Near-Infrared Absorption Features of Triplet-Pair States Assigned by Photoinduced-Absorption-Detected Magnetic Resonance

Abstract

Singlet fission proceeds through a manifold of triplet-pair states that are exceedingly difficult to distinguish spectroscopically. Here, we introduce a new implementation of photoinduced-absorption-detected magnetic resonance (PADMR) and use it to understand the excited-state absorption spectrum of a tri-2-pentylsilylethynyl pentadithiophene (TSPS-PDT) film. These experiments allow us to directly correlate magnetic transitions driven by RF with electronic transitions in the visible and near-infrared spectrum with high sensitivity. We find that the new near-infrared excited-state transitions that arise in thin films of TSPS-PDT are correlated with the magnetic transitions of T1, not 5TT. Thus, we assign these features to the excited-state absorption of 1TT, which is depleted when T1 states are driven to a spin configuration that forbids subsequent fusion. These results clarify the disputed origin of triplet-associated near-infrared absorption features in singlet-fission materials and demonstrate an incisive general purpose tool for studying the evolution of high-spin excited states.

Molecular Control of Triplet-Pair Spin Polarization and Its Optoelectronic Magnetic Resonance Probes

“…We propose that in TSPS-PDT, these NIR absorption features are dominated by a large population of 1 TT, and we are able to detect this magnetically dark singlet species in the PADMR experiment due to a kinetic coupling to geminate T + T species. 23 A remarkable implication of this observation is that, similar to the low-flux case in PLDMR of TES TIPS TT, T + T remains appreciably spin-correlated into the steady-state time scale of this experiment (hundreds of microseconds). While the main thrust of our work has been focused on understanding and preparing a pure magnetic sublevel population of 5 TT, these results highlight a different potential quantum resource: separated triplets that retain the spin memory of their formation, perhaps to the extent that they are an entangled pair.…”

“…Moreover, our analysis of the total f-PADMR (microwave frequency-swept) spectrum detected at visible wavelengths reveals that the transition peaking at 990 MHz is associated with T 1 states, not 5 TT states, which is just barely detectable in this system at 350 and 1200 MHz. 23 Prior work has assigned the NIR photoinduced absorption to n TT states largely on the basis that they do not appear when these molecules are sensitized with triplet states in solution, and they are kinetically correlated with the visible (600 nm) feature that is associated with the total triplet population. Figure 7 illustrates our hypothesis.…”

“…A new tool is needed to bridge the gap between optical and magnetic spectroscopy. To this end, we developed a novel implementation of photoinduced absorption-detected magnetic resonance 23 and applied it to understand the transient optical spectrum of a parallel-pair crystalline material, tri-2-pentylsilylethynyl pentadithiophene (TSPS-PDT). 25 Like TES TIPS TT, described in the preceding section, 4 TSPS-PDT was designed to form slip-stacked crystals with parallel principal molecular axes and minimal intermolecular coupling to inhibit n TT dissociation into consanguineous T + T states and to funnel the population toward 5 TT 0 , in accordance with the JDE model.…”

“…There are at least four photochemical pathways toward the vision articulated above: photoinitiated radical ion pairs, ,,, ligand-field state engineering, photoexcited triplets derived from intersystem crossing, , and singlet fission (SF). − ,,− Our work has focused on the last of these pathways due to its unique and potentially powerful properties, including ultrafast (sub-nanosecond) state preparation, the possibility of controlling spin evolution through both the inter- and intramolecular structure, and the demonstrated optical and electrical pathways toward ultrasensitive spin readout.…”

Multiexciton quintet state populations in a rigid pyrene-bridged parallel tetracene dimer

“…S14a †. Distinct ESA features for 1 TT and 5 TT were also observed by Pace et al 43 A more definitive method for connecting 5 TT photoinduced absorption and spin signatures involving optically detected magnetic resonance 44,45 is planned.…”