Color-Tunable Delayed Fluorescence and Efficient Spin–Orbit
Charge Transfer Intersystem Crossing in Compact Carbazole-Anthracene-Bodipy
Triads Employing the Sequential Electron Transfer Approach
posted on 2020-03-09, 22:13authored byZafar Mahmood, Maria Taddei, Noreen Rehmat, Laura Bussotti, Sandra Doria, Qinglin Guan, Shaomin Ji, Jianzhang Zhao, Mariangela Di Donato, Yanping Huo, Yong Heng Xing
Spin–orbit
charge transfer intersystem crossing (SOCT-ISC)
is a promising approach to develop heavy-atom-free triplet photosensitizers.
However, designing a strong visible-light harvesting heavy-atom-free
triplet photosensitizer with efficient ISC ability in various solvents
is still challenging. Most of the SOCT-ISC triplet photosensitizers
exhibit efficient ISC only in solvent of particular polarity. To address
this challenge, herein, two triads (BDP-AN-C-CZ and BDP-AN-N-CZ), composed of carbazole (CZ), anthracene (AN),
and bodipy (BDP) moieties, were devised. In these triads, the distance,
relative orientation, and position of CZ with respect to the AN moiety
were varied to study the effect on photophysical properties, especially
on SOCT-ISC efficiency. Electrochemical studies, steady-state, and
time-resolved spectroscopies confirmed a sequential photoinduced electron
transfer (PET) process in the triads. The fluorescence of the BDP
moiety is quenched and a red-shifted CT emission band is observed
in the triads, due to the enhanced PET effect, compared to the reference BDP-AN dyad. We observed that the SOCT-ISC yield can be enhanced
taking advantage of sequential electron transfer. The triad BDP-AN-C-CZ, in which the CZ moiety was directly linked to
the AN moiety, shows an efficient ISC ability both in low-polarity
and high-polarity solvents, and unity triplet quantum yield (ΦT) was observed in dichloromethane. Femtosecond transient absorption
spectroscopy confirmed the fast charge separation process (1.8 ps)
in BDP-AN-C-CZ as compared to the other triad BDP-AN-N-CZ (4.8 ps) and the reference BDP-AN dyad (7.7 ps). The
triads were used as triplet photosensitizers for triplet–triplet
annihilation (TTA) upconversion, and high upconversion quantum yield
(ΦUC = 18%) was observed. Interestingly, long-lived
(τDF = 118 μs) and solvent-dependent color-tunable
TTA delayed fluorescence was observed in the case of BDP-AN-C-CZ.