A Twist-Assisted Biphenyl Photosensitizer Passable Through Glucose Channel

Difluoroborate-based bichromophores: Symmetry relaxation and two-photon absorption

Given potential applications of multiphoton absorbers, in the present work we have studied the symmetry-relaxation effects in one- and two-photon absorption spectra in two bichromophore systems based on difluoroborate core linked by biphenylene or bianthracene moieties. We have employed a palette of experimental methods (synthesis, one- and two-photon spectroscopy, X-ray crystallography) and state-of-the-art computational methods to shed light on how symmetry relaxation, a result of twisting of building blocks, affects one- and two-photon absorption of the two studied fluorescent dyes. Electronic-structure calculations revealed that the planarity of central biphenyl moiety, as well as deviations from planarity up to 30-40 deg., ensure maximum values of two-photon transition strengths. Perpendicular arrangement of phenylene units in biphenylene moiety leads to 20% drop in the two-photon transition strengths. More detailed studies demonstrated that equilibrium structures of both compounds in chloroform solution show very different values of two-photon absorption cross sections at absorption band maxima, i.e. 224 GM for and 134 GM for biphenyle and bianthracene linkers, respectively. The latter value is in good agreement with experimental value obtained using Z-scan method. The difference in two-photon absorption cross section between both compounds can be rationalized based on equilibrium geometry differences, i.e. interplanar angle is 35 deg and 91 deg in the case of biphenylene and bianthracene moiety, respectively. It is thus not beneficial to introduce conformationally locked central linker based on bianthracene moiety.

Mapping the diffusion pattern of 1O2 along DNA duplex by guanine photooxidation with an appended biphenyl photosensitizer

To realize nucleic acid-targeting photodynamic therapy, a photosensitizer should be attached at the optimal position on a complementary oligonucleotide, where a guanine photooxidation is maximized. Here we show the photooxidation of 22 DNA duplexes with varied lengths between a 1O2-generating biphenyl photosensitizer attached at a midchain thymine in a strand and the single guanine reactant in the other strand. The best photooxidation efficiencies are achieved at 9, 10, and 21 base intervals, which coincides with the pitch of 10.5 base pairs per turn in a DNA duplex. The low efficiencies for near and far guanines are due to quenching of the biphenyl by guanine and dilution of 1O2 by diffusion, respectively. The 1O2-diffusion mapping along DNA duplex provides clues to the development of efficient and selective photosensitizer agents for nucleic acid-targeting photodynamic therapy, as well as an experimental demonstration of diffusion of a particle along cylindrical surface in molecular level.

Photodynamic Therapy by Glucose Transporter 1-Selective Light Inactivation

Chromophore-assisted light inactivation (CALI) was applied to molecule-targeted photodynamic therapy (PDT). In order to identify organic photosensitizers suitable for CALI, the carbonic anhydrase II (CAII) ligand, 4-sulfamoylbenzoic acid 1, was conjugated with several photosensitizers to produce compounds 2-7, whose CALI ability was evaluated by measuring their effect on CAII enzymatic activity. Di-iodinated BODIPY (I₂BODIPY) exhibited excellent CAII inactivation ability, similar to that of Ru(bpy)₃. The glucose-I₂BODIPY conjugate (8) was synthesized as an inactivation of glucose transporter 1 (GLUT1), a protein overexpressed in many cancer cells. Under light irradiation, 8 exhibited concentration-dependent cytotoxicity with half maximal inhibitory concentration (IC₅₀) values of 5.49, 11.14, and 8.73 μM, against human cervical carcinoma (HeLa), human lung carcinoma (A549), and human hepatocellular carcinoma (HepG2) cell lines, respectively. The GLUT1 inhibitor phloretin suppressed the cytotoxicity induced by 8 under light irradiation in a concentration-dependent manner. Western blot analysis indicated that GLUT1 was not detected in cell lines treated with 10 μM 8 under light irradiation. Furthermore, 8 reduced the levels of epidermal growth factor receptor tyrosine kinase (EGFR), phospho-ERK (Y204), and GLUT1 without affecting ERK, α-tubulin, and PCNA protein levels, whereas talaporfin sodium, a clinically approved photosensitizer for PDT, nonspecifically reduced intracellular protein levels in HeLa cells, indicating that 8 has a GLUT1-specific inactivation ability and causes light-induced cytotoxicity by modulating the EGFR/MAPK signaling pathway.

4'-Nitrobiphenyl thioglucoside as the Smallest, fluorescent photosensitizer with cancer targeting ligand

We have previously developed a glucose-linked biphenyl photosensitizer that can pass through glucose transporters, aiming for cancer-selective photodynamic therapy (PDT). Its small size (MW: 435) will allow oral administration and a fast clearance avoiding photosensitivity. However, its fluorescence efficiency was quite low, causing difficulty in monitoring cellular uptake. We thus synthesized a series of monosaccharide-linked biphenyl derivatives with a sulfur atom replacing an oxygen atom, in search of a photosensitizer with a brighter fluorescence. Among them, 4'-nitrobiphenyl thioglucoside showed a fluorescence emission extending to near infra-red region with a strength three times greater than that of the previous compound. This compound was found to have a higher ¹O₂-producing efficiency (Φ_Δ: 0.75) than the previous compound (Φ_Δ: 0.65). The thioglucoside indicated a significant photodamaging effect (IC₅₀: 250 μM) against cancer cells. Although the galactose and mannose analogs exerted similar photodamaging effects, they were moderately toxic in the dark at a concentration of 300 μM. The thioglucoside and thiomannoside were at least partially uptaken through glucose transporters as demonstrated by inhibition with cytochalasin B, whereas no inhibition was observed for the galactoside. The behavior of d-glucose toward the cellular uptakes of these photosensitizers was bipolar: inhibitory at a low concentration and recovery or acceleratory at a higher concentration. These results indicate that 4'-nitrobiphenyl thioglucoside is the smallest (MW: 393) cancer-targeting photosensitizer with a trackable fluorescence property.

Quantitative Structure-Property Relationship Modelling for the Prediction of Singlet Oxygen Generation by Heavy-Atom-Free BODIPY Photosensitizers*

Heavy-atom-free sensitizers forming long-living triplet excited states via the spin-orbit charge transfer intersystem crossing (SOCT-ISC) process have recently attracted attention due to their potential to replace costly transition metal complexes in photonic applications. The efficiency of SOCT-ISC in BODIPY donor-acceptor dyads, so far the most thoroughly investigated class of such sensitizers, can be finely tuned by structural modification. However, predicting the triplet state yields and reactive oxygen species (ROS) generation quantum yields for such compounds in a particular solvent is still very challenging due to a lack of established quantitative structure-property relationship (QSPR) models. In this work, the available data on singlet oxygen generation quantum yields (ΦΔ ) for a dataset containing >70 heavy-atom-free BODIPY in three different solvents (toluene, acetonitrile, and tetrahydrofuran) were analyzed. In order to build reliable QSPR model, a series of new BODIPYs were synthesized that bear different electron donating aryl groups in the meso position, their optical and structural properties were studied along with the solvent dependence of singlet oxygen generation, which confirmed the formation of triplet states via the SOCT-ISC mechanism. For the combined dataset of BODIPY structures, a total of more than 5000 quantum-chemical descriptors was calculated including quantum-chemical descriptors using density functional theory (DFT), namely M06-2X functional. QSPR models predicting ΦΔ values were developed using multiple linear regression (MLR), which perform significantly better than other machine learning methods and show sufficient statistical parameters (R=0.88-0.91 and q2 =0.62-0.69) for all three solvents. A small root mean squared error of 8.2 % was obtained for ΦΔ values predicted using MLR model in toluene. As a result, we proved that QSPR and machine learning techniques can be useful for predicting ΦΔ values in different media and virtual screening of new heavy-atom-free BODIPYs with improved photosensitizing ability.

Heavy-atom-free BODIPY photosensitizers with intersystem crossing mediated by intramolecular photoinduced electron transfer

Organic photosensitizers possessing efficient intersystem crossing (ISC) and forming long-living triplet excited states, play a crucial role in a number of applications. A common approach in the design of such dyes relies on the introduction of heavy atoms (e.g. transition metals or halogens) into the structure, which promote ISC via spin-orbit coupling interaction. In recent years, alternative methods to enhance ISC have been actively studied. Among those, the generation of triplet excited states through photoinduced electron transfer (PET) in heavy-atom-free molecules has attracted particular attention because it allows for the development of photosensitizers with programmed triplet state and fluorescence quantum yields. Due to their synthetic accessibility and tunability of optical properties, boron dipyrromethenes (BODIPYs) are so far the most perspective class of photosensitizers operating via this mechanism. This article reviews recently reported heavy-atom-free BODIPY donor-acceptor dyads and dimers which produce long-living triplet excited states and generate singlet oxygen. Structural factors which affect PET and concomitant triplet state formation in these molecules are discussed and the reported data on triplet state yields and singlet oxygen generation quantum yields in various solvents are summarized. Finally, examples of recent applications of these systems are highlighted.