arXiv subjects

Arshad Mehmood

Publications and source records attributed to Arshad Mehmood.

5 recordsLinked to original sources

arXiv2026

Vibrations Drive Ultrafast Intersystem Crossing of a Photoexcited Cr(III) Complex

The Cr(III) coordination complex serves as an archetypical 3d transition metal system for probing ultrafast excited-state dynamics with spin conversion due to its intrinsic intersystem crossing (ISC) pathway, $^{4}T_{2g} \rightarrow ^{2}E_{g}$, upon photoexcitation. Here we conduct ab initio molecular dynamics simulations in the $^{4}T_{2g}$ state of a model Cr(III) coordination complex, followed by analyses of multireference electronic structure properties. Across 50 trajectories, the compound retains a persistent Jahn-Teller distortion in the excited state, while exhibiting prominent symmetric metal-ligand bond stretching vibrations with frequencies of 219 cm$^{-1}$ and 465 cm$^{-1}$. State-averaged complete active space self-consistent field (SA-CASSCF) calculations obtain two corresponding normal modes at 225 cm$^{-1}$ and 487 cm$^{-1}$ with symmetric stretching character. The lower-frequency twisting/scissoring mode strongly modulates the $^{4}T_{2g}/^{2}E_{g}$ energy gap, periodically zeroing the energy gap, whereas spin-orbit coupling is essentially invariant to vibrational motion ($\approx$ 60-80 cm$^{-1}$). Furthermore, calculations of single-point excited-state absorption from $^{2}E_{g}$ to a higher ligand-to-metal charge-transfer (LMCT) state indicate that the coherences previously observed in transient absorption spectra arise from nuclear motion on the $^{2}E_{g}$ surface. These results provide insights into the correlation between vibrational motion and electronic transitions, which can facilitate rational molecular design of transition metal complexes with desired excited-state properties by leveraging ligand versatility.

physics.chem-ph↗

arXiv2025

Chemical and Conformational Control of the Spectroscopic Properties of Multi-Layer and Multi-Defect Carbon Dots

Carbon dots (CDs) are renowned for their bright and tunable photoluminescence (PL), stability, and biocompatibility, yet it remains challenging to link their heterogeneous structures to their spectroscopic properties. This study utilizes density functional theory (DFT) and time-dependent DFT (TD-DFT) to systematically investigate how the spectroscopic properties of complex CDs with multiple layers and multiple defects are determined by their structures and compositions. Calculations reveal that strongly oxidizing defects, such as carbonyl and carbonyl acetate, significantly redshift absorption and emission spectra. In contrast, less oxidizing defects, such as hydroxyl, behave as spectators with minimal impact on absorption and emission, except when they interact strongly with more oxidizing defects. We find that not only the excitation energy but also the excitation character itself is impacted by the presence of specific defects, and the pH-dependence of the spectroscopic properties can be attributed to their protonation state-dependent excitation character. We show that the twisting, sliding, and linker-mediated folding of surface-functionalized layers in CDs markedly alter excitation energies and characters, offering a molecular explanation for experimentally observed emission intermittency and polarization fluctuations. These insights provide strategies for optimizing CDs for various applications, including bioimaging, photocatalysis, and optoelectronic devices.

physics.chem-ph↗

arXiv2024

Simulating Ultrafast Transient Absorption Spectra from First Principles using a Time-Dependent Configuration Interaction Probe

Transient absorption spectroscopy (TAS) is among the most common ultrafast photochemical experiments, but its interpretation remains challenging. In this work, we present an efficient and robust method for simulating TAS signals from first principles. Excited-state absorption and stimulated emission (SE) signals are computed using time-dependent complete active space configuration interaction (TD-CASCI) simulations, leveraging the robustness of time-domain simulation to minimize electronic structure failure. We demonstrate our approach by simulating the TAS signal of 1$^\prime$-hydroxy-2$^\prime$-acetonapthone (HAN) from ab initio multiple spawning nonadiabatic molecular dynamics simulations. Our results are compared to gas-phase TAS data recorded from both jet-cooled ($T\sim$ 40 K) and hot ($\sim$ 403 K) molecules via cavity-enhanced transient absorption spectroscopy (CE-TAS). Decomposition of the computed spectrum allows us to assign a rise in the SE signal to excited-state proton transfer and the ultimate decay of the signal to relaxation through a twisted conical intersection. The total cost of computing the observable signal ($\sim$1700 graphics processing unit hours for $\sim$4 ns of electron dynamics) was markedly less than that of the ab initio multiple spawning calculations used to compute the underlying nonadiabatic dynamics.

physics.chem-ph↗

arXiv2024

IR Spectroscopy of Carboxylate-Passivated Semiconducting Nanocrystals: Simulation and Experiment

Surfaces of colloidal nanocrystals are frequently passivated with carboxylate ligands which exert significant effects on their optoelectronic properties and chemical stability. Experimentally, binding geometries of such ligands are typically investigated using vibrational spectroscopy, but the interpretation of the IR signal is usually not trivial. Here, using machine-learning (ML) algorithms trained on DFT data, we simulate an IR spectrum of a lead-rich PbS nanocrystal passivated with butyrate ligands. We obtain a good agreement with the experimental signal and demonstrate that the observed line shape stems from a very wide range of `tilted-bridge'-type geometries and does not indicate a coexistence of `bridging' and `chelating' binding modes as has been previously assumed. This work illustrates limitations of empirical spectrum assignment and demonstrates the effectiveness of ML-driven molecular dynamics simulations in reproducing IR spectra of nanoscopic systems.

physics.chem-ph↗

arXiv2023

Ultrafast internal conversion and photochromism in gas-phase salicylideneaniline

Salicylidenaniline (SA) is an archetypal system for excited-state intramolecular proton transfer (ESIPT) in non-planar systems. Multiple channels for relaxation involving both the keto and enol forms have been proposed after excitation to S$_1$ with near-UV light. Here we present transient absorption measurements of hot gas-phase SA, jet-cooled SA, and SA in Ar clusters using cavity-enhanced transient absorption spectroscopy (CE-TAS). Assignment of the spectra is aided by simulated TAS spectra, computed by applying time-dependent complete active space configuration interaction (TD-CASCI) to structures drawn from nonadiabatic molecular dynamics simulations. We find prompt ESIPT in all conditions followed by the rapid parallel generation of the trans-keto metastable photochrome state and fluorescent keto state in parallel. Increasing the internal energy increases the photochrome yield and decreases the fluorescent yield and fluorescent state lifetime observed in TAS. In Ar clusters, internal conversion of SA is severely hindered but the photochrome yield is unchanged. Taken together, these results are consistent with the photochrome being produced via the vibrationally excited keto population after ESIPT.

physics.chem-ph↗