Designing Excited States: Theory-Guided Access to Efficient Photosensitizers for Photodynamic Action

Effects of Heavy Iodine Atoms and π-Expanded Conjugation on Charge Transfer Dynamics in Carboxylic Acid BODIPY Derivatives as Triplet Photosensitizers

Borondipyrromethene (BODIPY) chromophores are composed of a functional-COOH group at meso position with or without a biphenyl ring, and their compounds with heavy iodine atoms at -2, -6 positions of the BODIPY indacene core were synthesized. The photophysical properties of the compounds were studied with steady-state absorption and fluorescence measurements. It was observed that the absorption band is significantly red-shifted, and fluorescence signals are quenched in the presence of iodine atoms. In addition to that, it was indicated that the biphenyl ring does not affect the spectral shifting in the absorption as well as fluorescence spectra. In an attempt to investigate the effect of π-expanded biphenyl moieties and heavy iodine atoms on charge transfer dynamics, femtosecond transient absorption spectroscopy measurements were carried out in the environment of the tetrahydrofuran (THF) solution. Based on the performed ultrafast pump-probe spectroscopy, BODIPY compounds with iodine atoms lead to intersystem crossing (ISC) and ISC rates were determined as 150 ps and 180 ps for iodine BODIPY compounds with and without π-expanded biphenyl moieties, respectively. According to the theoretical results, the charge transfer in the investigated compounds mostly appears to be intrinsic local excitations, corresponding to high photoluminescence efficiency. These experimental findings are useful for the design and study of the fundamental photochemistry of organic triplet photosensitizers.

BODIPY-Based Photodynamic Agents for Exclusively Generating Superoxide Radical over Singlet Oxygen

Developing Type-I photosensitizers is considered as an efficient approach to overcome the deficiency of traditional photodynamic therapy (PDT) for hypoxic tumors. However, it remains a challenge to design photosensitizers for generating reactive oxygen species by the Type-I process. Herein, we report a series of α,β-linked BODIPY dimers and a trimer that exclusively generate superoxide radical (O₂ ^-. ) by the Type-I process upon light irradiation. The triplet formation originates from an effective excited-state relaxation from the initially populated singlet (S₁ ) to triplet (T₁ ) states via an intermediate triplet (T₂ ) state. The low reduction potential and ultralong lifetime of the T₁ state facilitate the efficient generation of O₂ ^-. by inter-molecular charge transfer to molecular oxygen. The energy gap of T₁ -S₀ is smaller than that between ³ O₂ and ¹ O₂ thereby precluding the generation of singlet oxygen by the Type-II process. The trimer exhibits superior PDT performance under the hypoxic environment.

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.

Recent Advances in Activatable Organic Photosensitizers for Specific Photodynamic Therapy

Photodynamic therapy is an alternative modality for the therapy of diseases such as cancer in a minimally invasive manner. The essential photosensitizer, which acts as a catalyst when absorbing light, converts oxygen into cytotoxic reactive oxygen species that ablate malignant cells through apoptosis and/or necrosis, destroy tumor microvasculature, and stimulate immunity. An activatable photosensitizer whose photoactivity could be turned on by a specific disease biomarker is capable of distinguishing healthy cells from diseased cells, thereby reducing off-target photodamage. In this Minireview, we highlight progress in activatable organic photosensitizers over the past five years, including: (i) biorthogonal activatable BODIPYs; (ii) activatable Se-rhodamine with single-cell resolution; (iii) silicon phthalocyanine targeting oxygen tension; (iv) general D-π-A scaffolds; and (v) AIEgens. The potential challenges and opportunities for developing new types of activatable organic photosensitizers to overcome the hypoxia dilemmas of photodynamic therapy are discussed.

Spin-Orbit Charge-Transfer Intersystem Crossing in Anthracene-Perylenebisimide Compact Electron Donor-Acceptor Dyads and Triads and Photochemical Dianion Formation

Perylenebisimide (PBI)-anthracene (AN) donor-acceptor dyads/triad were prepared to investigate spin-orbit charge-transfer intersystem crossing (SOCT-ISC). Molecular conformation was controlled by connecting PBI units to the 2- or 9-position of the AN moiety. Steady-state, time-resolved transient absorption and emission spectroscopy revealed that chromophore orientation, electronic coupling, and dihedral angle between donor and acceptor exert a significant effect on the photophysical property. The dyad PBI-9-AN with orthogonal geometry shows weak ground-state coupling and efficient intersystem crossing (ISC, Φ_Δ =86 %) as compared with PBI-2-AN (Φ_Δ =57 %), which has a more coplanar geometry. By nanosecond transient absorption spectroscopy, a long-lived PBI localized triplet state was observed (τ_T =139 μs). Time-resolved EPR spectroscopy demonstrated that the electron spin polarization pattern of the triplet state is sensitive to the geometry and number of AN units attached to PBI. Reversible and stepwise generation of near-IR-absorbing PBI radical anion (PBI^-⋅ ) and dianion (PBI^2- ) was observed on photoexcitation in the presence of triethanolamine, and it was confirmed that selective photoexcitation at the near-IR absorption bands of PBI^.- is unable to produce PBI^2- .

Molecular Design of Highly Efficient Heavy-Atom-Free Triplet BODIPY Derivatives for Photodynamic Therapy and Bioimaging

Novel BODIPY photosensitizers were developed for imaging-guided photodynamic therapy. The introduction of a strong electron donor to the BODIPY core through a phenyl linker combined with the twisted arrangement between the donor and the BODIPY acceptor is essential for reducing the energy gap between the lowest singlet excited state and the lowest triplet state (ΔE_ST ), leading to a significant enhancement in the intersystem crossing (ISC) of the BODIPYs. Remarkably, the BDP-5 with the smallest ΔE_ST (ca. 0.44 eV) exhibited excellent singlet oxygen generation capabilities in both organic and aqueous solutions. BDP-5 also displayed bright emission in the far-red/near-infrared region in the condensed states. More importantly, both in vitro and in vivo studies demonstrated that BDP-5 NPs displayed a high potential for photodynamic cancer therapy and bioimaging.

Rational Design of Efficient Organic Phototherapeutic Agents via Perturbation Theory for Enhancing Anticancer Therapeutics

The development of efficient phototherapeutic agents (PTA) through rational and specific principles exhibits great potential to the biomedical field. In this study, a facile and rational strategy was used to design PTA through perturbation theory. According to the theory, both the intersystem crossing rate for singlet oxygen generation and nonradiative transition for photothermal conversion efficiency can be simultaneously enhanced by the rational optimization of donor-acceptor groups, heavy atom number, and their functional positions, which can effectively decrease the energy gap between the singlet and triplet states and increase the spin-orbit coupling constant. Finally, efficient PTA were obtained that showed excellent performance in multimode-imaging-guided synergetic photodynamic/photothermal therapy. This study therefore expands the intrinsic mechanism of organic PTA and should help guide the rational design of future organic PTA via perturbation theory.

Solvent-Sensitive Emitting Urea-Bridged bis-BODIPYs: Ready Access by a One-Pot Tandem Staudinger/Aza-Wittig Ureation

Herein we describe the synthesis, and computationally aided photophysical characterization of a new set of urea-bridged bis-BODIPY derivatives. These new dyads are efficiently obtained by a one-pot tandem Staudinger/aza-Wittig ureation protocol, from easily accessible meso-phenyl ortho-azidomethyl BODIPYs. These symmetric bis-BODIPYs outstand by a high absorption probability and excellent fluorescence and laser emission in less polar media. Nevertheless, this emission ability decreases in more polar media, which is ascribed to a light-induced charge-transfer from the urea spacer to the dipyrrin core, a process that can be modulated by appropriate changes in the substitution pattern of the BODIPY core. Furthermore, this ureation protocol can also be employed for the direct conjugation of our BODIPY-azides to amine-containing compounds, thus providing access to fluorescent non-symmetric ureas.

N-Bridged Annulated BODIPYs: Synthesis of Highly Fluorescent Blueshifted Dyes

A series of novel BODIPY dyes has been prepared through the introduction of an N-bridged annulated meso-phenyl ring at one of the β-positions of the BODIPY core. An unusual blueshift of the main spectral bands is observed, since the fusion of a meso-substituent results in a marked relative destabilization of the LUMO. The greater rigidity of the ring-fused structure leads to very high fluorescence quantum yields. The position of the main spectral bands can be fine-tuned by introducing electron withdrawing and donating groups onto the meso-phenyl ring.

Controllable Photodynamic Therapy Implemented by Regulating Singlet Oxygen Efficiency

With singlet oxygen (1O2) as the active agent, photodynamic therapy (PDT) is a promising technique for the treatment of various tumors and cancers. But it is hampered by the poor selectivity of most traditional photosensitizers (PS). In this review, we present a summary of controllable PDT implemented by regulating singlet oxygen efficiency. Herein, various controllable PDT strategies based on different initiating conditions (such as pH, light, H2O2 and so on) have been summarized and introduced. More importantly, the action mechanisms of controllable PDT strategies, such as photoinduced electron transfer (PET), fluorescence resonance energy transfer (FRET), intramolecular charge transfer (ICT) and some physical/chemical means (e.g. captivity and release), are described as a key point in the article. This review provide a general overview of designing novel PS or strategies for effective and controllable PDT.

Rational Design of Advanced Photosensitizers Based on Orthogonal BODIPY Dimers to Finely Modulate Singlet Oxygen Generation

The synthesis, photophysical characterization, and modeling of a new library of halogen-free photosensitizers (PS) based on orthogonal boron dipyrromethene (BODIPY) dimers are reported. Herein we establish key structural factors in order to enhance singlet oxygen generation by judiciously choosing the substitution patterns according to key electronic effects and synthetic accessibility factors. The photosensitization mechanism of orthogonal BODIPY dimers is demonstrated to be strongly related to their intrinsic intramolecular charge transfer (ICT) character through the spin-orbit charge-transfer intersystem crossing (SOCT-ISC) mechanism. Thus, singlet oxygen generation can be effectively modulated through the solvent polarity and the presence of electron-donating or withdrawing groups in one of the BODIPY units. The photodynamic therapy (PDT) activity is demonstrated by in vitro experiments, showing that selected photosensitizers are efficiently internalized into HeLa cells, exhibiting low dark toxicity and high phototoxicity, even at low PS concentration (0.05-5×10^-6  m).

A Bifunctional Photosensitizer for Enhanced Fractional Photodynamic Therapy: Singlet Oxygen Generation in the Presence and Absence of Light

The photosensitized generation of singlet oxygen within tumor tissues during photodynamic therapy (PDT) is self-limiting, as the already low oxygen concentrations within tumors is further diminished during the process. In certain applications, to minimize photoinduced hypoxia the light is introduced intermittently (fractional PDT) to allow time for the replenishment of cellular oxygen. This condition extends the time required for effective therapy. Herein, we demonstrated that a photosensitizer with an additional 2-pyridone module for trapping singlet oxygen would be useful in fractional PDT. Thus, in the light cycle, the endoperoxide of 2-pyridone is generated along with singlet oxygen. In the dark cycle, the endoperoxide undergoes thermal cycloreversion to produce singlet oxygen, regenerating the 2-pyridone module. As a result, the photodynamic process can continue in the dark as well as in the light cycles. Cell-culture studies validated this working principle in vitro.

Sensing and discrimination of cyanide and hydrogen sulfide using an 8-alkenyl-4,4-difluoro-4-bora-3a,4a-diaza-s-indacene derivative

8-(2-Phenylethenyl)BODIPY has been shown to be an appropriate chromo-fluorogenic probe for cyanide in H₂O.

Dual Functioning Thieno-Pyrrole Fused BODIPY Dyes for NIR Optical Imaging and Photodynamic Therapy: Singlet Oxygen Generation without Heavy Halogen Atom Assistance

We discovered a rare phenomenon wherein a thieno-pyrrole fused BODIPY dye (SBDPiR690) generates singlet oxygen without heavy halogen atom substituents. SBDPiR690 generates both singlet oxygen and fluorescence. To our knowledge, this is the first example of such a finding. To establish a structure-photophysical property relationship, we prepared SBDPiR analogs with electron-withdrawing groups at the para-position of the phenyl groups. The electron-withdrawing groups increased the HOMO-LUMO energy gap and singlet oxygen generation. Among the analogs, SBDPiR688, a CF3 analog, had an excellent dual functionality of brightness (82290 m(-1)  cm(-1) ) and phototoxic power (99170 m(-1)  cm(-1) ) comparable to those of Pc 4, due to a high extinction coefficient (211 000 m(-1)  cm(-1) ) and balanced decay (Φflu =0.39 and ΦΔ =0.47). The dual functionality of the lead compound SBDPiR690 was successfully applied to preclinical optical imaging and for PDT to effectively control a subcutaneous tumor.

Facile synthesis of dimeric BODIPY and its catalytic activity for sulfide oxidation under visible light

An efficient and green dimeric BODIPY-catalyzed sulfide oxidation under visible light is described.