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

## TL;DR

This paper introduces a new molecular design that stabilizes high-spin states for potential use in quantum technologies.

## Contribution

A novel molecular structure is proposed to stabilize quintet spin states through intramolecular geometry.

## Key findings

- The new dimers show dominant 5TT signals with suppressed relaxation pathways.
- Theoretical calculations confirm strong spin delocalization and binding energy in the dimers.
- The design principle enables kinetically trapping high-spin multiexciton states.

## 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.