From Terminal to Spiro-Phosphonium Acceptors, Remarkable Moieties to Develop Polyaromatic NIR Dyes.

Phosphonium-based compounds gain attention as promising photofunctional materials. As a contribution to the emerging field, we present a series of donor-acceptor ionic dyes, which were constructed by tailoring phosphonium (A) and extended π-NR2 (D) fragments to an anthracene framework. The alteration of the π-spacer of electron-donating substituents in species with terminal -+ PPh2 Me groups exhibits a long absorption wavelength up to λabs =527 nm in dichloromethane and shifted the emission to the near-infrared (NIR) region (λ=805 nm for thienyl aniline donor), although at low quantum yield (Φ<0.01). In turn, the introduction of a P-heterocyclic acceptor substantially narrowed the optical bandgap and improved the efficiency of fluorescence. In particular, the phospha-spiro moiety allowed to attain NIR emission (797 nm in dichloromethane) with fluorescence efficiency as high as Φ=0.12. The electron-accepting property of the phospha-spiro constituent outperformed that of the monocyclic and terminal phosphonium counterparts, illustrating a promising direction in the design of novel charge-transfer chromophores.

Comprehensive Thione-Derived Perylene Diimides and Their Bio-Conjugation for Simultaneous Imaging, Tracking, and Targeted Photodynamic Therapy.

In this study, the chromophore 3,4,9,10-perylenetetracarboxylic diimide (PDI) is anchored with phenyl substituents at the imide N site, followed by thionation, yielding a series of thione products 1S-PDI-D, 2S-cis-PDI-D, 2S-trans-PDI-D, 3S-PDI-D, and 4S-PDI-D, respectively, with n = 1, 2, 3, and 4 thione. The photophysical properties are dependent on the number of anchored thiones, where the observed prominent lower-lying absorption is assigned to the S0 → S2(ππ*) transition and is red-shifted upon increasing the number of thiones; the lowest-lying excited state is ascribed to a transition-forbidden S1(nπ*) configuration. All nS-PDIs are non-emissive in solution but reveal an excellent two-photon absorption cross-section of >800 GM. Supported by the femtosecond transient absorption study, the S1(nπ*) → T1(ππ*) intersystem crossing (ISC) rate is > 1012 s-1, resulting in ∼100% triplet population. The lowest-lying T1(ππ*) energy is calculated to be in the order of 1S-PDI-D > 2S-cis-PDI-D ∼ 2S-trans-PDI-D > 3S-PDI-D > 4S-PDI-D, where the T1 energy of 1S-PDI-D (1.10 eV) is higher than that (0.97 eV) of the 1O2 1Δg state. 1S-PDI-D is further modified by either conjugation with peptide FC131 on the two terminal sides, forming 1S-FC131, or linkage with peptide FC131 and cyanine5 dye on each terminal, yielding Cy5-1S-FC131. In vitro experiments show power of 1S-FC131 and Cy5-1S-FC131 in recognizing A549 cells out of other three lung normal cells and effective photodynamic therapy. In vivo, both molecular composites demonstrate outstanding antitumor ability in A549 xenografted tumor mice, where Cy5-1S-FC131 shows superiority of simultaneous fluorescence tracking and targeted photodynamic therapy.

Multiple Emission of Phosphonium Fluorophores Harnessed by the Pathways of Photoinduced Counterion Migration.

In the emerging field of intramolecular charge transfer induced counterion migration, we report the new insights into photophysical features of luminescent donor-acceptor phosphonium dyes (D-π-)n A+ [X- ] (π=-(C6 H4 )x -). The unique connectivity of the phosphorus atom affords multipolar molecules with a variable number of arms and the electronic properties of the acceptor group. In the ion-paired form, the transition from dipolar to quadrupolar configuration enhances the low energy migration-induced band by providing the additional pathways for anion motion. The multipolar architecture, adjustable lengths of the π-spacers and the nature of counterions allow for efficient tuning of the emission and achieving nearly pure white light with quantum yields around 30 %. The methyl substituent at the phosphorus atom reduces the rate of ion migration and suppresses the red shifted bands, simultaneously improving total emission intensity. The results unveil the harnessing of the multiple emission of phosphonium fluorophores by anion migration via structure and branching of donor-acceptor arms.

Fabrication of Circularly Polarized MR-TADF Emitters with Asymmetrical Peripheral-Lock Enhancing Helical B/N-Doped Nanographenes.

Circularly polarized thermally activated delayed fluorescence (CP-TADF) and multiple-resonance thermally activated delayed fluorescence (MR-TADF), which exhibit novel circularly polarized luminescence and excellent color fidelity, respectively, have gained immense popularity. In this study, integrated CP-TADF and MR-TADF (CPMR-TADF) are prepared by strategic design and synthesis of asymmetrical peripherally locked enantiomers, which are separated and denoted as (P,P″,P″)-/(M,M″,M″)-BN4 and (P,P″,P″)-/(M,M″,M″)-BN5 and exhibit TADF and circularly polarized light (CPL) properties. As the entire molecular frame participates in the frontier molecular orbitals, the resulting helical chirality of (+)/(-)-BN4- and (+)/(-)-BN5-based solution-processed organic light-emitting diodes (OLEDs) helps in achieving a narrow full width at half maximum (FWHM) of 49/49 and 48/48 nm and a high maximum external quantum efficiency (EQE) of 20.6%/19.0% and 22.0%/26.5%, respectively. Importantly, unambiguous circularly polarized electroluminescence signals with dissymmetry factors (gEL ) of +3.7 × 10-3 /-3.1 × 10-3 (BN4) and +1.9 × 10-3 /-1.6 × 10-3 (BN5) are obtained. The results indicate successful exploitation of CPMR-TADF-emitter-based OLEDs to exhibit three characteristics: high efficiency, color purity, and circularly polarized light.

The Observation of Interchain Motion in Self-Assembled Crystalline Platinum(II) Complexes: An Exquisite Case but By No Means the Only One in Molecular Solids.

In organic and organometallic solids, upon electronic excitation, most intermolecular structural relaxations follow a pathway along the π-π stacking direction or metal-metal bond with significant coupling strength. Differently, we discovered that the self-assembled platinum(II) complexes, Pt(fppz)2, exhibit an unusual interchain contraction. The ground-state and excited-state multiple local minima were distinguished by temperature-dependent excitation/emission spectra, indicating the existence of multiple local minima. The time-resolved emission decay revealed the excited-state structural relaxation lifetime with τobs = 41 ns at 298 K. Temperature-dependent X-ray diffraction analysis showed that the packing geometries contract 0.6 Å along the interchain direction (a-axis) at 50 K compared to the geometries at 298 K. Such structural displacements render the slow internal conversion rate in the excited states. We thus demonstrate the correlation between the packing geometries and the excited-state dynamics of the self-assembled Pt(II) complexes, shedding light on the unique direction of interchain structural deformation of the molecular aggregates.