Blue organic long-persistent luminescence via upconversion from charge-transfer to locally excited singlet state
Zesen Lin, Jin-Ting Ye, Shin Shinohara, Yuya Tanaka, Rengo Yoshioka, Chin‐Yiu Chan, Yi-Ting Lee, Xun Tang, Kirill Mitrofanov, Kai Wang, Hayato Ouchi, Liliia Moshniaha, Yemineni S. L. V. Narayana, Hisao Ishii, Xiaohong Zhang, Chihaya Adachi, Xiankai Chen, Ryota Kabe
Abstract
Long-persistent luminescence (LPL) materials have applications from safety signage to bioimaging; however, existing organic LPL (OLPL) systems do not align with human scotopic vision, which is sensitive to blue light. We present a strategy to blueshift the emissions in binary OLPL systems by upconverting the charge-transfer (CT) to a locally excited (LE) singlet state. Through rigorous steady-state and time-resolved photoluminescence spectroscopy and wavelength-resolved thermoluminescence measurements, we provide the direct experimental evidence for this upconversion in OLPL systems featuring small energy offsets between the lowest-energy CT and LE singlet states. These systems exhibited strong room temperature LPL, particularly when extrinsic electron traps are added. Importantly, the developed OLPL system achieved Class A (ISO 17398) LPL, matching well with human scotopic vision. The findings not only elucidate the role of small energy offsets in modulating LPL but also provide potential avenues for enhancing the efficiency and applicability of OLPL materials. Existing organic long-persistent luminescence systems do not align with human scotopic vision. Here, authors achieve blue-shifted emissions in binary systems by upconverting charge-transfer to locally excited singlet state, with a strong luminescence under international standard for safety signs.