5-Hydroxyflavone (5HF) is a naturally occurring flavonol with a hydroxyl group at the C5 position and shows unusual proton-transfer properties with a very low fluorescence quantum yield, which justifies its role as a natural UV filter. Using the CASPT2//CASSCF method to study the mechanistic photophysics of its two-water hydrogen-bonded complex 5HF-2H2O (referred to as 5HF-2W), we have identified four competitive S2(ππ*) radiationless relaxation channels from the Franck-Condon (FC) point. The first is barrierless excited-state intramolecular proton transfer (ESIPT) to generate the 1ππ*-T tautomer, which further evolves toward the nearby 1ππ*/S0-T conical intersection and then deactivates back to the S0 state, followed by favorable reverse ground-state proton transfer. The second is indirect 1ππ*→3ππ* intersystem crossing (ISC) mediated by the dark 1nπ* state. In this route, the 1ππ* state first hops to the 1nπ* state via the 1ππ*/1nπ*-N conical intersection, followed by 1nπ*→3ππ* ISC at the 1nπ*/3ππ*/3nπ*-N intersection structure to reach the 3ππ* state, enhanced by the CASPT2-computed large 1nπ*/3ππ* spin-orbit coupling (SOC) of 30.2 cm-1. The generated 3ππ* state undergoes ESIPT by overcoming a 3.5 kcal/mol energy barrier to yield the 3ππ*-T tautomer, which subsequently runs into the nearby 3ππ*/S0-T crossing point and hops to the S0 state. The third is similar to the second one, but its ISC is relayed by the 3nπ* state. At the 1nπ*/3ππ*/3nπ*-N intersection structure, it first transfers to the 3nπ* state (1nπ*/3nπ* SOC: 17.4 cm-1) and then hops to the 3ππ* state through 3nπ*→3ππ* internal conversion (IC) at the 3nπ*/3ππ*-N conical intersection, which is followed by direct ISC from T1 to S0 via the 3ππ*/S0-N crossing point. The last one is direct 1ππ*→S0 IC from the FC region through the 1ππ*/S0-N conical intersection. This work contributes to the understanding of the photophysics of 5HF-based flavonoids and their analogues.

Phosphorescence properties and bioimaging of pyridine auxiliary ligand cyclometalated Pt(II) complex with multiphoton absorption

Understanding how substitution topology governs excited-state processes is central to the rational design of high-performance organic emitters. Herein, eighteen symmetrically substituted D-A-D molecules were designed by combining a naphthalimide acceptor with nine electron-donating groups, and their electronic and photophysical properties were systematically investigated using density functional theory. Symmetric donor substitution at the C3,C6 and C4,C5 positions of the naphthalimide core was considered to elucidate regio-dependent structure-property relationships. In general, the 4,5-disubstituted derivatives exhibit smaller singlet-triplet energy gaps (ΔEST) in the ground state, except for the diphenylamine derivative, primarily due to increased donor-acceptor twisting. In contrast, several 3,6-disubstituted derivatives bearing strong donors also display reduced ΔEST values. Molecules with smaller root-mean-square deviations between optimized S1 and S0 geometries undergo reduced structural reorganization upon excitation, which is expected to suppress non-radiative decay via internal conversion and favour higher radiative rates. Accordingly, the 3,6-disubstituted derivatives generally show larger oscillator strengths and higher radiative decay rates than their 4,5-substituted counterparts. Notably, 3,6-disubstituted systems incorporating diphenylamine and 10,11-dihydro-5H-dibenzo[b,f]azepine exhibit particularly high radiative rates. Intersystem crossing (ISC) rates are typically larger for the 3,6-disubstituted derivatives, with deviations attributable to differences in spin-orbit coupling matrix elements, while the 5H-dibenzo[b,f]azepine derivatives show consistently high ISC rates irrespective of the substitution pattern. Reverse intersystem crossing (RISC) is most efficient for phenoxazine- and 5-phenyl-5,10-dihydrophenazine-based systems, largely independent of regio-substitution. Overall, while 3,6-disubstitution often promotes stronger radiative and ISC processes, donor identity plays a decisive role in facilitating RISC and enabling TADF behaviour. These results provide clear design insights into how regio-controlled substitution and donor selection jointly govern fluorescence, phosphorescence, and TADF pathways in naphthalimide-based emitters.

The realization of high-efficiency thick-film organic solar cells (OSCs) is crucial for scalable manufacturing yet remains challenging due to limited exciton diffusion. Here, we introduce a magnetic strategy by incorporating two-dimensional (2D) ferromagnetic MoPS3 nanocrystal into the active layer to manipulate exciton spin dynamics. We demonstrate that the interaction between the ferromagnetic MoPS3 nanocrystal and excitons promotes the formation of weak intrinsic magnetic fields within the active layer. These fields effectively promote the intersystem crossing (ISC) from short-lived singlet excitons to long-lived triplet excitons, thereby extending exciton diffusion length and reducing non-radiative recombination losses. Consequently, MoPS3 nanocrystal doped D18-Cl:L8-BO devices achieve power conversion efficiencies of 20.37% at an active layer thickness of 100 nm and 19.36% (19.13% certified value) at an active layer thickness of 300 nm, representing one of the highest reported values for thick-film ( > 300 nm) OSCs. Universal applicability is demonstrated with power conversion efficiencies of 20.91%/19.63% (D18:L8-BO) and 19.13%/17.92% (PM6:Y6) at 100/300 nm. This work establishes 2D ferromagnetic MoPS3 nanocrystal as effective spin manipulators in organic semiconductors and provides a universal strategy to overcome the critical thickness-performance trade-off in OSCs.

Controllable photodynamic therapy (PDT)-triggered ferroptosis is a highly effective precisely controlled tumor therapy. However, conventional "always-on" type-I PDT agents suffer from poor tumor specificity, limiting therapeutic precision. Herein, a novel tumor-penetrating phosphorescent nanoagent (iRGD-mediated organometallic complex nanoparticles, iPNs) was developed for hypoxia-activated near-infrared-II (NIR-II) "turn-on" bioimaging and type-I PDT. The prominent intersystem crossing (ISC) property of the organometallic complex (PdTCPP) facilitated both hypoxia-activated NIR-II phosphorescence and efficient type-I PDT in iPNs. Moreover, conjugation with the iRGD peptide enhanced the tumor penetration of the nanoagent. The hypoxia-activated NIR-II phosphorescence of iPNs, combined with their enhanced tumor penetration capability, resulted in a 5-fold higher tumor-to-normal tissue (T/NT) ratio than that induced by conventional NIR-II probes. This significantly improved tumor specificity. Subsequently, iPNs-triggered type-I PDT significantly promoted the intracellular accumulation of superoxide anion radicals (O2 •-) and hydroxyl radicals (·OH) in cancer cells, thereby inhibiting tumor growth. Proteomic analysis of both cells and tumor tissues further revealed the key ferroptosis and apoptosis pathways. Overall, this study demonstrates that the organometallic complex nanoagent with hypoxia-activated NIR-II bioimaging capability has the potential to guide precision type-I PDT to induce ferroptosis.

We theoretically investigated vibronic coupling responsible for nonradiative transitions, i.e., internal conversion (IC) and intersystem crossing (ISC), in xanthone. The nonradiative decay pathway of aromatic ketones is often debated because of their fast ISC. Xanthone in the gas phase follows a pathway that obeys El-Sayed's rule, namely, IC from the 1ππ* to 1nπ* states and ISC from the 1nπ* to 3ππ* states, of which a simple pathway is adequate for analyzing vibronic structures. We employed an expression for the nonradiative rate constant based on Fermi's golden rule within the mixed-spin crude adiabatic approximation, which has the advantage that both IC and ISC can be considered as equally vibronically induced transitions. Our calculations showed that the IC from the 1ππ* to 1nπ* state was faster than ISC channels because of stronger vibronic coupling and less favorable spin-orbit (SO) coupling to nearby triplets. In addition, the ISC from 1nπ* to 3ππ* was faster than that from 1nπ* to 3nπ* because of the large structural displacement and small energy gap. This study can provide guidelines for determining whether IC or ISC dominates, depending on the balance between vibronic coupling, SO coupling, and the energy gap, thereby informing molecular design for controlled nonradiative decay. ISC can dominate its IC counterpart in a xanthone derivative by tuning the singlet-triplet energy gap and/or the SO coupling.

Designing efficient organic near-infrared (NIR) photosensitizers (PSs) is crucial for improving photodynamic therapy (PDT) against tumors. However, their practical application is often hindered by suboptimal performance and an incomplete understanding of intersystem crossing (ISC) dynamics. Herein, we propose a terminal-group modulation strategy for constructing A-D-A'-D-A-type NIR PSs with an enhanced ISC efficiency. Three π-conjugated oligomers (O1-O3) were synthesized by integrating identical D-A'-D cores with distinct terminal acceptor units. The resulting nanoparticles (ONPs 1 - ONPs 3) exhibited comparable morphology, particle size, optical absorption, and emission profiles. Notably, ONPs 1 demonstrated substantially superior reactive oxygen species (ROS) generation compared with those of ONPs 2 and ONPs 3. Theoretical calculations revealed that the benzene terminal group in ONPs 1 significantly enhanced ISC efficiency (up to 28%), attributed to a reduced singlet-triplet energy gap (ΔEST), diminished oscillator strength (f), and an increased spin-orbit coupling (SOC) constant (λ). These features facilitate efficient conversion of singlet (S1) excitons to triplet (T1) states, thereby promoting either energy transfer to molecular oxygen or electron transfer to surrounding acceptors, ultimately boosting ROS production during PDT. Consequently, ONPs 1 achieved the highest ROS generation capability (6.8-fold higher than indocyanine green, ICG) and a markedly enhanced singlet oxygen (1O2) yield (2.2% vs 0.2% for ICG). In addition to 1O2, ONPs 1 was also confirmed to generate hydroxyl radicals (•OH). Collectively, these advantages enable ONPs 1 to achieve potent type-I and type-II synergistic PDT efficacy in both in vitro and in vivo models. This work provides a rational design guideline for developing high-performance organic NIR photosensitizers with enhanced ISC efficiency for advanced photodynamic cancer therapy.

Photon-driven pyroptosis represents an advanced and promising modality in anti-tumor immunotherapy, offering new avenue in combating cancer recurrence and metastasis. Essential metal-complex shows great potential in photoimmunotherapy, which however hardly generates reactive oxygen species (ROS) to active photoimmunotherapy under light irradiation due to low spin-orbit coupling (SOC) resulting in intersystem crossing (ISC) being blocked. To address this challenge, we innovatively proposed the concept of essential-metal ion as an electron-withdrawing group to modulate the donor-acceptor (D-A) system, thereby significantly improving ROS generation efficiency. As a proof of concept, we designed and synthesized a series of Zn2+-complexes through constructing enhanced D-A systems. Zn2+ coordination efficiently enhances the ISC by lowering the excited singlet-triplet energy gap (ΔES1-T1), leading to exceptionally high ROS generation efficiency through Type I mechanisms to overcome hypoxia and the immunosuppression of tumor microenvironment. In these Zn2+-complexes, Zn-TPY-TPA-DTZ can photodynamically damage lysosomes to trigger pyroptosis, which further induce anti-tumor immune responses through the promotion of immunogenic cell death (ICD), and ultimately generates a robust anti-tumor immune response and curbed the proliferation of 4T1 tumors in vivo. This study provides systematic guidance for the rational design of advanced Zn2+-complexes, propelling advancements in cancer treatment strategies.

Singlet oxygen and superoxide anion radicals represent two key active species in photocatalytic reactions. However, most conventional catalysts depend exclusively on one of these species or necessitate altered reaction conditions to access different active intermediates. In this work, we designed and synthesized three novel two-dimensional polyimide-based covalent organic frameworks (COFs), in which the electron-donating phthalocyanine and electron-accepting benzothiadiazole units are closely stacked, forming an alternating π-column architecture. These donor-acceptor COFs exhibit pronounced spatial charge separation, which not only promotes charge-carrier separation and electron transport but also narrows the energy gap between singlet and triplet excited states, enhances intersystem crossing (ISC) efficiency, and enables the simultaneous and efficient execution of both electron-transfer and energy-transfer processes. Consequently, this study not only expands the potential for functionalizing highly stable 2D COFs but also offers new perspectives on the conversion of solar energy into chemical energy using COF-based systems.

We studied the intersystem crossing (ISC) of neutral, monoanion, and dianion of a series of phenyl, pyrazole, and triazole fused perylene bisimides (PBI) derivatives. The neutral compounds exhibit high fluorescence quantum yields (ΦF = 66.5%-91.9%) and low singlet oxygen quantum yields (ΦΔ < 7.8%), with no triplet state signals detected via femtosecond transient absorption (fs-TA) or nanosecond transient absorption (ns-TA) spectral measurements. Time-resolved electron paramagnetic resonance (TREPR) revealed an e,e,e,a,a,a electron spin polarization (ESP) for the T1 state of these PBIs. Calculations of spin-orbit coupling matrix elements (SOCMEs) and g-tensor anisotropy both confirm the weak spin-orbit coupling (SOC) of the molecules, accounting for their low ISC efficiency. Pulse electron paramagnetic resonance (pulse EPR) measurements of radical anions showed that PBI derivatives lacking pyrazole/triazole moieties have much longer transverse relaxation times (Tm = 2.5-2.6 µs) than the other compounds. Additionally, ns-TA spectra confirmed ISC in PBI dianions, which possess long-lived triplet states (213-558 µs). These species can be used as super strong reductants in photocatalytic redox organic reactions.

Long-lived triplet excitons in polymers are crucial for driving oxygen reduction reaction (ORR) in photocatalytic H2O2 production, but their formation is typically limited by weak spin-orbit coupling (SOC) and a large singlet-triplet splitting energy (ΔEST). Here we present a "dual-polarized" strategy in a novel donor-acceptor (D-A) polymer (MQDP) containing S═N─C and C═N─C linkages. MQDP polymer was prepared via supramolecular precursor polymerization of acenaphthenequinone (AQ), dibenzothiophene-5-oxide (DPO), and melem (ME). Compared to the single-polarized analogue MQP (τp = 672 µs), the dual-polarized units in MQDP enhance SOC and reduce ΔEST, thereby generating multiple singlet-to-triplet intersystem crossing (ISC) transfer channelsto produce long-lived triplet excitons (τp = 868 µs). In addition, the dual-polarized MQDP enriches surface-active sites for O2 adsorption, effectively reducing the energy barrier for ORR. The MQDP achieves a remarkable H2O2 generation rate of 15.38 mmol g-1 h-1 under visible light irradiation and ambient air, nearly 1.9 times higher than that of MQP (8.13 mmol g-1 h-1). These findings demonstrate the effectiveness of the dual-polarized design in tuning exciton dynamics and surface reactivity of D-A polymers for enhanced photocatalysis.

ABSTRACT Long‐lived triplet excitons in polymers are crucial for driving oxygen reduction reaction (ORR) in photocatalytic H 2 O 2 production, but their formation is typically limited by weak spin‐orbit coupling (SOC) and a large singlet–triplet splitting energy (Δ E ST ). Here we present a “dual‐polarized” strategy in a novel donor–acceptor (D–A) polymer (MQDP) containing S═N─C and C═N─C linkages. MQDP polymer was prepared via supramolecular precursor polymerization of acenaphthenequinone (AQ), dibenzothiophene‐5‐oxide (DPO), and melem (ME). Compared to the single‐polarized analogue MQP ( τ p = 672 µs), the dual‐polarized units in MQDP enhance SOC and reduce Δ E ST , thereby generating multiple singlet‐to‐triplet intersystem crossing (ISC) transfer channels to produce long‐lived triplet excitons ( τ p = 868 µs). In addition, the dual‐polarized MQDP enriches surface−active sites for O 2 adsorption, effectively reducing the energy barrier for ORR. The MQDP achieves a remarkable H 2 O 2 generation rate of 15.38 mmol g −1 h −1 under visible light irradiation and ambient air, nearly 1.9 times higher than that of MQP (8.13 mmol g −1 h −1 ). These findings demonstrate the effectiveness of the dual‐polarized design in tuning exciton dynamics and surface reactivity of D–A polymers for enhanced photocatalysis.

Naphthalene-to-azulene isoelectronic structural reconstruction in perylene (P), perylenediimide (PDI), and its chalcogenides (X-PDI, X = O, S, Se), similar to the Stone-Wales defect in graphene, may significantly alter the intrinsic electronic structure and thus poses scientific curiosity about how and to what extent their structure-function relationships change with such reconstruction. Structural, electronic, and photophysical properties for the reconstructed analogues of P (rP) and X-PDI (X-rPDI) are studied for the first time, adopting polarization-consistent optimally tuned range-separated hybrid (OT-RSH) in toluene. All X-rPDIs, including rP are found to be planar and dynamically stable, with thermodynamic formation energies comparable to those of their pristine congeners, indicating synthetic feasibility. The complex interplay of chalcogens and reconstruction produces an increased electronic gap in S/Se-rPDI compared to their respective PDI analogues, which, in competition with varied exciton binding energy in X-rPDIs produce red- and blue-shifted lowest excited singlet (S1) and triplet (T1 > 1.0 eV), respectively. Optically forbidden S1 in rP and all X-rPDIs, with closely lying optically bright Sn suggests fluorescence turn-off. Similar ππ* excitonic characters and lesser chalcogen contributions yield relatively smaller intersystem crossing (ISC) rates for X-rPDIs than X-PDIs. The rate increases down the chalcogen group for both X-rPDIs and X-PDIs due to gradually increased heavy-atom effects. Interestingly, reconstruction lowers the excited singlet-triplet gap and generates nonzero spin-orbit coupling, yielding ∼4 orders higher ISC rates in rP and O-rPDI compared to their pristine analogues. Further, while S-rPDI shows ∼6 orders smaller rate than S-PDI, both Se-rPDI and Se-PDI display remarkably high ISC rates (∼1012-1013 s-1). Importantly, Se-rPDI with moderately high energy T1 and a considerably large ISC rate, could serve as a better triplet photosensitizer than Se-PDI. These insights into the reconstruction-tailored structure-function relationships will help to design new azulene-based functional organic molecules.

p-Benzoyl-l-phenylalanine (pBzF) is a widely used noncanonical amino acid (ncAA) that expands the chemical repertoire of proteins. Its benzophenone (BP) chromophore undergoes near-quantitative intersystem crossing (ISC) to a triplet state, furnishing a highly efficient, site-addressable photoreactive handle. Beyond photochemistry, the bulky, hydrophobic side chain introduces distinct steric and electronic effects that enable new reactivity in protein active sites. Genetic incorporation of pBzF in vivo, including directed evolution, has unlocked applications ranging from site-specific photo-crosslinking for interaction mapping to engineering antibody fragments, sharpening monoclonal antibody (mAb) epitope recognition, and creating protein-based photocatalysts. pBzF has also proved powerful for mechanistic studies by stabilizing short-lived intermediates. More recently, pBzF-containing proteins have been leveraged in light-driven transformations, including [2+2] photocycloadditions, deracemizations, and dehalogenations, and in the construction of artificial photosynthetic systems. This review critically discusses these advances and establishes pBzF as a versatile photochemical and structural motif for building proteins with non-natural, light-responsive, and catalytically competent functions.

A new study is presented to elucidate the photodynamics of model carbonyl-substituted polyaromatics targeting the relevance of carbonyl-group orientation and torsional re-equilibration on intersystem crossing (ISC). Our experiments focused on 9-acetylanthracene (9AA) using femtosecond resolved spectroscopy. In the ground state of this molecule, steric interactions force the carbonyl substituent into a near-perpendicular orientation relative to the aromatic system. The time-resolved signals from 9AA show that ISC takes place after spectral shifts that reflect the evolution of the carbonyl group to a slanted geometry as it adjusts to a dihedral angle of around 40° with respect to the aromatic plane. Depending on the solvent, in 9AA manifold crossing takes place on the 3 to 25 ps time-scale. On the other hand, for 2-acetylanthracene (2AA) which is coplanar in both S0 and S1, the emission lifetimes can reach several nanoseconds. Analysis of these systems at the highest available theoretical levels reveals further insights into the excited-state dynamics. For 9AA and in contrast with previous publications, it is established that for all relevant geometries, the first excited singlet retains a ππ* character and decays through ISC with no involvement of other singlet states. The manifold crossing involves the interaction with the triplet manifold through states which's transition orbitals are partially localized at the acetyl substituent. Specifically, the slanted geometry of the carbonyl group in 9AA and the potential energy surface around the equilibrium S1 geometry implies significant spin-orbit interactions and accelerated manifold-crossings. The present results highlight the relevance of substituent reorientation and their slanted geometries which appear to be a dominant feature in carbonyl and nitrated aromatic systems which show rapid ISC dynamics. In the article, we include details on the differences in the mechanisms operating in these two kinds of systems which show the fastest ISC rates among organic chromophores.

The photophysical properties of a BODIPY derivative with the highly twisted molecular structure of anthracene-fused boron-dipyrromethene (AN-BDP) were studied with steady-state and time-resolved spectroscopic methods. The fused anthryl and the BDP units in AN-BDP units both adopt distorted geometry (with ca. 10° of torsion), and there is large dihedral angle between the two units (ca. 49.7°). Interestingly, the fluorescence quantum yields are highly dependent on the solvent polarity (59~3%, from toluene to acetonitrile), yet the fluorescence emission wavelength does not change in different solvents. Nanosecond transient absorption spectra indicate that the triplet state is long-lived, with an intrinsic triplet state lifetime of 551 μs. Interestingly the severely twisted structure only shows a moderate intersystem crossing (ISC) yield (10%). Femtosecond transient absorption spectra indicate slow ISC (>1.5 ns), which is in agreement with the fluorescence lifetime (2.3 ns). Time-resolved electron paramagnetic resonance (TREPR) spectra show smaller zero-field-splitting D and E tensors as (-71.4 mT, 16.7 mT, respectively) compared to the triplet state of the iodinated native BDP (D = -104.6 mT, E = 22.8 mT), inferring that the triplet-state wave function of the new compound is delocalized over the twisted molecular framework. The theoretical computation indicated a solvent-polarity-dependent energy barrier for the relaxed S1 state to a conical interaction (CI) of the S1 and the S0 state potential curves, which agrees with the weaker fluorescence in polar solvents.

Recent development in the field of organic luminescent materials has gained tremendous attention, particularly due to the materials which exhibit efficient room temperature phosphorescence (RTP). The extensive research in the field has led to variety of metal-free pure organic systems exhibiting excellent performance such as high quantum yield, longer lifetime, and afterglow at ambient conditions. A wide range of strategies were developed to enhance the RTP efficiency, among which the incorporation of chalcogen atoms such as O, S, Se, and Te at bridging position in pure organic system has emerged as a promising strategy. The bridging of the chalcogens atoms not only facilitates the strong spin orbit coupling (SOC) and efficient intersystem crossing (ISC), but also provide more rigid molecular framework to reduce the nonradiative decay pathways and stabilizes the triplet excitons, resulting in RTP with higher quantum yield and longer lifetime. This review focuses on the development of bridged chalcogen-containing pure organic RTP materials with various strategies to achieve more efficient RTP and their applications. In particular, we discuss and compare a broad range of chalcogen containing organic phosphors, highlighting their structural differences and design strategies that affect the phosphorescence under ambient condition.

Multifunctional near-infrared fluorescent probe for imaging of hydrogen peroxide and photodynamic therapy of amyloid-β aggregation in Alzheimer's disease.

ABSTRACT Gel‐based room‐temperature phosphorescence (RTP) materials have garnered significant attention due to their promising applications in flexible electronics and photonics. However, the inherent swollen state and porous architecture of such gels often promote intense molecular motion and facilitate oxygen diffusion, which can severely quench phosphorescence under ambient conditions. In this work, we report a versatile strategy for constructing high‐performance organic RTP materials by leveraging organic aerogels, which exhibit superior luminescent, mechanical, and thermal properties. Owing to their structural advantages, these organic aerogels possess a three‐dimensional rigid framework that enhances intersystem crossing (ISC) efficiency and promotes multiple intermolecular interactions, thereby enabling efficient RTP with an ultralong phosphorescent lifetime of up to 1007 ms. Notably, the resulting RTP aerogels demonstrate exceptional structural robustness (compression modulus of 1 MPa), excellent thermal insulation (peak heat release rate reduced to 31.1 kW/m 2 ), and outstanding flame retardancy (limiting oxygen index exceeding 90%), positioning them among the most multifunctional organic aerogels reported to date. Given their balanced combination of RTP performance, mechanical resilience, and thermal stability, these phosphorescent aerogels represent a highly promising platform for the development of advanced, multifunctional organic RTP materials.

A series of 1-methylpyridinium-substituted brominated distyryl-BODIPY dyes, PyBXI (X = H, M, or Br), was synthesized to achieve cooperative singlet oxygen (1O2) production through qualitatively different dual intersystem crossing (ISC) pathways: spin-orbit charge-transfer ISC (SOCT-ISC) and heavy-atom-induced ISC. Upon photoexcitation of the PyBXI dyes, charge-transfer states were preferentially formed through photoinduced electron transfer from the distyryl-BODIPY core to the 1-methylpyridinium moiety, however, followed by nonradiative charge recombination rather than the desired SOCT-ISC. This resulted in negligible fluorescence and 1O2 quantum yields in the non-brominated dye PyBHI. The introduction of bromine atoms improved 1O2 quantum yield from 0.0034 for the mono-brominated dye PyBMI to 0.0061 for the di-brominated dye PyBBrI, attributable to the heavy atom effect. Nonetheless, the 1O2 production efficiency of these dyes remained limited, as photoinduced electron transfer was considered to occur nearly two orders of magnitude faster than singlet-to-triplet ISC. In vitro assays using MCF-7 and HeLa cells demonstrated that PyBBrI induced significant cell death, with IC50 values of ca. 95 and 220 nM, respectively, confirming its potential for use in cancer therapy.