Spin-Orbit Charge-Transfer Intersystem Crossing of Compact Naphthalenediimide-Carbazole Electron-Donor-Acceptor Triads

Compact electron donor-acceptor triads based on carbazole (Cz) and naphthalenediimide (NDI) were prepared to study the spin-orbit charge-transfer intersystem crossing (SOCT-ISC). By variation of the molecular conformation and electron-donating ability of the carbazole moieties, the electronic coupling between the two units was tuned, and as a result charge-transfer (CT) absorption bands with different magnitudes were observed (ε = 4000-18 000 M^-1 cm^-1). Interestingly, the triads with NDI attached at the 3-C position or with a phenyl spacer at the N position of the Cz moiety, thermally activated delayed fluorescence (TADF) was observed. Femtosecond transient absorption (fs-TA) spectroscopy indicated fast electron transfer (0.8-1.5 ps) from the Cz to NDI unit, followed by population of the triplet state (150-600 ps). Long-lived triplet states (up to τ_T = 45-50 μs) were observed for the triads. The solvent-polarity-dependent singlet-oxygen quantum yield (Φ_Δ) is 0-26%. Time-resolved electron paramagnetic resonance (TREPR) spectral study of TADF molecules indicated the presence of the ³CT state for NDI-Cz-Ph (zero-field-splitting parameter D = 21 G) and an ³LE state for NDI-Ph-Cz (D = 586 G). The triads were used as triplet photosensitizers in triplet-triplet annihilation upconversion by excitation into the CT absorption band; the upconversion quantum yield was Φ_UC = 8.2%, and there was a large anti-Stokes shift of 0.55 eV. Spatially confined photoexcitation is achieved with the upconversion using focusing laser beam excitation, and not the normally used collimated laser beam, i.e., the upconversion was only observed at the focal point of the laser beam. Photo-driven intermolecular electron transfer was demonstrated with reversible formation of the NDI^-• radical anion in the presence of the sacrificial electron donor triethanolamine.

An Ultra-Long-Lived Triplet Excited State in Water at Room Temperature: Insights on the Molecular Design of Tridecafullerenes

Suitably engineered molecular systems exhibiting triplet excited states with very long lifetimes are important for high-end applications in nonlinear optics, photocatalysis, or biomedicine. We report the finding of an ultra-long-lived triplet state with a mean lifetime of 93 ms in an aqueous phase at room temperature, measured for a globular tridecafullerene with a highly compact glycodendrimeric structure. A series of three tridecafullerenes bearing different glycodendrons and spacers to the C60 units have been synthesized and characterized. UV/Vis spectra and DLS experiments confirm their aggregation in water. Steady-state and time-resolved fluorescence experiments suggest a different degree of inner solvation of the multifullerenes depending on their molecular design. Efficient quenching of the triplet states by O2 but not by waterborne azide anions has been observed. Molecular modelling reveals dissimilar access of the aqueous phase to the internal structure of the tridecafullerenes, differently shielded by the glycodendrimeric shell.

Highly luminescent palladium(ii) complexes with sub-millisecond blue to green phosphorescent excited states. Photocatalysis and highly efficient PSF-OLEDs

Palladium(ii) complexes supported by tetradentate [N^C^C^N] and [O^N^C^N] ligand systems display sky blue to red phosphorescence with emission quantum yields and emission lifetimes up to 0.64 and 272 μs, respectively. Femtosecond time-resolved fluorescence (fs-TRF) measurements on these Pd(ii) complexes reveal a fast intersystem crossing from singlet to triplet manifolds with time constants of 0.6-21 ps. DFT/TDDFT calculations revealed that, as a result of the spiro-fluorene and bridging tertiary amine units of the ligands, the T₁ excited state is more ligand-localized and has smaller structural distortion, leading to slower non-radiative decay as well as radiative decay of T₁ → S₀ transition and thereby highly emissive, long-lived triplet excited states. The Pd(ii) complexes have been found to be efficient catalysts for visible light-driven, reductive C-C bond formation from unactivated alkyl bromides with conversions and yields of up to 90% and 83%, respectively. These complexes have also been employed as photosensitizers for [2 + 2] cycloaddition of styrenes, with conversions and yields comparable to those of the reported Ir(iii) complexes. Both green and sky blue organic-light emitting devices (OLEDs) have been generated with these Pd(ii) complexes as guest emitters. Maximum external quantum efficiencies (EQE) of up to 16.5% have been achieved in the sky blue OLEDs. The long emission lifetimes render the Pd(ii) complexes good sensitizers for phosphor-sensitized fluorescent OLEDs (PSF-OLEDs). By utilizing these phosphorescent Pd(ii) complexes as sensitizers, highly efficient green and yellow PSF-OLEDs having high EQE (up to 14.3%), high colour purity and long operation lifetimes, with 90% of initial luminance (LT₉₀) for more than 80 000 h, have been realized.

Triplet-Triplet Annihilation Photon Upconversion in Polymer Thin Film: Sensitizer Design

Efficient visible-to-UV photon upconversion via triplet-triplet annihilation (TTA) is accomplished in polyurethane (PU) films by developing new, powerful photosensitizers fully functional in the solid-state matrix. These rationally designed triplet sensitizers feature a bichromophoric scaffold comprising a tris-cyclometalated iridium(III) complex covalently tethered to a suitable organic small molecule. The very rapid intramolecular triplet energy transfer from the former to the latter is pivotal for achieving the potent sensitizing ability, because this process out-competes the radiative and nonradiative decays inherent to the metal complex and produces long-lived triplet excitons localized with the acceptor moiety readily available for intermolecular transfer and TTA. Nonetheless, compared to the solution state, the molecular diffusion is greatly limited in solid matrices, which even creates difficulty for the Dexter-type intramolecular energy transfer. This is proven by the experimental results showing that the sensitizing performance of the bichromophoric molecules strongly depends on the spatial distance separating the donor (D) and acceptor (A) units and that incorporating a longer linker between the D and A evidently curbs the TTA upconversion efficiency in PU films. Using a rationally optimized sensitizer structure in combination with 2,7-di-tert-butylpyrene as the annihilator/emitter, the doped polyurethane (PU) films demonstrate effective visible-to-UV upconverted emission signal under noncoherent-light irradiation, attaining an upconversion quantum yield of 2.6%. Such quantum efficiency is the highest value so far reported for the visible-to-UV TTA systems in solid matrices.

Iridium-Based High-Sensitivity Oxygen Sensors and Photosensitizers with Ultralong Triplet Lifetimes

The photophysics of a series of bichromophoric molecules featuring an intramolecular triplet energy transfer between a triscyclometalated iridium(III) complex and covalently linked organic group are studied. By systematically varying the energy gap (0.1-0.3 eV) between the donor (metal complex) and acceptor (pyrene unit), reversible triplet energy transfer processes with equilibrium constant K ranging from ca. 500 to 40 000 are established. Unique photophysical consequences of such large K values are observed. Because of the highly imbalanced forward and backward energy transfer rates, triplet excitons dominantly populate the acceptor moiety in the steady state, giving rise to ultralong luminescence lifetimes up to 1-4 ms. Because the triscyclometalated Ir and triplet pyrene groups both impart relatively small nonradiative energy loss, decent phosphorescence quantum yields (Φ = 0.1-0.6) are attained in spite of the exceptionally prolonged excited states. By virtue of such precious combination of long-lived triplet state and high Φ, these bichromophoric molecules can serve as highly sensitive luminescent sensors for detecting trace amount of O2 and as potent photosensitizers for producing singlet oxygen even under low-oxygen content conditions.

MLCT sensitizers in photochemical upconversion: past, present, and potential future directions

This frontier presentation highlights the historical development of MLCT sensitizers in photochemical upconversion while indentifying current state-of-the-art and exciting opportunities in this arena moving towards the future.

Singlet oxygen generation by cyclometalated complexes and applications

While cyclometalated complexes have been extensively studied for optoelectronic applications, these compounds also represent a relatively new class of photosensitizers for the production of singlet oxygen. Thus far, singlet oxygen generation from cyclometalated Ir and Pt complexes has been studied in detail. In this review, photophysical data for singlet oxygen generation from these complexes are presented, and the mechanism of (1) O2 generation is discussed, including evidence for singlet oxygen generation via an electron-transfer mechanism for some of cyclometalated Ir complexes. The period from the first report of singlet oxygen generation by a cyclometalated Ir complex in 2002 through August 2013 is covered in this review. This new class of singlet oxygen photosensitizers may prove to be rather versatile due to the ease of substitution of ancillary ligands without loss of activity. Several cyclometalated complexes have been tethered to zeolites, polystyrene, or quantum dots. Applications for photooxygenation of organic molecules, including "traditional" singlet oxygen reactions (ene reaction, [4 + 2] and [2 + 2] cycloadditions) as well as oxidative coupling of amines are presented. Potential biomedical applications are also reviewed.

Cyclometalated Ir(iii) complexes with styryl-BODIPY ligands showing near IR absorption/emission: preparation, study of photophysical properties and application as photodynamic/luminescence imaging materials

Heteroleptic C^N cyclometalated iridium(iii) complexes showing strong NIR absorption were prepared.