Impact of iodine loading and substitution position on intersystem crossing efficiency in a series of ten methylated-meso-phenyl-BODIPY dyes

Triplet-Triplet Annihilation Upconverting Liposomes: Mechanistic Insights into the Role of Membranes in Two-Dimensional TTA-UC

Triplet-triplet annihilation upconversion (TTA-UC) implemented in nanoparticle assemblies is of emerging interest in biomedical applications, including in drug delivery and imaging. As it is a bimolecular process, ensuring sufficient mobility of the sensitizer and annihilator to facilitate effective collision in the nanoparticle is key. Liposomes can provide the benefits of two-dimensional confinement and condensed concentration of the sensitizer and annihilator along with superior fluidity compared to other nanoparticle assemblies. They are also biocompatible and widely applied across drug delivery modalities. However, there are relatively few liposomal TTA-UC systems reported to date, so systematic studies of the influence of the liposomal environment on TTA-UC are currently lacking. Here, we report the first example of a BODIPY-based sensitizer TTA-UC system within liposomes and use this system to study TTA-UC generation and compare the relative intensity of the anti-Stokes signal for this system as a function of liposome composition and membrane fluidity. We report for the first time on time-resolved spectroscopic studies of TTA-UC in membranes. Nanosecond transient absorption data reveal the BODIPY-perylene dyad sensitizer has a long triplet lifetime in liposome with contributions from three triplet excited states, whose lifetimes are reduced upon coinclusion of the annihilator due to triplet-triplet energy transfer, to a greater extent than in solution. This indicates triplet energy transfer between the sensitizer and the annihilator is enhanced in the membrane system. Molecular dynamics simulations of the sensitizer and annihilator TTA collision complex are modeled in the membrane and confirm the co-orientation of the pair within the membrane structure and that the persistence time of the bound complex exceeds the TTA kinetics. Modeling also reliably predicted the diffusion coefficient for the sensitizer which matches closely with the experimental values from fluorescence correlation spectroscopy. The relative intensity of the TTA-UC output across nine liposomal systems of different lipid compositions was explored to examine the influence of membrane viscosity on upconversion (UC). UC showed the highest relative intensity for the most fluidic membranes and the weakest intensity for highly viscous membrane compositions, including a phase separation membrane. Overall, our study reveals that the co-orientation of the UC pair within the membrane is crucial for effective TTA-UC within a biomembrane and that the intensity of the TTA-UC output can be tuned in liposomal nanoparticles by modifying the phase and fluidity of the liposome. These new insights will aid in the design of liposomal TTA-UC systems for biomedical applications.

Engineering Metal Halide Perovskite Nanocrystals with BODIPY Dyes for Photosensitization and Photocatalytic Applications

The sensitization of surface-anchored organic dyes on semiconductor nanocrystals through energy transfer mechanisms has received increasing attention owing to their potential applications in photodynamic therapy, photocatalysis, and photon upconversion. Here, we investigate the sensitization mechanisms through visible-light excitation of two nanohybrids based on CsPbBr3 perovskite nanocrystals (NC) functionalized with borondipyrromethene (BODIPY) dyes, specifically 8-(4-carboxyphenyl)-1,3,5,7-tetramethyl-4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (BDP) and 8-(4-carboxyphenyl)-2,6-diiodo-1,3,5,7-tetramethyl-4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (I2-BDP), named as NC@BDP and NC@I2-BDP, respectively. The ability of I2-BDP dyes to extract hot hole carriers from the perovskite nanocrystals is comprehensively investigated by combining steady-state and time-resolved fluorescence as well as femtosecond transient absorption spectroscopy with spectroelectrochemistry and quantum chemical theoretical calculations, which together provide a complete overview of the phenomena that take place in the nanohybrid. Förster resonance energy transfer (FRET) dominates (82%) the photosensitization of the singlet excited state of BDP in the NC@BDP nanohybrid with a rate constant of 3.8 ± 0.2 × 1010 s-1, while charge transfer (64%) mediated by an ultrafast charge transfer rate constant of 1.00 ± 0.08 × 1012 s-1 from hot states and hole transfer from the band edge is found to be mainly responsible for the photosensitization of the triplet excited state of I2-BDP in the NC@I2-BDP nanohybrid. These findings suggest that the NC@I2-BDP nanohybrid is a unique energy transfer photocatalyst for oxidizing α-terpinene to ascaridole through singlet oxygen formation.

Spin-Vibronic Coupling Controls the Intersystem Crossing of Iodine-Substituted BODIPY Triplet Chromophores

4,4-Difluoro-4-borata-3a-azonia-4a-aza-s-indacene (BODIPY) dyes are extensively used in various applications of their triplet states, ranging from photoredox catalysis, through triplet sensitization to photodynamic therapy. However, the rational design of BODIPY triplet chromophores by ab initio modelling is limited by their strong interactions of spin, electronic and vibrational dynamics. In particular, spin-vibronic coupling is often overlooked when estimating intersystem crossing (ISC) rates. In this study, a combined experimental and theoretical approach using spin-vibronic coupling to correctly describe ISC in BODIPY dyes was developed. For this purpose, seven π-extended BODIPY derivatives with iodine atoms in different positions were examined. It was found that the heavy-atom effect of iodine atoms is site specific, causing high triplet yields in only some positions. This site-specific ISC was explained by El-Sayed rules, so both the contribution and character of the molecular orbitals involved in the excitation must be considered when predicting the ISC rates. Overall, the rational design of BODIPY triplet chromophores requires using (i) the high-quality electronic structure theory, including both static and dynamical correlations; and (ii) the two-component wave function Hamiltonian, and rationalizing; and (iii) ISC based on the character of the molecular orbitals of heavy atoms involved in the excitation, expanding El-Sayed rules beyond their traditional applications.

Switchable Optical Properties of Dyes and Nanoparticles in Electrowetting Devices

The optical properties of light-absorbing materials in optical shutter devices are critical to the use of such platforms for optical applications. We demonstrate switchable optical properties of dyes and nanoparticles in liquid-based electrowetting-on-dielectric (EWOD) devices. Our work uses narrow-band-absorbing dyes and nanoparticles, which are appealing for spectral-filtering applications targeting specific wavelengths while maintaining device transparency at other wavelengths. Low-voltage actuation of boron dipyromethene (BODIPY) dyes and nanoparticles (Ag and CdSe) was demonstrated without degradation of the light-absorbing materials. Three BODIPY dyes were used, namely Abs 503 nm, 535 nm and 560 nm for dye 1 (BODIPY-core), 2 (I2BODIPY) and 3 (BODIPY-TMS), respectively. Reversible and low-voltage (≤20 V) switching of dye optical properties was observed as a function of device pixel dimensions (300 × 900, 200 × 600 and 150 × 450 µm). Low-voltage and reversible switching was also demonstrated for plasmonic and semiconductor nanoparticles, such as CdSe nanotetrapods (abs 508 nm), CdSe nanoplatelets (Abs 461 and 432 nm) and Ag nanoparticles (Abs 430 nm). Nanoparticle-based devices showed minimal hysteresis as well as faster relaxation times. The study presented can thus be extended to a variety of nanomaterials and dyes having the desired optical properties.

Near-infrared light-triggered prodrug photolysis by one-step energy transfer

Prodrug photolysis enables spatiotemporal control of drug release at the desired lesions. For photoactivated therapy, near-infrared (NIR) light is preferable due to its deep tissue penetration and low phototoxicity. However, most of the photocleavable groups cannot be directly activated by NIR light. Here, we report a upconversion-like process via only one step of energy transfer for NIR light-triggered prodrug photolysis. We utilize a photosensitizer (PS) that can be activated via singlet-triplet (S-T) absorption and achieve photolysis of boron-dipyrromethene (BODIPY)-based prodrugs via triplet-triplet energy transfer. Using the strategy, NIR light can achieve green light-responsive photolysis with a single-photon process. A wide range of drugs and bioactive molecules are designed and demonstrated to be released under low-irradiance NIR light (100 mW/cm2, 5 min) with high yields (up to 87%). Moreover, a micellar nanosystem encapsulating both PS and prodrug is developed to demonstrate the practicality of our strategy in normoxia aqueous environment for cancer therapy. This study may advance the development of photocleavable prodrugs and photoresponsive drug delivery systems for photo-activated therapy.

Transforming Dyes into Fluorophores: Exciton-Induced Emission with Chain-like Oligo-BODIPY Superstructures

Herein we present a systematic study demonstrating to which extent exciton formation can amplify fluorescence based on a series of ethylene-bridged oligo-BODIPYs. A set of non- and weakly fluorescent BODIPY motifs was selected and transformed into discrete, chain-like oligomers by linkage via a flexible ethylene tether. The prepared superstructures constitute excitonically active entities with non-conjugated, Coulomb-coupled oscillators. The non-radiative deactivation channels of Internal Conversion (IC), also combined with an upstream reductive Photoelectron Transfer (rPET) and Intersystem Crossing (ISC) were addressed at the monomeric state and the evolution of fluorescence and (non-)radiative decay rates studied along the oligomeric series. We demonstrate that a "masked" fluorescence can be fully reactivated irrespective of the imposed conformational rigidity. This work challenges the paradigm that a collective fluorescence enhancement is limited to sterically induced motional restrictions.