organic semiconductors CHEMICAL SCIENCE Jha, A., Duan, H., Tiwari, V., Thorwart, M., Miller, R. 2018; 9 (19): 4468-4476

  Abstract

  Doping is an extremely important process where intentional insertion of impurities in semiconductors controls their electronic properties. In organic semiconductors, one of the convenient, but inefficient, ways of doping is the spin casting of a precursor mixture of components in solution, followed by solvent evaporation. Active control over this process holds the key to significant improvements over current poor doping efficiencies. Yet, an optimized control can only come from a detailed understanding of electronic interactions responsible for the low doping efficiencies. Here, we use two-dimensional nonlinear optical spectroscopy to examine these interactions in the course of the doping process by probing the solution mixture of doped organic semiconductors. A dopant accepts an electron from the semiconductor and the two ions form a duplex of interacting charges known as ion-pair complexes. Well-resolved off-diagonal peaks in the two-dimensional spectra clearly demonstrate the electronic connectivity among the ions in solution. This electronic interaction represents a well resolved electrostatically bound state, as opposed to a random distribution of ions. We developed a theoretical model to recover the experimental data, which reveals an unexpectedly strong electronic coupling of ∼250 cm-1 with an intermolecular distance of ∼4.5 Å between ions in solution, which is approximately the expected distance in processed films. The fact that this relationship persists from solution to the processed film gives direct evidence that Coulomb interactions are retained from the precursor solution to the processed films. This memory effect renders the charge carriers equally bound also in the film and, hence, results in poor doping efficiencies. This new insight will help pave the way towards rational tailoring of the electronic interactions to improve doping efficiencies in processed organic semiconductor thin films.

  View details for DOI 10.1039/c8sc00758f

  View details for Web of Science ID 000432598400011

  View details for PubMedID 29896388

  View details for PubMedCentralID PMC5956981