Application of a bodipy-C70 dyad in triplet-triplet annihilation upconversion of perylene as a metal-free photosensitizer

Multiple Pathways in the Triplet States Population for a Naphthalenediimide-C60 Dyad

The link of an antenna dye with an electron spin converter, in this case naphthalenediimide and C60, produces a system with a rich photophysics including the detection of more than one triplet state on the long timescale (tens of μs). Beside the use of optical spectroscopies in the ns and in the fs time scale, we used time-resolved Electron Paramagnetic Resonance (TREPR) to study the system evolution following photoexcitation. TREPR keeps track of the formation path of the triplet states through specific spin polarization patterns observed in the spectra. The flexibility of the linker and solvent polarity play a role in favouring either electron transfer or energy transfer processes.

Efficient Red-to-Blue Triplet-Triplet Annihilation Upconversion Using the C70-Bodipy-Triphenylamine Triad as a Heavy-Atom-Free Triplet Photosensitizer

Triplet-triplet annihilation upconversion (TTA-UC) with heavy-atom-free organic triplet photosensitizers has attracted extensive attention recently, however, the successful examples with absorption in red and first near-infrared (NIR-I, 650-900 nm) region are still insufficient. Herein, we conducted a new TTA-UC system of perylene using C70-bodipy-triphenylamine triad (C70-BDP-T) as the heavy-atom-free photosensitizer. Efficient red-to-blue (663 to 450 nm) TTA-UC was achieved in this system with an anti-Stokes shift of 0.88 eV and a quantum yield up to 5.2% (out of a 50% maximum) in deaerated toluene. Notably, this is the highest quantum yield to date in similar TTA-UC systems with heavy-atom-free organic photosensitizers. Using steady-state and transient absorption spectroscopy, together with cyclic voltammogram and quantum chemical calculations, photophysical and photochemical mechanisms were elucidated. Specifically, two triplet triads, C70-3BDP*-T and 3C70*-BDP-T, were produced efficiently upon photoexcitation, with lifetimes of 2.0 ± 0.1 and 32.2 ± 0.3 μs, respectively. Electron transfer and recombination mechanisms were confirmed to play crucial roles in the formation of these triplets, instead of intersystem crossing. Our results shed light on the superiority of fullerenes in the development of heavy-atom-free photosensitizers.

Panchromatic Light-Absorbing [70]Fullerene-Perylene-BODIPY Triad with Cascade of Energy Transfer as an Efficient Singlet Oxygen Sensitizer

A panchromatic light-absorbing [70]fullerene-perylene-BODIPY triad (C₇₀-P-B) was synthesized and applied as a heavy atom-free organic triplet photosensitizer for photooxidation. The photophysical processes were comprehensively investigated by the methods of steady-state spectroscopy, time-resolved spectroscopy, as well as theoretical calculations. C₇₀-P-B shows a strong absorption ability from 300-620 nm. Efficient cascading intramolecular singlet-singlet energy transfer in C₇₀-P-B was confirmed by the luminescence study. The backward triplet excited state energy transfer from C₇₀ moiety to perylene then occurs to populate ³perylene*. Thus, the triplet excited states of C₇₀-P-B are distributed on both C₇₀ and perylene moiety with lifetimes of 23 ± 1 μs and 175 ± 17 μs, respectively. C₇₀-P-B exhibits excellent photooxidation capacity, and its yield of singlet oxygen reaches 0.82. The photooxidation rate constant of C₇₀-P-B is 3.70 times that of C₇₀-Boc and 1.58 times that of MB, respectively. The results in this paper are useful for designing efficient heavy atom-free organic triplet photosensitizers for practical application in photovoltaics, photodynamic therapy, etc.

Heavy Atom-Free Triplet Photosensitizers: Molecular Structure Design, Photophysical Properties and Application in Photodynamic Therapy

Photodynamic therapy (PDT) is a promising method for the treatment of cancer, because of its advantages including a low toxicity, non-drug-resistant character, and targeting capability. From a photochemical aspect, a critical property of triplet photosensitizers (PSs) used for PDT reagents is the intersystem crossing (ISC) efficiency. Conventional PDT reagents are limited to porphyrin compounds. However, these compounds are difficult to prepare, purify, and derivatize. Thus, new molecular structure paradigms are desired to develop novel, efficient, and versatile PDT reagents, especially those contain no heavy atoms, such as Pt or I, etc. Unfortunately, the ISC ability of heavy atom-free organic compounds is usually elusive, and it is difficult to predict the ISC capability of these compounds and design novel heavy atom-free PDT reagents. Herein, from a photophysical perspective, we summarize the recent developments of heavy atom-free triplet PSs, including methods based on radical-enhanced ISC (REISC, facilitated by electron spin-spin interaction), twisted π-conjugation system-induced ISC, the use of fullerene C₆₀ as an electron spin converter in antenna-C₆₀ dyads, energetically matched S₁/T_n states-enhanced ISC, etc. The application of these compounds in PDT is also briefly introduced. Most of the presented examples are the works of our research group.

Surface Plasmon Enhanced Upconversion Fluorescence in Short-Wave Infrared for In Vivo Imaging of Ovarian Cancer

Short-wave infrared (SWIR; 850-1700 nm) upconversion fluorescence enables "autofluorescence-free" imaging with minimal tissue scattering, yet it is rarely explored due to the lack of strongly emissive SWIR upconversion fluorophores. In this work, we apply SWIR upconversion fluorescence for in vivo imaging with exceptional image contrast. Gold nanorods (AuNRs) are used to enhance the SWIR upconversion emission of small organic dyes, forming a AuNR-dye nanocomposite (NC). A maximal enhancement factor of ∼1320, contributed by both excitation and radiative decay rate enhancement, is achieved by varying the dye-to-AuNR ratio. In addition, the upconversion emission intensity of both free dyes and AuNR-dye NCs depends linearly on the excitation power, indicating that the upconversion emission mechanism remains unchanged upon enhancement, and it involves one-photon absorption. Moreover, the SWIR upconversion emission shows a significantly higher signal contrast than downconversion emission in the same emission window in a nonscattering medium. Finally, we apply the surface plasmon enhanced SWIR upconversion fluorescence for in vivo imaging of ovarian cancer, demonstrating high image contrast and low required dosage due to the suppressed autofluorescence.

The TDDFT Excitation Energies of the BODIPYs; The DFT and TDDFT Challenge Continues

The derivatives of 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (BODIPY) are pivotal ingredients for a large number of functional, stimuli-responsive materials and therapeutic molecules based on their photophysical properties, and there is a urgent need to understand and predict their optical traits prior to investing a large amount of resources in preparing them. Density functional theory (DFT) and time-dependent DFT (TDDFT) computations were performed to calculate the excitation energies of the lowest-energy singlet excited state of a large series of common BODIPY derivatives employing various functional aiming at the best possible combination providing the least deviations from the experimental values. Using the common "fudge" correction, a series of combinations was investigated, and a methodology is proposed offering equal or better performances than what is reported in the literature.

Broadband Visible Light-Absorbing [70]Fullerene-BODIPY-Triphenylamine Triad: Synthesis and Application as Heavy Atom-Free Organic Triplet Photosensitizer for Photooxidation

A broadband visible light-absorbing [70]fullerene-BODIPY-triphenylamine triad (C70-B-T) has been synthesized and applied as a heavy atom-free organic triplet photosensitizer for photooxidation. By attaching two triphenylmethyl amine units (TPAs) to the π-core of BODIPY via ethynyl linkers, the absorption range of the antenna is extended to 700 nm with a peak at 600 nm. Thus, the absorption spectrum of C70-B-T almost covers the entire UV-visible region (270-700 nm). The photophysical processes are investigated by means of steady-state and transient spectroscopies. Upon photoexcitation at 339 nm, an efficient energy transfer (ET) from TPA to BODIPY occurs both in C70-B-T and B-T, resulting in the appearance of the BODIPY emission at 664 nm. Direct or indirect (via ET) excitation of the BODIPY-part of C70-B-T is followed by photoinduced ET from the antenna to C70, thus the singlet excited state of C70 (1C70*) is populated. Subsequently, the triplet excited state of C70 (3C70*) is produced via the intrinsic intersystem crossing of C70. The photooxidation ability of C70-B-T was studied using 1,5-dihydroxy naphthalene (DHN) as a chemical sensor. The photooxidation efficiency of C70-B-T is higher than that of the individual components of C70-1 and B-T, and even higher than that of methylene blue (MB). The photooxidation rate constant of C70-B-T is 1.47 and 1.51 times as that of C70-1 and MB, respectively. The results indicate that the C70-antenna systems can be used as another structure motif for a heavy atom-free organic triplet photosensitizer.

Solvent effects on triplet-triplet annihilation upconversion kinetics of perylene with a Bodipy-phenyl-C60 photosensitizer

The solvent effect usually plays an important role in triplet-triplet annihilation (TTA) upconversion processes. In this work, we have studied the TTA upconversion kinetics of perylene with Bodipy-phenyl-C60 as the triplet photosensitizer in five solvents, 1,4-dioxane, dichlorobenzene, chlorobenzene, toluene, and tetrahydrofuran (THF). Although no significant solvent effect was observed in steady-state absorption and fluorescence emission spectra, the overall TTA upconversion quantum yields showed a profound dependence on solvent properties, i.e. 4.9% in 1,4-dioxane, 7.1% in dichlorobenzene, 6.7% in chlorobenzene, 4.6% in toluene, and 2.2% in THF (the maximum of 50%). Each elementary reaction step involved in the overall process was analyzed by applying femtosecond and nanosecond time-resolved transient absorption spectroscopy, revealing that the fluorescence emission of perylene was more significantly affected by the solvents in contrast to the other steps. Moreover, an extra intramolecular energy-transfer pathway of Bodipy-phenyl-C60 was found via the formation of charge-separated states in dichlorobenzene, chlorobenzene, and THF solvents, once being excited. These conclusions provide valuable clues to choose the most favorable solvent for the higher TTA upconversion efficiency in related applications.

Rigid axially symmetrical C60-BODIPY triplet photosensitizers: effect of bridge length on singlet oxygen generation

Two rigid axially symmetrical C₆₀-BODIPY systems with different bridge lengths have been synthesized and the dyad with short bridge generates a higher quantum yield of singlet oxygen.

Diketopyrrolopyrrole–fullerene C60 architectures as highly efficient heavy atom-free photosensitizers: synthesis, photophysical properties and photodynamic activity

New diketopyrrolopyrrole–C₆₀ architectures were synthesized as potential heavy atom-free photosensitizers in photodynamic inactivation of microorganisms.

Near-infrared to violet triplet-triplet annihilation fluorescence upconversion of Os(ii) complexes by strong spin-forbidden transition

Three Os(ii) complexes were synthesized with ligands 2,2'-dipyridyl (dipy), 1,10-phenanthroline monohydrate (phen), and 4,7-diphenyl-1,10-phenanthroline (diphen), and applied as triplet photosensitizers for triplet-triplet annihilation (TTA) fluorescence upconversion. The strong spin-orbital coupling made direct spin-forbidden transition of S₀-T₁ feasible. Lifetimes of the lowest triplet state of these complexes were determined to be 107 ns, 373 ns, and 386 ns for Os-dipy, Os-phen, and Os-diphen, respectively, using nanosecond transient absorption spectra. From steady-state phosphorescence emission spectra, energies of the triplet states were derived to be 1.75 eV, 1.80 eV, and 1.74 eV for Os-dipy, Os-phen, and Os-diphen, respectively. Using these photosensitizers, strong upconverted fluorescence of the triplet acceptors, 9,10-diphenylanthracene (DPA), perylene, and 9,10-bis(phenethynyl) anthracene (BPEA), was observed in the visible to violet range. In particular, fluorescence emission with the largest anti-Stokes shift of 1.14 eV was observed for the Os-phen/DPA system, and the upconverted quantum yield was determined as 5.9% in deoxygenated dichloroethane. Additionally, upconversion was determined in air using mixtures of dichloroethane and DMSO solvents, and the maximal quantum yield was measured to be 4.5% for Os-phen/DPA.