Excited-State Planarization in Donor–Bridge Dye Sensitizers: Phenylene versus Thiophene Bridges

Abstract

Donor-π-acceptor complexes for solar energy conversion are commonly composed of a chomophore donor and a semiconductor nanoparticle acceptor separated by a π bridge. The electronic coupling between the donor and acceptor is known to be large when the π systems of the donor and bridge are coplanar. However, the accessibility of highly coplanar geometries in the excited state is not well understood. In this work, we clarify the relationship between the bridge structure and excited-state donor-bridge coplanarization by comparing rhodamine sensitizers with either phenylene (O-Ph) or thiophene (O-Th) bridge units. Using a variety of optical spectroscopic and computational techniques, we model the S excited-state potential surfaces of O-Ph and O-Th along the dihedral coordinate of donor-bridge coplanarization, τ. We find that O-Th accesses a nearly coplanar (τ = 8°) global minimum geometry in S where significant intramolecular charge transfer (ICT) character is developed. The S coplanar geometry is populated in <10 ps and is stable for ca. 1 ns. Importantly, O-Ph is sterically hindered from rotation along τ and therefore remains at its initial S equilibrium geometry far from coplanarity (τ = 56°). Our results demonstrate that donor-bridge dye sensitizers utilizing thiophene bridges should facilitate strong donor-acceptor coupling by an ultrafast and stabilizing coplanarization mechanism in S. The coplanarization will result in strong donor-acceptor coupling, potentially increasing the electron transfer efficiency. These findings provide further explanation for the success of thiophene as a bridge unit and can be used to guide the informed design of new molecular sensitizers.

Two-Dimensional Impulsively Stimulated Resonant Raman Spectroscopy of Molecular Excited States

“…Furthermore, Raman spectroscopy benefits from resonance enhancements of specific chromophore signatures and thereby provides a route to comprehensively investigate vibrational energy flow during reactive transformations by selectively probing specific environments [25]. In particular, time-domain impulsive stimulated Raman scattering (ISRS) [26][27][28][29][30][31][32][33][34][35][36] offers several advantages over its frequency-domain analogues for the detection of vibronic features, especially for low-frequency modes, by efficiently removing elastic scattering contributions and background noise [37][38][39][40]. Its multidimensional extension, 2D-ISRS, has been theoretically proposed initially [41] and realized in both nonresonant [42,43] and resonant [44,45] implementations to study ground-state intramolecular vibrational anharmonicities, nonlinear corrections to the molecular polarizability, product-reactant correlations, and solvation dynamics, up to the recent realization of single-pulse 2D spectroscopy by means of appropriately shaped light pulses [46], with possible applications theoretically suggested for the x-ray domain [47].…”

Devising Efficient Red‐Shifting Strategies for Bioimaging: A Generalizable Donor‐Acceptor Fluorophore Prototype

“…The push‐pull structure is usually established by incorporating the electron‐donating groups (EDGs) and electron‐withdrawing groups (EWGs) on the opposite ends of the parent backbone. The conjugated fluorophore core could then act as the ‐linker to effectively promote ICT in the excited state …”

Genuine Dynamics vs Cross Phase Modulation Artifacts in Femtosecond Stimulated Raman Spectroscopy

“…During the past decade, the FSRS capabilities to unveil femtosecond structural changes have been road tested for a large scenario of samples, spanning from biochemical compounds to condensed-matter systems. Particularly, FSRS provided huge insights on the study of photoreaction processes, − on different protein structural evolution upon photoexcitation, − on vibrational energy redistribution, , and on the wavepacket evolution on excited state surfaces. − In addition, FSRS has also been applied to investigate condensed matter compounds, with potential huge outcomes in the field of photonics, for measuring lattice dynamics in semiconductor nanocrystals, for directly monitoring the ultrafast long-range charge separation, and for unraveling photoinduced modification of magnetic properties. , …”