  Quantum sensors have notably advanced high-sensitivity magnetic field
detection. Here, we report quantum sensors constructed from polarized
spin-triplet electrons in photoexcited organic chromophores, specifically
focusing on pentacene-doped para-terphenyl (${\approx}$0.1%). We demonstrate
essential quantum sensing properties at room temperature: electronic optical
polarization and state-dependent fluorescence contrast, by leveraging
differential pumping and relaxation rates between triplet and ground states. We
measure high optically detected magnetic resonance (ODMR) contrast
${\approx}16.8\%$ of the triplet states at room temperature, along with long
coherence times under spin echo and CPMG sequences, $T_2{=}2.7\mu$s and
$T_2^{DD}{=}18.4\mu$s respectively, limited only by the triplet lifetimes. The
material offers several advantages for quantum sensing, including the ability
to grow large ($cm$-scale) crystals at low cost, the absence of paramagnetic
impurities, and the diamagnetism of electronic states used for sensing when not
optically illuminated. Utilizing pentacene as a representative of a broader
class of spin triplet-polarizable organic molecules, this study highlights new
potential for quantum sensing in chemical systems.
