Assessment of Water-Soluble π-Extended Squaraines as One- and Two-Photon Singlet Oxygen Photosensitizers: Design, Synthesis, and Characterization

POSS engineering of squaraine nanoparticle with high photothermal conversion efficiency for photothermal therapy

Photothermal therapy (PTT) has been extensively studied due to its promising therapeutic effects and potential for development in cancer treatments. Central to PTT is the development of photothermal agents (PTAs). This study presents a novel nanoparticle called POSS-SQ, which satisfies the necessary conditions to function as a PTA. Comprised of squaraine (SQ) and polyhedral oligomeric sesquisiloxane (POSS), POSS-SQ NPs exhibit strong near-infrared (NIR) absorption and high photothermal conversion efficiency (PCE) attributable to the intermolecular electron transfer in SQ. Furthermore, POSS when modified with polyethylene glycol (PEG) through "click" chemistry, effectively enhances cell permeability and biocompatibility of the nanoparticles. Photothermal experiments reveal that POSS-SQ NPs demonstrate concentration and laser power dependence, with a PCE of 67.2%. In vitro and in vivo experiments confirm the excellent biosafety and tumor growth inhibition potential of POSS-SQ NPs under laser irradiation, attributed to the synergistic effects of enhanced cell permeability and exceptional photothermal properties. This research highlights the possibility of obtaining PTAs with high PCE and excellent biocompatibility by combining SQ-N and POSS, offering a new approach for designing and developing more efficient PTAs to enhance better PTT outcomes.

Enhanced Photodynamic Therapy: A Review of Combined Energy Sources

Photodynamic therapy (PDT) has been used in recent years as a non-invasive treatment for cancer, due to the side effects of traditional treatments such as surgery, radiotherapy, and chemotherapy. This therapeutic technique requires a photosensitizer, light energy, and oxygen to produce reactive oxygen species (ROS) which mediate cellular toxicity. PDT is a useful non-invasive therapy for cancer treatment, but it has some limitations that need to be overcome, such as low-light-penetration depths, non-targeting photosensitizers, and tumor hypoxia. This review focuses on the latest innovative strategies based on the synergistic use of other energy sources, such as non-visible radiation of the electromagnetic spectrum (microwaves, infrared, and X-rays), ultrasound, and electric/magnetic fields, to overcome PDT limitations and enhance the therapeutic effect of PDT. The main principles, mechanisms, and crucial elements of PDT are also addressed.

Norsquaraine endowed with anticancer and antibacterial activities

BACKGROUND

Photodynamic therapy (PDT) is a method for the treatment of cancer. Furthermore, PDT can also be used for the eradication of bacteria. The photo-sensitizing drug, a.k.a photosensitizer, is critical for the success of PDT. Although norsquaraines are analogs of squaraine dyes, they are overlooked as photosensitizers.

METHODS

In this work, synthesis, characterization, bioimaging and in vitro PDT applications of a new norsquaraine dye 1 were described. We also prepared nanoparticles from norsquaraine 1 and Pluronic F127 to obtain 1@F127.

RESULTS

Norsquaraine 1 boosted the generation of reactive oxygen species over a wide range of pH (pH 8.0, 7.0, 6.0, and 2.2.). Furthermore, 1 was internalized by epidermoid laryngeal carcinoma Hep-2 (Hep-2) cells and used for fluorescence imaging. Remarkably, norsquaraine 1 destroyed most of the cancer cells (ca. 77% to 89%) after illumination with red light. Most strikingly, 1 successfully inhibited the growth of Methicillin-resistant Staphylococcus aureus (MRSA) upon illumination. Last but not least, photodynamic sterilization of tomato juice, an acidic beverage, was feasible using 1 as a photo sterilizer. Nano formulation of 1 with Pluronic F127 provided 1@F127 nanoparticles. It is lucid that 1@F127 nanoparticles permeate into Hep-2 cells and boost the generation of ROS upon illumination.

CONCLUSION

Norsquaraine 1 shows superior features as a photosensitizer pertinent to PDT in a wide range of pH. This norsquaraine is endowed with anticancer and antibacterial activities. Which should be further evaluated.

Synthesis and Strong Solvatochromism of Push-Pull Thienylthiazole Boron Complexes

The solvatochromic behavior of two donor-π bridge-acceptor (D-π-A) compounds based on the 2-(3-boryl-2-thienyl)thiazole π-linker and indandione acceptor moiety are investigated. DFT/TD-DFT calculations were performed in combination with steady-state absorption and emission measurements, along with electrochemical studies, to elucidate the effect of two different strongly electron-donating hydrazonyl units on the solvatochromic and fluorescence behavior of these compounds. The Lippert–Mataga equation was used to estimate the change in dipole moments (Δµ) between ground and excited states based on the measured spectroscopic properties in solvents of varying polarity with the data being supported by theoretical studies. The two asymmetrical D-π-A molecules feature strong solvatochromic shifts in fluorescence of up to ~4300 cm⁻¹ and a concomitant change of the emission color from yellow to red. These changes were accompanied by an increase in Stokes shift to reach values as large as ~5700–5800 cm⁻¹. Quantum yields of ca. 0.75 could be observed for the N,N-dimethylhydrazonyl derivative in nonpolar solvents, which gradually decreased along with increasing solvent polarity, as opposed to the consistently reduced values obtained for the N,N-diphenylhydrazonyl derivative of up to ca. 0.20 in nonpolar solvents. These two push–pull molecules are contrasted with a structurally similar acceptor-π bridge-acceptor (A-π-A) compound.

Electron‐Rich EDOT Linkers in Tetracationic bis‐Triarylborane Chromophores: Influence on Water Stability, Biomacromolecule Sensing, and Photoinduced Cytotoxicity

Three novel tetracationic bis‐triarylboranes with 3,4‐ethylenedioxythiophene (EDOT) linkers, and their neutral precursors, showed significant red‐shifted absorption and emission compared to their thiophene‐containing analogues, with one of the EDOT‐derivatives emitting in the NIR region. Only the EDOT‐linked trixylylborane tetracation was stable in aqueous solution, indicating that direct attachment of a thiophene or even 3‐methylthiophene to the boron atom is insufficient to provide hydrolytic stability in aqueous solution. Further comparative analysis of the EDOT‐linked trixylylborane tetracation and its bis‐thiophene analogue revealed efficient photo‐induced singlet oxygen production, with the consequent biological implications. Thus, both analogues bind strongly to ds‐DNA and BSA, very efficiently enter living human cells, accumulate in several different cytoplasmic organelles with no toxic effect but, under intense visible light irradiation, they exhibit almost instantaneous and very strong cytotoxic effects, presumably attributed to singlet oxygen production. Thus, both compounds are intriguing theranostic agents, whose intracellular and probably intra‐tissue location can be monitored by strong fluorescence, allowing switching on of the strong bioactivity by well‐focused visible light.

Picolylamine-functionalized benz[e]indole squaraine dyes: Synthetic approach, characterization and in vitro efficacy as potential anticancer phototherapeutic agents

Squaraine dyes are a family of compounds known for their relevant photophysical and photochemical properties potentially useful as photosensitizing agents. Since pyridines have been introduced into the skeleton of several families of compounds to enhance their pharmacological activity, and this approach had not yet been performed on squaraines, novel dyes derived from benz[e]indole functionalized with picolyl- and dipicolylamine and N-ethyl and -hexyl chains were designed and synthesized. After being fully characterized, their interaction with human albumin was in vitro and in silico evaluated. Dyes were further assessed for their phototoxicity activity, and the most interesting ones were studied regarding cell localization and induction of morphological cell changes, genotoxicity, apoptosis and cell cycle arrest. The molecules with N-ethyl chains showed the greatest in vitro light-dependent cytotoxic effects, particularly the zwitterionic squaraine dye and the one bearing a single pyridine unit, which also exhibited a more significant interaction with human albumin. Phenotypically, the cells incubated with these squaraines became smaller and rounded after irradiation, the effects varying with the tested concentration. Genotoxic effects were observed even without irradiation, being more evident for the N-ethyl picolylamine-derived dye. The fluorescence emitted by Rhodamine 123 largely coincided with that emitted by the dyes, suggesting that they are found preferentially in mitochondria. After irradiation, an increase in the subG1 population was verified by propidium iodide-staining analysis by flow cytometry, indicative of cell death by apoptosis.

Squaraine probes for the bimodal staining of lipid droplets and endoplasmic reticulum imaging in live cells

We report the synthesis of a library of asymmetric squaraines and their application as superior bimodal “on-demand” fluorescence probes for lipid drolet and endoplasmic reticulum in cancer cells.

A Sequential Dual-Model Strategy Based on Photoactivatable Metallopolymer for On-Demand Release of Photosensitizers and Anticancer Drugs

The synergistic combination of chemotherapy and photodynamic therapy has attracted considerable attention for its enhanced antitumoral effects; however, it remains challenging to successfully delivery photosensitizers and anticancer drugs while minimizing drug leakage at off-target sites. A red-light-activatable metallopolymer, Poly(Ru/PTX), is synthesized for combined chemo-photodynamic therapy. The polymer has a biodegradable backbone that contains a photosensitizer Ru complex and the anticancer drug paclitaxel (PTX) via a singlet oxygen (1 O2 ) cleavable linker. The polymer self-assembles into nanoparticles, which can efficiently accumulate at the tumor sites during blood circulation. The distribution of the therapeutic agents is synchronized because the Ru complex and PTX are covalently conjugate to the polymer, and off-target toxicity during circulation is also mostly avoided. Red light irradiation at the tumor directly cleaves the Ru complex and produces 1 O2 for photodynamic therapy. Sequentially, the generated 1 O2 triggers the breakage of the linker to release the PTX for chemotherapy. Therefore, this novel sequential dual-model release strategy creates a synergistic chemo-photodynamic therapy while minimizing drug leakage. This study offers a new platform to develop smart delivery systems for the on-demand release of therapeutic agents in vivo.

Recent Strategies to Develop Innovative Photosensitizers for Enhanced Photodynamic Therapy

This review presents a robust strategy to design photosensitizers (PSs) for various species. Photodynamic therapy (PDT) is a photochemical-based treatment approach that involves the use of light combined with a light-activated chemical, referred to as a PS. Attractively, PDT is one of the alternatives to conventional cancer treatment due to its noninvasive nature, high cure rates, and low side effects. PSs play an important factor in photoinduced reactive oxygen species (ROS) generation. Although the concept of photosensitizer-based photodynamic therapy has been widely adopted for clinical trials and bioimaging, until now, to our surprise, there has been no relevant review article on rational designs of organic PSs for PDT. Furthermore, most of published review articles in PDT focused on nanomaterials and nanotechnology based on traditional PSs. Therefore, this review aimed at reporting recent strategies to develop innovative organic photosensitizers for enhanced photodynamic therapy, with each example described in detail instead of providing only a general overview, as is typically done in previous reviews of PDT, to provide intuitive, vivid, and specific insights to the readers.

Synthesis of near-infrared absorbing and fluorescent bis(pyrrol-2-yl)squaraines and their halochromic properties

1,2-Squaraines (partially conjugated neutral structure) showed blue-shifted λmax compared to the corresponding 1,3-squaraines (fully conjugated zwitterionic structure) and the mono-protonated thiophene-fused 1,3-squaraine showed λmax at 1007 nm.

Water-soluble chiral tetrazine derivatives: towards the application of circularly polarized luminescence from upper-excited states to photodynamic therapy

A new family of water-soluble chiral tetrazine derivatives 1 and 2 is reported. Spectroscopic studies reveal that the derivatives violate Kasha's rule and emit from their upper-excited states (S n , n > 1). The transition assignments are supported by time-dependent density functional theory calculations. More importantly, both chromophores exhibit anisotropy factors on the order of ∼10-3 to 10-4 for circular dichroism and circularly polarized luminescence (CPL) from upper-excited states. Additionally, the nonplanar geometry of the derivatives induces a significant yield of triplet excited states. Transient absorption spectroscopic measurements reveal high triplet quantum yields of ∼86% for 1 and ∼81% for 2. Through in vitro studies, we demonstrate that the derivatives can be used as photodynamic therapy (PDT) agents, providing a highly efficient form of cancer therapy. This study is the first demonstration of simple organic molecules with CPL from upper-excited states and efficient PDT.

Perylenetetracarboxy-3,4:9,10-diimide derivatives with large two-photon absorption activity

Perylenetetracarboxy-3,4:9,10-diimides with large TPA cross-sections.

A molecular probe based on pyrimidine imidazole derivatives for stable super-resolution endoplasmic reticulum imaging in living cells

Multi-functional florescent dyes capable of acting as molecular probes in living systems under two-photon microscopy, as well as super-resolution nanoscopy, are of great interest.

Spectral properties of ionic benzotristhiazole based donor-acceptor NLO-phores in polymer matrices and their one- and two-photon cellular imaging ability

A series of ionic benzotristhiazolium (BTT) push-pull chromophores, with different nitrogen donor groups and different lengths of conjugated bridges, was successfully doped in polar polymer matrices (PVC and PSS). The spectral (photophysical) properties of their low concentration thin polymeric films are compared with those in solution and are discussed in terms of matrix polarity/viscosity influence, specific polymer-chromophore interaction, structure-spectral property relationship and Twisted Intramolecular Charge-Transfer (TICT) state formation. The elimination of a non-emissive phantom and TICT state formation by restricted intramolecular rotations in the polymer matrix or viscous solvent results in a relatively high Φ_F of all the investigated NLO-phores; particularly for near-infrared NIR molecular rotors bearing diphenylamino and julolidine donor groups. Because of cationic characteristics, small molecular weight, calculated high second hyperpolarizability and significant emission efficiency dependence on surroundings' viscosity (rigidochromic effect), two dyes were chosen as candidates for potential fluorescent probes for one-photon (1P) and two photon (2P) cellular imaging. The selected BTT NLO-phore with a julolidine donor is promising as a mitochondria-specific fluorescent small molecular probe for live cell super-resolution imaging with low cytotoxicity and good photostability, and is also potentially suitable for super-resolution STED imaging.

Beyond the Barriers of Light Penetration: Strategies, Perspectives and Possibilities for Photodynamic Therapy

Photodynamic therapy (PDT) is a photochemistry based treatment modality that involves the generation of cytotoxic species through the interactions of a photosensitizer molecule with light irradiation of an appropriate wavelength. PDT is an approved therapeutic modality for several cancers globally and in several cases has proved to be effective where traditional treatments have failed. The key parameters that determine PDT efficacy are 1. the photosensitizer (nature of the molecules, selectivity, and macroscopic and microscopic localization etc.), 2. light application (wavelength, fluence, fluence rate, irradiation regimes etc.) and 3. the microenvironment (vascularity, hypoxic regions, stromal tissue density, molecular heterogeneity etc.). Over the years, several groups aimed to monitor and manipulate the components of these critical parameters to improve the effectiveness of PDT treatments. However, PDT is still misconstrued to be a surface treatment primarily due to the limited depths of light penetration. In this review, we present the recent advances, strategies and perspectives in PDT approaches, particularly in cancer treatment, that focus on increasing the 'damage zone' beyond the reach of light in the body. This is enabled by a spectrum of approaches that range from innovative photosensitizer excitation strategies, increased specificity of phototoxicity, and biomodulatory approaches that amplify the biotherapeutic effects induced by photodynamic action. Along with the increasing depth of understanding of the underlying physical, chemical and physiological mechanisms, it is anticipated that with the convergence of these strategies, the clinical utility of PDT will be expanded to a powerful modality in the armamentarium for the management of cancer.

Squaraines bearing halogenated moieties as anticancer photosensitizers: Synthesis, characterization and biological evaluation

We report the synthesis and characterization of a series of symmetrical indolenine-based squaraine dyes along with the evaluation of their singlet oxygen generation efficiency. The photodynamic activity of these new photosensitizers has been evaluated on a human tumor fibrosarcoma (HT-1080) cell line. The cytotoxicity increased over time and is induced by the photoactivation of bromo (Br-C4) and iodio (I-C4) long carbon chain squaraine dyes and the consequent increase in reactive oxygen species (ROS) production (p < 0.001), which leads to necrosis 6 h after treatment. Induction of cytochrome c release, DNA damage and up-regulation of GPX1, NQO1 and SOD2 mRNA gene expression after PDT were investigated.

Design, synthesis, linear and nonlinear photophysical properties of novel pyrimidine-based imidazole derivatives

Novel pyrimidine imidazole derivatives with flexible ether chains have been synthesised and evaluated for their cell imaging performanceviaphotophysical investigations and theoretical calculations.

On the Potential Use of Squaraine Derivatives as Photosensitizers in Photodynamic Therapy: A TDDFT and RICC2 Survey

A time-dependent density functional theory (TDDFT) and the second-order approximated coupled-cluster model with the resolution of identity approximation (RICC2) studies are reported here for some classes of squaraine derivatives. These compounds have a sharp electronic band, ranging from the visible to near-red part of the spectrum, with an high molar absorption coefficient. These features make them potential photosensitizers in the photodynamic therapy of cancer (PDT), in which a light source, a photosensitizer, and molecular oxygen ((3)O2) are combined to give cytotoxic singlet oxygen ((1)O2) as a final result in a photochemical process. For the examined structures, the introduction of different substituents (electron donating, electron withdrawing, or fused rings) in the parent molecule, in order to give different squaraine derivatives, changes the maximum absorption wavelength (λmax) from 620 to 730 nm, giving a near-red absorbing photosensitizer that can better penetrate human tissue to damage tumor cells. Theoretical results, obtained from both TDDFT/PBE0 and RICC2, are able to reproduce qualitatively the substitution effect on λmax, resulting in a useful tool for testing different structure modifications and, in general, for the molecular design of PDT photosensitizers. Calculated vertical excitation energies (singlet-singlet transitions) generally agree with experimental data within 0.3 eV. The singlet oxygen generation ability of these compounds requires that their triplet energy, for a type II reaction mechanism, should be greater than 0.98 eV. Theoretical triplet energies from the RICC2 method suggests that this requisite is fulfilled for all compounds, though the results are generally overestimated with respect to experiment by 0.7 eV, whereas TDDFT/PBE0 triplet energies, which are underestimated within 0.2 eV in few cases, lie close to the above-mentioned limit and can be considered suitable for PDT applications.

Nanocomposite-Based Photodynamic Therapy Strategies for Deep Tumor Treatment

Photodynamic therapy (PDT), as an emerging clinically approved modality, has been used for treatment of various cancer diseases. Conventional PDT strategies are mainly focused on superficial lesions because the wavelength of illumination light of most clinically approved photosensitizers (PSs) is located in the UV/VIS range that possesses limited tissue penetration ability, leading to ineffective therapeutic response for deep-seated tumors. The combination of PDT and nanotechnology is becoming a promising approach to fight against deep tumors. Here, the rapid development of new PDT modalities based on various smartly designed nanocomposites integrating with conventionally used PSs for deep tumor treatments is introduced. Until now many types of multifunctional nanoparticles have been studied, and according to the source of excitation energy they can be classified into three major groups: near infrared (NIR) light excited nanomaterials, X-ray excited scintillating/afterglow nanoparticles, and internal light emission excited nanocarriers. The in vitro and in vivo applications of these newly developed PDT modalities are further summarized here, which highlights their potential use as promising nano-agents for deep tumor therapy.

Two-photon absorption properties and (1)O2 generation ability of Ir complexes: an unexpected large cross section of [Ir(CO)2Cl(4-(para-di-n-butylaminostyryl)pyridine)]

The new complexes cis-[Ir(CO)2Cl(4-(para-di-n-butylaminostyryl)pyridine)] () and [Ir(cyclometallated-2-phenylpyridine)2(4,4'-(para-di-n-butylaminostyryl)-2,2'-bipyridine)][PF6] () were synthesized and fully characterized along with the known complex Ir(cyclometallated-2-phenylpyridine)2(5-Me-1,10-phenanthroline)][PF6] (). Remarkably, complex , with an Ir(i) centre, displays fluorescence - as opposed to the phosphorescence typical of many Ir(iii) complexes - with a modestly high quantum yield in solution, opening a new route for the design of iridium-based emitters which should not be limited to the +3 oxidation state. It is also characterized by an unexpectedly large two-photon absorption (TPA) cross section, an order of magnitude higher than that previously reported for Ir(iii) or Pt(ii) complexes. The great potential of cyclometallated Ir(iii) complexes for photodynamic therapy was confirmed, with and showing a good singlet oxygen generation ability, coupled with a modest TPA activity for .

Effective Two-Photon Excited Photodynamic Therapy of Xenograft Tumors Sensitized by Water-Soluble Bis(arylidene)cycloalkanone Photosensitizers

A series of bis(arylidene)cycloalkanone photosensitizers modified by polyethylene glycol (PEG) have been studied for two-photon excited photodynamic therapy (2PE-PDT). As compared with their prototype compounds, these PEGylated photosensitizers show enhanced water solubilities while their photophysical and photochemical properties, including linear absorption, two-photon absorption, fluorescence, and singlet oxygen quantum yield, remain unaltered. In vitro behaviors (cellular uptake, subcellular localization, photocytotoxicity in both PDT and 2PE-PDT) of these photosensitizers reveal that an optimized lipid-water partition coefficient can be obtained by adjusting the length and position of the PEG chains. Among them, the photosensitizer modified asymmetrically by two tetraethylene glycol chains presents the best performance as a 2PE-PDT candidate. Selective blood-vessel closure and obvious therapeutic effect in inhibiting the growth of tumors are confirmed by in vivo 2PE-PDT after intravenous injection of this photosensitiezer. The survival periods of treated tumor-bearing mice are significantly prolonged. This study demonstrates the feasibility of using a simple molecule to construct a potential candidate for 2PE-PDT.

Near-IR squaraine dye-loaded gated periodic mesoporous organosilica for photo-oxidation of phenol in a continuous-flow device

Periodic mesoporous organosilica (PMO) has been widely used for the fabrication of a variety of catalytically active materials. We report the preparation of novel photo-responsive PMO with azobenzene-gated pores. Upon activation, the azobenzene gate undergoes trans-cis isomerization, which allows an unsymmetrical near-infrared squaraine dye (Sq) to enter into the pores. The gate closure by cis-trans isomerization of the azobenzene unit leads to the safe loading of the monomeric dye inside the pores. The dye-loaded and azobenzene-gated PMO (Sq-azo@PMO) exhibits excellent generation of reactive oxygen species upon excitation at 664 nm, which can be effectively used for the oxidation of phenol into benzoquinone in aqueous solution. Furthermore, Sq-azo@PMO as the catalyst was placed inside a custom-built, continuous-flow device to carry out the photo-oxidation of phenol to benzoquinone in the presence of 664-nm light. By using the device, about 23% production of benzoquinone with 100% selectivity was achieved. The current research presents a prototype of transforming heterogeneous catalysts toward practical use.

Rational design of small indolic squaraine dyes with large two-photon absorption cross section

Small organic dyes with large two-photon absorption (TPA) cross sections (δ) are more desirable in many applications compared with large molecules. Herein, we proposed a facile theoretical method for the fast screening of small organic molecules as potential TPA dyes. This method is based on a theoretical analysis to the natural transition orbitals (NTOs) directly associated with the TPA transition. Experimental results on the small indolic squaraine dyes (ISD) confirmed that their TPA cross sections is strongly correlated to the delocalization degree of the NTOs of the S₂ excited states. Aided by this simple and intuitive method, we have successfully designed and synthesized a small indolic squaraine dye (ISD) with a remarkable δ value above 8000 GM at 780 nm. The ISD dye also exhibits a high singlet oxygen generation quantum yield about 0.90. The rationally designed TPA dye was successfully applied in both two-photon excited fluorescence cell imaging and in vivo cerebrovascular blood fluid tracing.

Synthesis and characterization of near-infrared absorbing water soluble squaraines and study of their photodynamic effects in DLA live cells

Here, we report the synthesis, photophysical properties and photodynamic effects in DLA live cells of three water soluble squaraine dyes, viz. bisbenzothiazolium squaraine dyes SQMI and SQDI with iodine in one and both benzothiazolium units, respectively, and an unsymmetrical squaraine dye ASQI containing iodinated benzothiazolium and aniline substituents. The diiodinated SQDI showed an anomalous trend in both fluorescence and triplet quantum yields over the monoiodinated SQMI, with SQDI showing higher fluorescence and lower triplet quantum yields compared to SQMI. Nanosecond laser flash photolysis of SQDI and SQMI indicated the formation of triplet excited states with quantum yield of 0.19 and 0.26, respectively. On photoirradiation, both the SQDI and SQMI generate singlet oxygen and it was observed that both dyes undergoing oxidation reactions with the singlet oxygen generated. ASQI which exhibited a lower triplet quantum yield of 0.06 was, however, stable and did not react with the singlet oxygen generated. In vitro cytotoxicity studies of these dyes in DLA live cells were performed using Trypan blue dye exclusion method and it reflect an order of cytotoxicity of SQDI>SQMI>ASQI. Intracellular generation of the ROS was confirmed by dichlorofluorescein assay after the in vitro PDT.

On the origin of the very strong two-photon activity of squaraine dyes – a standard/damped response theory study

In the present work, we report the mechanism of a very large increase in the two-photon (TP) activity of squaraine based molecules upon changing the substituents.

Bovine serum albumin nanoparticles with fluorogenic near-IR-emitting squaraine dyes

Two squaraine (SQ) dyes, N-propanesulfonate-benzothiazolium squaraine (SQ-1) and N-propanesulfonate-benzoindolium squaraine (SQ-2), were synthesized with sulfonate groups to increase water solubility. Both dyes are almost nonfluorescent in aqueous solution with fluorescent quantum yields of 0.03, but exhibited fluorescence enhancement after noncovalently binding with bovine serum albumin (BSA). Upon addition of BSA, the fluorescence intensity increased by ca. a factor of 10, along with a 10-fold extension in the fluorescence lifetime. SQ-1 and SQ-2 interacted with BSA efficiently and appeared to show a preference for binding at site II, which involves combinational effects of electrostatic and hydrophobic interactions. The fluorogenic squaraine dyes were then used to label BSA, forming BSA-based nanoparticles (NPs) through noncovalent binding. The resulting BSA-SQ NPs exhibited enhanced near-IR fluorescence and reduced aggregation of the squaraine moiety. The BSA-SQ NPs were used for cell incubation and bioimaging studies. Confocal fluorescent images were obtained for HCT 116 cells incubated with the BSA-SQ NPs and LysoSensor Green, demonstrating the utility of the NP probes for intracellular imaging. This strategy ovecomes the generally low fluorescence emission of SQ dyes in water and aggregation-reduced fluorescence, providing a versatile strategy for sensing and imaging in biological environments.

Controlled aggregation and enhanced two-photon absorption of a water-soluble squaraine dye with a poly(acrylic acid) template

Controlling the aggregation behavior of organic dyes is important for understanding and exploring supramolecular assembly utilizing the specific characteristics of aggregation. Regulating J-aggregation by electrostatic interactions between anionic polyelectrolytes and cationic dyes has gained growing interest. Here, we report the formation of J-aggregates of a water-soluble cationic squaraine dye, 4-(pyridinium-1-yl)butylbenzothiazolium squaraine (SQ), using poly(acrylic acid) sodium salt (PAA-Na) as a template. Electrostatic interactions between the PAA-Na polyelectrolyte and the cationic SQ dye enhanced J-aggregation; the absorbance of the resulting J-band with the polyelectrolyte template was much sharper than the absorbance of the J-aggregate formed using a high concentration of NaCl. Significantly, removal of the polyelectrolyte PPA-Na template by the introduction of calcium ions, which can form stronger ionic binding with carboxylate groups, dissociated J-aggregates, freeing the SQ molecules back to unaggregated or lower aggregate forms. To demonstrate the reversibility of the J-aggregate formation cycle, an in situ experiment was conducted that showed 60% reversibility of the second cycle. In addition, an enhancement by ca. 23 times per repeat unit of the two-photon absorption (2PA) cross section was observed at 920 nm for the polyelectrolyte template-SQ J-aggregate compared to unaggregated or lower aggregate SQ. These results suggest a prominent role of polyelectrolyte templated SQ J-aggregation in the enhancement of 2PA efficiency and provide a means of modulating supramolecular assembly.

Protein triggered fluorescence switching of near-infrared emitting nanoparticles for contrast-enhanced imaging

Sub-100 nm colloidal particles which are surface-functionalized with multiple environmentally-sensitive moieties have the potential to combine imaging, early detection, and the treatment of cancer with a single type of long-circulating "nanodevice". Deep tissue imaging is achievable through the development of particles which are surface-modified with fluorophores that operate in the near-infrared (NIR) spectrum and where the fluorophore's signal can be maximized by "turning-on" the fluorescence only in the targeted tissue. We present a general approach for the synthesis of NIR emitting nanoparticles that exhibit a protein triggered activation/deactivation of the emission. Dispersing the particles into an aqueous solution, such as phosphate buffered saline (PBS), resulted in an aggregation of the hydrophobic fluorophores and a cessation of emission. The emission can be reinstated, or activated, by the conversion of the surface-attached fluorophores from an aggregate to a monomeric species with the addition of an albumin. This activated probe can be deactivated and returned to a quenched state by a simple tryptic digestion of the albumin. The methodology for emission switching offers a path to maximize the signal from the typically weak quantum yield inherent in NIR fluorophores.

Thiosquaraine rotaxanes: synthesis, dynamic structure, and oxygen photosensitization

Thiosquaraine dyes have sulfur atoms instead of oxygens on the central squaraine core, and they are powerful singlet oxygen photosensitizers. Stability studies show that they are susceptible to attack by nucleophiles. This problem was circumvented by preparing a mechanically interlocked thiosquaraine rotaxane. NMR studies of the rotaxane indicate an unusual dynamic molecular structure due to a nonsymmetrical coconformation. Upon irradiation with red light, the thiosquaraine rotaxane generates the same amount of singlet oxygen as the known photosensitizer methylene blue.

Multi-photon absorption effect and intra-molecular charge transfer of donor-π-acceptor chromophore ethyl p-amino benzoate

Fourier transform (FT)-Raman and infrared (IR) spectra of the nonlinear optical (NLO) material ethyl p-amino benzoate (EPAB) have been recorded and analyzed. The geometry and harmonic vibrational wavenumbers are calculated with the help of B3LYP density functional theory method. The detailed interpretation of the vibrational spectra has been carried out with the aid of normal coordinate analysis following the scaled quantum mechanical force field methodology. Stability of the molecule arising from hyperconjugative interactions leading to its NLO activity and charge delocalization have been analyzed using natural bond orbital (NBO) analysis. Employing the open-aperture z-scan technique, NLO absorption of the sample has been studied in two excitation regimes, using 100 fs and 5 ns laser pulses respectively. It is found that EPAB is a three-photon absorber for 100 fs pulses at the excitation wavelength of 800 nm. For ns pulses at 532 nm it exhibits strong optical limiting, indicating possible photonics applications.

Enhanced intersystem crossing rate in polymethine-like molecules: sulfur-containing squaraines versus oxygen-containing analogues

Two different approaches to increase intersystem crossing rates in polymethine-like molecules are presented: traditional heavy-atom substitution and molecular levels engineering. Linear and nonlinear optical properties of a series of polymethine dyes with Br- and Se-atom substitution, and a series of new squaraine molecules, where one or two oxygen atoms in a squaraine bridge are replaced with sulfur atoms, are investigated. A consequence of the oxygen-to-sulfur substitution in squaraines is the inversion of their lowest-lying ππ* and nπ* states leading to a significant reduction of singlet-triplet energy difference and opening of an additional intersystem channel of relaxation. Experimental studies show that triplet quantum yields for polymethine dyes with heavy-atom substitutions are small (not more than 10%), while for sulfur-containing squaraines these values reach almost unity. Linear spectroscopic characterization includes absorption, fluorescence, quantum yield, anisotropy, and singlet oxygen generation measurements. Nonlinear characterization, performed by picosecond and femtosecond laser systems (pump-probe and Z-scan measurements), includes measurements of the triplet quantum yields, excited state absorption, two-photon absorption, and singlet and triplet state lifetimes. Experimental results are in agreement with density functional theory calculations allowing determination of the energy positions, spin-orbital coupling, and electronic configurations of the lowest electronic transitions.