Noreen Rehmat

Transient absorption spectroscopy is used to study events that take place upon photoexcitation and reveal the processes such as electron transfer, energy transfer, intersystem crossing, isomerization, proton transfer, exciplex formation, etc .

We present a red light-activated zinc^II bis(dipyrrin) symmetry breaking charge transfer (SBCT) architecture, showing a large molar absorption coefficient (ε = 15.4 × 10⁴ M⁻¹ cm⁻¹), high reactive singlet oxygen generation efficiency (Φ_Δ ≈ 0.8) and long-lived triplet state (τ_T = 150 μs) compared to the donor–acceptor analogue dipyrrin-BF₂ complex, highlighting the superiority of the SBCT approach. For the first time, we demonstrated the potential of a SBCT scaffold in red-light-induced methyl methacrylate (MMA) polymerization, using a dual photocatalyst excitation approach.

Bodipy (BDP)‐perylenebisimide (PBI) donor–acceptor dyads/triad were prepared to study the spin–orbit charge‐transfer intersystem crossing (SOCT‐ISC). For BDP‐PBI‐3, in which BDP was attached at the imide position of PBI, higher singlet oxygen quantum yield (Φ_Δ=85 %) was observed than the bay‐substituted derivative BDP‐PBI‐1 (Φ_Δ=30 %). Femtosecond transient absorption spectra indicate slow Förster resonance energy transfer (FRET; 40.4 ps) and charge separation (CS; 1.55 ns) in BDP‐PBI‐3, while for BDP‐PBI‐1, CS takes 2.8 ps. For triad BDP‐PBI‐2, ultrafast FRET (149 fs) and CS (4.7 ps) process were observed, the subsequent charge recombination (CR) takes 5.8 ns and long‐lived ³PBI* (179.8 μs) state is populated. Nanosecond transient absorption spectra of BDP‐PBI‐3 show that the CR gives upper triplet excited state (³BDP*) and subsequently, via a slow intramolecular triplet energy transfer (14.5 μs), the ³PBI* state is finally populated, indicating that upper triplet state is involved in SOCT‐ISC. Time‐resolved electron paramagnetic resonance spectroscopy revealed that both radical pair ISC (RP ISC) and SOCT‐ISC contribute to the ISC. A rare electron spin polarization of (e, e, e, e, e, e) was observed for the triplet state formed via the RP ISC mechanism, due to the S−T₊₁/T₀ states mixing.

Effective triplet–triplet annihilation photon upconversion (TTA-UC) is usually difficult to achieve under aerobic conditions, but is highly desirable for practical applications in which durability is greatly needed to sustain environmental loads and expand the lifespan of the relevant devices. Here, we developed a novel upcoversion organogel with platinum octaethylporphyrin (PtOEP) as sensitizer, 9,10-diphenylanthracene (DPA) as acceptor and n-octanal-modified polyvinyl alcohol (PVA) as structural matrix. The acquired organogel demonstrated self-healing ability and malleability. The exquisite combination of these components affords a variety of features, such as high oxygen resistance, high transparency, strong mechanics, and malleability, benefitting from the dynamic features of the noncovalently crosslinked network and highly compact morphology as well as the enhanced contact probability of the chromophores. The organogel shows effective green-to-blue upconversion, with a quantum yield Φ_UC of up to 6.7% in the aerobic environment. The upconversion efficiency of the organogel can maintain over 67% of its original value after long-term sealed storage (>180 days). This research provides a simple and effective strategy for the development of high-efficiency upconversion solid carrier materials.

Perylenebisimide (PBI)–anthracene (AN) donor–acceptor dyads/triad were prepared to investigate spin–orbit charge‐transfer intersystem crossing (SOCT‐ISC). Molecular conformation was controlled by connecting PBI units to the 2‐ or 9‐position of the AN moiety. Steady‐state, time‐resolved transient absorption and emission spectroscopy revealed that chromophore orientation, electronic coupling, and dihedral angle between donor and acceptor exert a significant effect on the photophysical property. The dyad PBI‐9‐AN with orthogonal geometry shows weak ground‐state coupling and efficient intersystem crossing (ISC, Φ_Δ=86 %) as compared with PBI‐2‐AN (Φ_Δ=57 %), which has a more coplanar geometry. By nanosecond transient absorption spectroscopy, a long‐lived PBI localized triplet state was observed (τ_T=139 μs). Time‐resolved EPR spectroscopy demonstrated that the electron spin polarization pattern of the triplet state is sensitive to the geometry and number of AN units attached to PBI. Reversible and stepwise generation of near‐IR‐absorbing PBI radical anion (PBI^−⋅) and dianion (PBI²⁻) was observed on photoexcitation in the presence of triethanolamine, and it was confirmed that selective photoexcitation at the near‐IR absorption bands of PBI^.− is unable to produce PBI²⁻.

Zinc(II) bis(dipyrrin) complexes, which feature intense visible absorption and efficient symmetry breaking charge transfer (SBCT) are outstanding candidates for photovoltaics but their short lived triplet states limit applications in several areas. Herein we demonstrate that triplet excited state dynamics of bis(dipyrrin) complexes can be efficiently tuned by attaching electron donating aryl moieties at the 5,5′‐position of the complexes. For the first time, a long lived triplet excited state (τ_T=296 μs) along with efficient ISC ability (Φ_Δ=71 %) was observed for zinc(II) bis(dipyrrin) complexes, formed via SBCT. The results revealed that molecular geometry and energy gap between the charge transfer (CT) state and triplet energy levels strongly control the triplet excited state properties of the complexes. An efficient triplet–triplet annihilation upconversion system was devised for the first time using a SBCT architecture as triplet photosensitizer, reaching a high upconversion quantum yield of 6.2 %. Our findings provide a blueprint for the development of triplet photosensitizers based on earth abundant metal complexes with long lived triplet state for revolutionary photochemical applications.

Perylenebisimide-carbazole (PBI–Cz) dyads were prepared to study charge-recombination (CR) induced intersystem crossing (ISC) in electron donor/acceptor dyads. The distance and the mutual orientation of the perylenebisimide (PBI) and carbazole (Cz) moieties were varied to study their effect on photophysical properties. Steady state and time-resolved optical spectroscopies show that electronic coupling between the electron donor and acceptor is negligible at the ground state. The fluorescence of the PBI moiety is strongly quenched in the dyads, and a larger separation between the donor and the acceptor results in less fluorescence quenching. The Gibbs free energy changes of the electron transfer and the energy level of the charge transfer state were studied using the electrochemical and optical spectra data. The singlet oxygen quantum yields (Φ_Δ) are up to 72% for dyads with the shortest separation between the donor and the acceptor. Nanosecond transient absorption spectra confirmed the formation of the PBI-localized long lived triplet state (the lifetime is up to 190 μs). Notably non-orthogonal dyads show efficient spin orbit charge transfer (SOCT-ISC), which is different from the previously proposed orthogonal molecular structure for SOCT-ISC. Time-resolved electron paramagnetic resonance (TREPR) spectroscopy shows that all three dyads give the same electron spin polarization of eae/aea, and thus the radical pair ISC (RP ISC) mechanism is excluded, which is different from the previously reported PBI-phenothiazine analogues. Efficient and reversible transformation of dyads to their radical anion was observed in the presence of sacrificial electron donor triethanolamine in an inert atmosphere with photo-irradiation.