Single-Carbon Bridged Pentacene Dimers Enable Efficient Singlet Fission and Quintet State Stabilization

Abstract

Singlet fission (SF) offers a promising avenue for quantum information science, as it generates spin-entangled triplet pairs with quintet character (⁵TT) upon photoexcitation, enabling access to multilevel spin qubit states beyond the traditional two-level systems. However, the ⁵TT state often decays via several pathways: (1) dissociation into isolated triplets; (2) triplet-triplet annihilation back into the singlet manifold; or (3) spin conversion to lower-multiplicity triplet pair states. These competing relaxation channels pose a major challenge for stabilizing ⁵TT. Here, we introduce a novel molecular design that prolongs ⁵TT lifetime by anchoring two pentacene chromophores to the same carbon (C9) position of a fluorene bridge, yielding FlePc2 and FlePhPc2. This single-point attachment enforces a near-parallel intramolecular geometry, promoting strong through-space spin interactions that hinder dissociation. Field-swept electron spin echo (FS-ESE) measurements reveal dominant ⁵TT signals, indicative of suppressed relaxation pathways. Theoretical calculations predict a substantial binding energy for the reported dimers, accompanied by significant spin density delocalization across both pentacenes, thereby rationalizing ⁵TT stabilization. These findings establish a molecular design principle for kinetically trapping high-spin multiexciton states, paving the way for spin-based quantum technologies.

1. Syntheses and Structures of Molecules Studied in This Work

1

X-ray structure of FlePc2.

2

Steady-state absorption and emission spectra of reported compounds recorded in toluene.

3

NTO analysis of S₁S₀ state for (a) FlePc2 and (b) FlePhPc2 at their S₀ optimized structures.

4

Fluorescence lifetime measurements (a, c, e) conducted at a concentration of ∼10 μM and fs-TA measurements (b, d, f) conducted at ∼100 μM in toluene for PhTIPSPc, FlePc2, and FlePhPc2, respectively.

5

(a, b) Selected single-wavelength kinetic traces (data points) and corresponding model fits (solid lines) obtained from global analysis of fs-TA data for FlePc2 in toluene. Notably, the identical decay time constants observed in both fs-TA and TCSPC measurements (Figure c) suggest that these processes correspond to precursor–successor dynamics. (c, d) show the results of the global analysis, including the species-associated spectra and the corresponding time-dependent concentration profiles, respectively. (e) illustrates the simplified kinetic model used to describe the SF process within a 1.6 ns observation window. All the measurements were conducted at ∼298 K.

6

FS-ESE spectra recorded for (a) FlePc2 and FlePhPc2, and (b) PhTIPSPc under CW laser excitation at 532 nm. All samples were measured in flash-frozen toluene at 80 K. The concentrations were 0.3 mM for the dimeric species FlePc2 and FlePhPc2, and 0.6 mM for the monomeric species PhTIPSPc. Measurements for FlePc2 and FlePhPc2 were performed using identical scan parameters to allow direct comparison of signal intensities. In (a), the slightly higher baseline noise in the FlePc2 trace arises from the longer integration window used to capture its broader spin–echo signal, not from differences in receiver gain or detection sensitivity.

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(a) Left panel: ⁵TT simulation of the FS-ESE spectrum for FlePc2 using optimized spin Hamiltonian parameters. Right panel: Simulated individual transitions within the quintet spin manifold, illustrating detailed spin polarization characteristics. (b) Upper panel: Pulse sequence utilized in the nutation experiments. Lower panel: Experimental Rabi oscillations measured at magnetic fields of 3280 and 3509 G for FlePc2, recorded in flash-frozen toluene at 80 K under 532 nm CW laser excitation. (c) Frequency-domain spectra derived from the Fourier transform analysis of the recorded Rabi oscillations confirm identical nutation frequencies at both field positions.

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Spin density distribution of the FlePc2 of ⁵TT and its enlarged view, obtained by ωB97x-D/6–31G­(d,p). Isovalue for spin density is 0.0004.