Our research focus on molecular photochemistry and photophysics in triplet states' field.

Zhao’s group studies the electron transfer (charge separation and charge recombination), energy transfer (via Förster and Dexter mechanisms), intersystem crossing (ISC) and electron spin dynamics of organic compounds and transition metal complexes, with steady state and time-resolved optical and magnetic spectroscopies.

Zhao’s laboratory devised series of transition metal complexes containing visible light-harvesting organic chromophores. These complexes show strong absorption of visible light (molar absorption coefficients are up to 100,000 M-1 cm-1) and long-lived triplet excited states (lifetimes are up to 100 microseconds). The principles are to replace the low-lying MLCT states of the conventional transition metal complexes with the ligand-localized LE state, thus the transition probability concerning the photoexcitation and the decay from the triplet state to the ground state were changed. These complexes have been demonstrated to be more efficient in triplet-triplet annihilation upconversion (TTA-UC), photocatalytic hydrogen production, luminescence oxygen sensing and photodynamic therapy. Other applications of these novel transition metal complexes, where intermolecular electron transfer or energy transfer are involved, may be also improved by using these novel photosensitizers.

The group also studied triplet-triplet annihilation upconversion (TTA-UC) in recent years. We developed series of new triplet photosensitizers showing strong absorption of visible light and long-lived triplet state, include the transition metal complexes and organic compounds, and these new triplet photosensitizers were used for TTA-UC and the efficient UC were observed. Based on tuning the triplet state properties, the group also developed external stimuli-response TTA-UC, for instance the photo-switchable TTA-UC, and the biologically-active small molecules activated TTA-UC systems, which are responsive to H2O2, thiols, H+, etc. These strategies were also used for activatable photodynamic therapy (PDT).

The group is also keen to develop novel triplet photosensitizers based on ISC by using the electron spin converter, charge recombination, energy matching S1/Tn states (n>1), and electron spin-spin exchange between a persistent radical and the organic chromophore (radical enhanced ISC, EISC). These research will inspire more studies on developing new triplet photosensitizers.

Recently the group is interested in electron spin dynamics concerning photophysical processes. Electron spin controls most photochemical and photophysical processes, such as the photo-induced charge separation, charge recombination, and photo-cleavage and subsequent radical recombination, etc. Through collaborations with groups in Italy, Russia, Germany, Japan and China, we studied the electron spin selective ISC of electron donor/acceptor dyads, and the ISC via spin-orbit coupling and charge recombination, etc.

The research in the group is based on various steady state and time-resolved optical spectroscopies, for instance, the UV-Vis absorption, fluorescence/phosphorescence excitation/emission spectra, luminescence lifetime and time-resolved luminescence spectra (0.1 ns ~ 10 s), temperature-variable absorption and luminescence spectra (300 K ~ 77 K), nanosecond transient absorption spectra (10 ns ~ 1 s); femtosecond transient absorption spectra, and time-resolved electron paramagnetic resonance (TREPR) spectra. The groups is also interested in understanding of the photophysics with the aid of theoretical computations. The group is well equipped with steady state and time-resolved optical spectrometers.