Abstract
Understanding charge transfer (CT) dynamics in molecular donor-acceptor (D-A) dyads can provide insight into developing efficient D-A molecules for capturing solar energy. Here, we characterize the excited-state evolution of a julolidine-BODIPY (Jul-BD) D-A system with an emissive CT state using time-resolved fluorescence, femtosecond transient absorption, and two-dimensional electronic spectroscopies. Comparison of these results with those from phenyl-BODIPY (Ph-BD) allows us to identify the dynamics at play during CT state formation and its subsequent conversion to either a fully charge-separated or triplet state. Photoexcitation of Jul-BD in tetrahydrofuran results in the formation of an initial emissive CT state that relaxes before fully charge-separating. In contrast, Jul-BD in toluene exhibits similar CT state dynamics, albeit at slower timescales, before decaying to a terminal triplet species. Quantum beat analysis at early times in both solvents shows several vibronic modes, which are corroborated using density functional theory (DFT) calculations. For Ph-BD, a single 220 cm-1 compression mode about the single bond linking the phenyl to BODIPY modulates their orbital overlap. Three active vibronic modes, 147, 174, and 214 cm-1, are found in Jul-BD, regardless of the dielectric constant of the medium. These motions correspond to compression and torsional motions along the single bond joining Jul to BD and are responsible for the evolution of the spontaneous and stimulated emission features in the time-resolved spectroscopic data, which is further supported by time-dependent DFT calculations of the steady-state absorption spectrum of the Jul-BD as a function of increasing D-A dihedral core angle. These findings show how torsional and compression motions can play a pivotal role in intramolecular CT between a D and an A linked by a single bond.
| Original language | English (US) |
|---|---|
| Pages (from-to) | 2946-2957 |
| Number of pages | 12 |
| Journal | Journal of Physical Chemistry A |
| Volume | 127 |
| Issue number | 13 |
| DOIs | |
| State | Published - Apr 6 2023 |
Funding
The authors would like to thank Dr. Joseph Christensen for useful discussions and his conceptual contributions to this study. This work was supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, under Award DE-FG02-99ER14999. J.P.O. is supported by the U.S. Department of Defense through the National Defense Science and Engineering Graduate (NDSEG) Fellowship Program. X-ray crystallography, H nuclear magnetic resonance (NMR) spectroscopy, and mass spectrometry were conducted in IMSERC facilities at Northwestern University, which have received support from the Soft and Hybrid Nanotechnology Experimental (SHyNE) Resource (NSF ECCS-2025633), NSF CHE-1048773, Northwestern University, the State of Illinois, and the International Institute for Nanotechnology (IIN). 1 The authors would like to thank Dr. Joseph Christensen for useful discussions and his conceptual contributions to this study. This work was supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, under Award DE-FG02-99ER14999. J.P.O. is supported by the U.S. Department of Defense through the National Defense Science and Engineering Graduate (NDSEG) Fellowship Program. X-ray crystallography, 1H nuclear magnetic resonance (NMR) spectroscopy, and mass spectrometry were conducted in IMSERC facilities at Northwestern University, which have received support from the Soft and Hybrid Nanotechnology Experimental (SHyNE) Resource (NSF ECCS-2025633), NSF CHE-1048773, Northwestern University, the State of Illinois, and the International Institute for Nanotechnology (IIN).
ASJC Scopus subject areas
- Physical and Theoretical Chemistry