Quantum Yield Of Generation Research Articles

Unravelling the intricacies of fluorescence enhancement in heptamethine cyanine dyes induced by heavy halogen atoms

The classic heavy-atom effect refers to the photophysical phenomenon observed in fluorophores, where the interaction with heavy atoms quenches fluorescence emission. Many fluorophores containing heavy halogen atoms, such as xanthenes, BODIPYs, porphyrins, and anthracenes, adhere to this effect, while the photophysical behavior of others, e.g. halogenated heptamethine cyanine dyes, is poorly studied having contradicting results. In this study, we explored for the first time unexpected fluorescence enhancement induced by heavy halogen atoms in heptamethine dyes, contrasting the classic heavy-atom effect. To this end, a series of novel heptamethine cyanine dyes were synthesized and their photophysical properties were experimentally and theoretically investigated. Heavy halogen atoms were found to significantly affect the fluorescence behaviour of heptamethine dyes depending on the number and position of the attached halogens. Thus, the introduction of bromine and iodine atoms at the non-conjugated positions to the chromophore moieties led to an increase in the fluorescence quantum yields, which contradicts the classic principle and could be explained by heavy halogen-induced vibration restrictions. The value of halogen atom-promoted singlet oxygen generation quantum yield ΦΔ is also not straightforward and depends on the number, position and weight of the halogen atom. The breaking of the classic heavy-atom effect in halogenated heptamethine dyes provides an effective strategy to substantially increase the fluorescence intensity of long-wavelength cyanine dyes, providing new functional advantages for fluorescence imaging, diagnostics, and photodynamic therapy, among other applications.

The (un)known issue with using rose bengal as a standard of singlet oxygen photoproduction.

Rose bengal (RB) is a widely used photosensitizer for determining quantum yields of singlet oxygen generation. While it is known to aggregate in polar environments at concentrations above 2 μM, the relationship between RB concentration and singlet oxygen photogeneration remains unclear. This study investigates the shift from monomeric to dimeric RB with increasing concentration and its impact on singlet oxygen generation in D2O-based solutions and DMPC liposomes. Absorbance maxima for RB were observed at 514 nm (dimer) and 549 nm (monomer), with ionic environments influencing aggregation rates. Singlet oxygen phosphorescence showed non-linear dependency above 2 μM, indicating the effects of aggregation. Results suggest that RB concentrations should be kept at 1 μM or lower in photochemical studies to avoid aggregation-related discrepancies in singlet oxygen yield determination. These findings highlight the importance of considering RB aggregation in photochemical research and medical applications.

Photovoltaic Molecule-Based Phototheranostics With A-D-A Structure for Near-Infrared Fluorescence Imaging-Guided Synergetic Photodynamic and Photothermal Therapy of Tumors.

Phototheranostics with near-infrared fluorescence and reactive oxygen species generation ability and high photothermal conversion efficiency (PCE) plays a significant role in fluorescence imaging-guided synergetic photodynamic and photothermal therapy of tumors. Here, a star molecule in organic photovoltaic materials, NCBDT-4C with an A-D-A conjugated structure, was assembled with DSPE-PEG-NH2 to prepare water dispersive nanoparticles (NPs). The prepared NCBDT-4Cl NPs exhibited a maximum NIR absorption peak at 764 nm and a maximum fluorescence peak at 798 nm. These NPs could generate superoxide anion, singlet oxygen (1O2), and heat under 808 nm laser irradiation. The 1O2 generation quantum yield and PCE of the NPs were 37.5% and 53.6%, respectively. The combination of photodynamic and photothermal therapy of cancer was demonstrated invitro and invivo. This work presents the advanced application of organic photovoltaic materials in cancer phototherapy.

Non-peripheral octasubstituted zinc(II) phthalocyanines bearing pyridinepropoxy substituents – Antibacterial, anticancer photodynamic and sonodynamic activity

The novel non-peripheral octa-substituted zinc(II) phthalocyanines with 3- and 4-pyridinepropoxy substituents were synthesized via cyclization of substituted phthalonitriles and further characterized. Their photodynamic and sonodynamic activity were then assessed toward bacteria and cancer cells. Additionally, inhibition activity against common human enzymes was evaluated. The singlet oxygen generation (with 1,3-diphenylisobenzofuran – DPBF as an unspecific chemical quencher of singlet oxygen) were measured under light irradiation, whereas under ultrasounds (1 MHz, 3 W) the stability of DPBF in the presence of sensitizer was evaluated. Both phthalocyanines revealed high photostability in DMSO and moderate in DMF, whereas the sonostability in DMF was moderate. Calculated light-induced singlet oxygen generation quantum yields were similar for both compounds and oscillated around 0.33 in DMF and 0.67 in DMSO. Sonodynamic manner revealed moderately high DPBF decomposition upon 1 MHz. Significant bacterial reduction was noted in both photodynamic and sonodynamic manner, reaching >3 log reduction against MRSA and S. epidermidis. Both compounds showed ca. 50 % viability reduction toward hypopharyngeal tumor (FaDu). Moreover, up to 60 % viability reduction was observed in squamous cell carcinoma (SCC-25). In summary, this molecular building of the efficient phthalocyanine-based sensitizer is a potential therapeutic for photodynamic and sonodynamic applications.

Electrodeposition of Photosensitive Layers Based on Conducting Polymers and Zinc Phthalocyaninate, Their Structure and Photoelectrical Properties

Photosensitive hybrid layers were obtained by electrochemical polymerization of pyrrole and 3,4-ethylenedioxythiophene (EDOT) in the presence of water-soluble sodium salt of zinc octa(3ʹ,5ʹ-dicarboxyphenoxy)phthalocyaninate (ZnPc) containing 16 ionogenic carboxylate groups. It was found that the process of electrodeposition of hybrid layers most effectively occurs in galvanostatic and potentiostatic modes on the sublayer of the PEDOT-polyacid complex. The electronic and chemical structure and morphology of hybrid layers of polypyrrole (PPy) obtained in the presence of ZnPc were studied. Possible reasons are considered that the measured values of photosensitivity and external quantum yield of charge carrier generation in PPy-ZnPc are several times higher than in PEDOT-ZnPc.

Exploiting G-Quadruplex-DNA Damage as a Tool to Quantify Singlet Oxygen Production.

G-Quadruplexes (G4s) are highly dynamic and polymorphic nucleic acid structures that can adopt a variety of conformations. When exposed to oxidative conditions, more specifically singlet oxygen, the guanosine nucleobases can be oxidized, which in turn can affect the conformation and folding of the G4. Based on this peculiar phenomenon, it is rationalized that G4s can serve as quantification sensors for the production of singlet oxygen. Here, a method for determining the quantum yield of singlet oxygen generation for visible as well as UV-light excited photosensitizers, using a short G4 DNA sequence, readily available from common DNA companies, as a biological and water-soluble probe, is presented.

О возможности использования иттербиевого комплекса 2,4-ди (α-метоксиэтил)дейтеропорфирина IX для тераностики новообразований поверхностной локализации

The dynamics of the photosensitized singlet oxygen formation for the ytterbium 2,4-di(α-methoxyethyl)deuteroporphyrin IX complex (Yb-DMDP IX) and for its metal-free form under laser excitation on a highly sensitive laser fluorimeter was studied. It has been shown that the quantum yield of singlet oxygen generation for Yb-DMDP IX (11%) is significantly lower than in the case of DMDP IX itself (40%), which experimentally confirms the low phototoxicity of the Yb complex. However, the established 11% quantum yield of 1O2 indicates the possible presence of DMDP IX in the main substance of Yb-DMDP IX, leading to residual phototoxicity. Using high-performance liquid chromatography, the quantitative ratios of the Yb-complex and its metal-free form (10%) were established. Thus, the presented Yb-complex can also be used for cancer theranostics, since after carrying out IR luminescence diagnostics (in the 900–1100 nm spectral range), it is also possible to subsequently carry out the PDT procedure in the absorption band of the porphyrin complex metal-free form at the 630 nm wavelength. The Yb-DMDP IX effectiveness for the PDT procedure can be significantly increased by treating this compound with ascorbic acid after the IR luminescence diagnostics procedure.

Phosphorus (V) 15-crown-5-phthalocyanine as a promising non-toxic near-IR photoinitiator in aqueous medium

The search for novel water soluble red and near-IR photoinitiators for biomedical applications is a very important task. Here we describe the synthesis and photophysical properties of a water soluble complex of P(V) with tetra-15-crown-5-phthalocyanine bearing diethylene glycol monoethyl ether as axial ligand (15C5PcP-gly). In addition to intense near-IR absorption, compound 15C5PcP-gly is characterized by a high quantum yield of singlet oxygen generation with moderate photostability. The combination of these properties with the discovered low dark and light induced cytotoxicity and negligible intracellular accumulation makes the complex 15C5PcP-gly promising for biophotopolymerization. Taking advantage of all the benefits of the 15C5PcP-gly complex, we demonstrate for the first time its successful application in the bulk photocuring of a water soluble polyethylene glycol diacrylate-based system with simultaneous preservation of mammalian cells viability.

Hydrophilic Biocompatible Fluorescent Organic Nanoparticles as Nanocarriers for Biosourced Photosensitizers for Photodynamic Therapy.

Most photosensitizers of interest for photodynamic therapy-especially porphyrinoids and chlorins-are hydrophobic. To circumvent this difficulty, the use of nanocarriers is an attractive strategy. In this perspective, we have developed highly water-soluble and biocompatible fluorescent organic nanoparticles (FONPs) made from citric acid and diethyltriamine which are then activated by ethlynene diamine as nanoplatforms for efficient photosensitizers (PSs). Purpurin 18 (Pp18) was selected as a biosourced chlorin photosensitizer combining the efficient single oxygen generation ability and suitable absorption in the biological spectral window. The simple reaction of activated FONPs with Pp18, which contains a reactive anhydride ring, yielded nanoparticles containing both Pp18 and Cp6 derivatives. These functionalized nanoparticles combine solubility in water, high singlet oxygen generation quantum yield in aqueous media (0.72) and absorption both in the near UV region (FONPS) and in the visible region (Soret band approximately 420 nm as well as Q bands at 500 nm, 560 nm, 660 nm and 710 nm). The functionalized nanoparticles retain the blue fluorescence of FONPs when excited in the near UV region but also show deep-red or NIR fluorescence when excited in the visible absorption bands of the PSs (typically at 520 nm, 660 nm or 710 nm). Moreover, these nanoparticles behave as efficient photosensitizers inducing colorectal cancer cell (HCT116 and HT-29 cell lines) death upon illumination at 650 nm. Half maximal inhibitory concentration (IC50) values down to, respectively, 0.04 and 0.13 nmol/mL were observed showing the potential of FONPs[Cp6] for the PDT treatment of cancer. In conclusion, we have shown that these novel biocompatible nanoparticles, which can be elaborated from biosourced components, both show deep-red emission upon excitation in the red region and are able to produce singlet oxygen with high efficiency in aqueous environments. Moreover, they show high PDT efficiency on colorectal cancer cells upon excitation in the deep red region. As such, these functional organic nanoparticles hold promise both for PDT treatment and theranostics.

Push–pull biphenyl–based iodonium salts: Highly sensitive one-component photoinitiators for photopolymerization under UV–visible LEDs

Diaryliodonium salts play an important role as highly efficient photoinitiators in the field of photopolymerizations. They can rapidly photodecompose to produce free radicals and super acids; thus, can initiate highly efficient photocuring of acrylic and epoxy monomers and resins. However, the absorption of commercialized diphenyliodonium salts is poorly matched with popular LED light sources, and the participation of sensitizers is usually required to achieve efficient photoexcitation. Here, we synthesized five new phenyl biphenyl iodonium salts with different electron-pushing and electron-withdrawing substituents, e.g., methoxy and CN, at the biphenyl substituent. Our results demonstrate that all synthesized iodonium salts have strong absorption in the range (λmax = 258–352 nm; εmax = 21,300–29,900 M−1 cm−1). The photochemistry of these synthesized iodonium salt is strongly influenced by substituents on the biphenyl ring. All iodonium salts can undergo rapid photolysis under light excitation, with higher quantum yield of photoacid generation reaching 0.25 and can quickly initiate cationic photopolymerization of epoxy monomers. Furthermore, free radicals are generated during photolysis, which can successfully trigger free radical polymerization. These iodonium salts with good absorbance in the UVA band demonstrate their potential as excellent photoinitiators under 365–405 nm LED excitation.

Exploring a Mitochondria Targeting, Dinuclear Cyclometalated Iridium (III) Complex for Image-Guided Photodynamic Therapy in Triple-Negative Breast Cancer Cells.

Photodynamic therapy (PDT) has emerged as an efficient and noninvasive treatment approach utilizing laser-triggered photosensitizers for combating cancer. Within this rapidly advancing field, iridium-based photosensitizers with their dual functionality as both imaging probes and PDT agents exhibit a potential for precise and targeted therapeutic interventions. However, most reported classes of Ir(III)-based photosensitizers comprise mononuclear iridium(III), with very few examples of dinuclear systems. Exploring the full potential of iridium-based dinuclear systems for PDT applications remains a challenge. Herein, we report a dinuclear Ir(III) complex (IRDI) along with a structurally similar monomer complex (IRMO) having 2-(2,4-difluorophenyl)pyridine and 4'-methyl-2,2'-bipyridine ligands. The comparative investigation of the mononuclear and dinuclear Ir(III) complexes showed similar absorption profiles, but the dinuclear derivative IRDI exhibited a higher photoluminescence quantum yield (Φp) of 0.70 compared to that of IRMO (Φp = 0.47). Further, IRDI showed a higher singlet oxygen generation quantum yield (Φs) of 0.49 compared to IRMO (Φs = 0.28), signifying the enhanced potential of the dinuclear derivative for image-guided photodynamic therapy. In vitro assessments indicate that IRDI shows efficient cellular uptake and significant photocytotoxicity in the triple-negative breast cancer cell line MDA-MB-231. In addition, the presence of a dual positive charge on the dinuclear system facilitates the inherent mitochondria-targeting ability without the need for a specific targeting group. Subcellular singlet oxygen generation by IRDI was confirmed using Si-DMA, and light-activated cellular apoptosis via ROS-mediated PDT was verified through various live-dead assays performed in the presence and absence of the singlet oxygen scavenger NaN3. Further, the mechanism of cell death was elucidated by an annexin V-FITC/PI flow cytometric assay and by investigating the cytochrome c release from mitochondria using Western blot analysis. Thus, the dinuclear complex designed to enhance spin-orbit coupling with minimal excitonic coupling represents a promising strategy for efficient image-guided PDT using iridium complexes.

Optimizing radical yield from free chlorine with tailored UV light emitting diode emission spectra

Novel UV sources, which do not contain mercury, provide the opportunity for enhancement of current oxidation technologies through spectral optimization, minimizing inefficiencies that currently limit conventional technology. Wastewater reuse is the primary full-scale application of UV advanced oxidation processes (AOPs) in practice but any background absorbance and the low molar absorption by conventional radical promoters (hydrogen peroxide) have historically limited their system efficiency, resulting in the underutilization of photons in a reactor. This bench-scale research evaluated use of longer wavelength UV light emitting diodes (265, 280, and 300 nm) matched with free chlorine to optimize the utilization of photons for advanced oxidation. Free chlorine possesses large absorption bands in the 280 to 300 nm range in basic pH waters which are common in carbon-based reuse and was used to experimentally verify quantum yields of hydroxyl radical generation across the UV LED peak emission wavelengths. pH- and wavelength-dependent fluence-based rate constants were experimentally derived using Nitrobenzene and Benzoic acid as probe compounds and evaluated to determine the contribution of the hydroxyl and chlorine radical. Reclaimed water taken from various advanced treatment steps was treated with this UV LED AOP to investigate how background absorbance affects radical generation and contaminant transformation kinetics. In addition, alternative performance metrics to evaluate hydroxyl radical production at different incident fluence rates and different rates of photon absorption at unique wavelengths across varying background UV absorbance levels were assessed.

Luminescence of In(III)Cl-etioporphyrin-I.

The luminescent and photophysical properties of the etioporphyrin-I complex with indium(III) chloride, InCl-EtioP-I were experimentally studied at room and liquid nitrogen temperatures in pure and mixed toluene solutions. At 77 K, in a 1:2 mixture of toluene with diethyl ether, the quantum yield of phosphorescence reaches 10.2%, while the duration of phosphorescence is 17 ms. At these conditions, the ratio of phosphorescence-to-fluorescence integral intensities is equal to 26.1, which is the highest for complexes of this type. At 298 K, the quantum yield of the singlet oxygen generation is maximal in pure toluene (81%). Quantum-chemical calculations of absorption and fluorescence spectra at temperatures of 77 K and 298 K qualitatively coincide with the experimental data. The InCl-EtioP-I compound will further be used as a photoresponsive material in thin-film optoelectronic devices.

Spectroscopic Study on CdS/Ni/KNbO3: Confirming Ni Effect to Photocatalytic Activity.

Herein, we report the structural and photophysical properties of CdS/Ni/KNbO3 composites with a quantum yield for photocatalytic H2 generation that is CdS and Ni amount dependent. The nonstoichiometric KNbO3 (1:1.1) structure indicates the defect at the K site, which is Ni-occupied during its deposit process. It exhibits a tendency like a Ni-doped characteristic up to 0.1 wt % Ni and then forms a Ni cluster in case the Ni amount exceeds 0.1 wt %. The related structural and photophysical properties of CdS/Ni/KNbO3 are examined with Fourier transform infrared, X-ray diffraction, ultraviolet-visible absorption, and luminescence spectral analysis. It demonstrates the CdS/Ni/KNbO3 composites to be an efficient light conversion caused by efficient charge/electron transfer between KNbO3 and CdS via doped Ni. The photocatalytic activity of CdS/Ni/KNbO3 exhibits a CdS and Ni amount dependency. The best photocatalytic activity for H2 generation is obtained with 0.1 wt % Ni and 2.9 wt % CdS as it gradually declines with the excess Ni amount than 0.1 wt % caused by a formed Ni cluster.

Protein Photodamaging Activity and Photocytotoxic Effect of an Axial-Connecting Phosphorus(V)porphyrin Trimer.

An axial-connecting trimer of the porphyrin phosphorus(V) complex was synthesized to evaluate the relaxation process of the photoexcited state and the photosensitizer activity. The photoexcitation energy was localized on the central unit of the phosphorus(V)porphyrin trimer. The photoexcited state of the central unit was relaxed through a process similar to that of the monomer phosphorus(V)porphyrin. The excited state of this axially connected type of phosphorus(V)porphyrin trimer was not deactivated through intramolecular electron transfer. The singlet oxygen generation quantum yield of the trimer was almost the same as that of the monomer. The phosphorus(V)porphyrin, trimer, and monomer bound to human serum albumin and oxidized the tryptophan residue via singlet oxygen generation and electron transfer during visible light irradiation. The photocytotoxicity of these phosphorus(V)porphyrins on two cell lines was examined. The monomer induced photocytotoxicity; however, the trimer did not show cytotoxicity with or without photoirradiation. In summary, the photoexcited state of the trimer was almost the same as that of the monomer, and these phosphorus(V)porphyrins demonstrated a similar protein-photodamaging activity. The difference in association between the photosensitizer molecules and cells is the key factor of phototoxicity by these phosphorus(V)porphyrins.

Spectroscopic study of methylene blue photophysical properties in biological media

A spectroscopic study of the photophysical properties of methylene blue (MB) in aqueous solutions was carried out. Absorption and fluorescence spectra as well as fluorescence lifetime were recorded. The concentration dependence of the intensity and shape of the spectra allowed establishing the ranges of MB concentrations for in vitro and in vivo studies at which aggregation is not observed (up to 0.01 mM, which corresponds to 3.2 mg/kg). Studies of photodegradation in biological media showed that photobleaching of more than 80% in plasma and culture media is observed already at a dose of 5 J/cm2 , while in water at this concentration and dose photobleaching is not yet observed, and at a dose of 50 J/cm2 photobleaching of MB is about 30%. It was found that in media containing proteins and having an alkaline pH, photobleaching occurs significantly faster than in neutral aqueous media. The ionic strength of the solution has no effect on the photobleaching rate. Such photobleaching is caused by the photodegradation of MB rather than the transition to the leucoform.The efficiency of singlet oxygen generation and photodynamic activity were evaluated in vitro. In the investigated range of MB concentrations, the efficiency of singlet oxygen generation is rather low, because positively charged MB binds to negatively charged cell membranes, which leads to a change in the type of photodynamic reaction. The emergence of other reactive oxygen species (ROS), different from singlet oxygen, in cells has been demonstrated. The generation of ROS and the low quantum yield of singlet oxygen generation indicate the tendency of MB to provide the type I photosensitization mechanism (electron transfer with the formation of semi-reduced and semi-oxidized MB+ radicals) rather than to the type II mechanism (energy transfer to oxygen with the formation of singlet oxygen) in biological media and in vivo.

Tetramethylalloxazines as efficient singlet oxygen photosensitizers and potential redox-sensitive agents

Tetramethylalloxazines (TMeAll) have been found to have a high quantum yield of singlet oxygen generation when used as photosensitizers. Their electronic structure and transition energies (S0 → Si, S0 → Ti, T1 → Ti) were calculated using DFT and TD-DFT methods and compared to experimental absorption spectra. Generally, TMeAll display an energy diagram similar to other derivatives belonging to the alloxazine class of compounds, namely π,π* transitions are accompanied by closely located n,π* transitions. Photophysical data such as quantum yields of fluorescence, fluorescence lifetimes, and nonradiative rate constants were also studied in methanol (MeOH), acetonitrile (ACN), and 1,2-dichloroethane (DCE). The transient absorption spectra were also analyzed. To assess cytotoxicity of new compounds, a hemolytic assay was performed using human red blood cells (RBC) in vitro. Subsequently, fluorescence lifetime imaging experiments (FLIM) were performed on RBC under physiological and oxidative stress conditions alone or in the presence of TMeAll allowing for pinpointing changes caused by those compounds on the intracellular environment of these cells.

Regulating 1O2 generation from heavy-atom-free triplet photosensitizers based on thiophene-fused BODIPY

Triplet photosensitizers (PSs) used in photodynamic therapy (PDT) exhibit a strong relationship between the intersystem crossing (ISC) and generation of singlet oxygen (1O2). However, the effective design of PS molecules, particularly, heavy-atom-free PS, remains challenging due to limited understanding of the correlation between ISC rate (kISC) and the quantum yield of 1O2 generation (ΦΔ). We present a case study for designing heavy-atom-free PSs (R–SB) via a combination of experimental and theoretical investigations. Electron-donating groups were introduced to the thiophene-fused BODIPY core. The measured ΦΔ increased as the calculated kISC increased. The remarkable agreement between the experimental and theoretical findings improved our understanding of the ISC process in R–SB structures and established a framework for the potential future molecular design of heavy-atom-free PSs based on the BODIPY core. Among the various R–SB structures synthesized, carbazolyl benzene (Cbz) -SB exhibited fluorescence emission and was the most effective for 1O2 generation. Cbz-SB was further utilized for mitochondria-targeted fluorescence bioimaging and the PDT of HeLa cells, demonstrating its potential applicability in the respective areas.

Synthesis, Electrochemical and Photochemical Properties of Sulfanyl Porphyrazine with Ferrocenyl Substituents.

Ferrocene is useful in modern organometallic chemistry due to its versatile applications in material sciences, catalysis, medicinal chemistry, and diagnostic applications. The ferrocene moiety can potentially serve many purposes in therapeutics and diagnostics. In the course of this study, (6-bromo-1-oxohexyl)ferrocene was combined with dimercaptomaleonitrile sodium salt to yield a novel maleonitrile derivative. Subsequently, this compound was subjected to an autocyclotetramerization reaction using the Linstead conditions in order to obtain an octaferrocenyl-substituted magnesium(II) sulfanyl porphyrazine. Following that, both compounds-the maleonitrile derivative and the porphyrazine derivative-were subjected to physicochemical characterization using UV-Vis, ES-TOF, MALDI-TOF, and one-dimensional and two-dimensional NMR spectroscopy. Moreover, the sulfanyl porphyrazine was subjected to various photophysical studies, including optical absorption and emission measurements, as well as the evaluation of its photochemical properties. Values of singlet oxygen generation quantum yields were obtained in different organic solvents. The electrochemical properties of the synthesized compounds were studied using cyclic voltammetry. According to the electrochemical results, the presence of electron-withdrawing oxohexyl groups attached to ferrocene afforded significantly more positive oxidation potentials of the ferrocene-based redox process up to 0.34 V vs. Fc+/Fc.

Effect of an amine functionalization of phenanthroline ligand associated with an ester-pyridine ligand on excited-state properties of Re(I) complex. Interplay between two 3MLCT

In this work, the synthesis, characterization and photophysical properties of rhenium(I) polypyridyl compound, fac-[Re(et-isonic)(NH2phen)(CO)3]PF6, where NH2phen = 5-amino-1,10-phenanthroline and et-isonic = ethyl isonicotinate, are reported. The compound exhibited a broad absorption band around 300–500 nm that by Time-Dependent Density Functional Theory (TD-DFT) corresponds to a mixture of ILNH2phen and MLCTRe→NH2phen transitions and a contribution of the charge transfer transition MLCTRe→et-isonic. Contrary to what is expected, the change of Cl (λmax = 490 and 565 nm; ϕ = 0.002; τ = 0.49 ± 0.07 μs and < 0.020 μs) for et-isonic ligand (λmax = 590 nm; ϕ = 0.003; τ = 0.43 ± 0.03 μs and 0.044 ± 0.014 μs), a π-acceptor ligand, promoted a bathochromic shift of the emission maxima in acetonitrile solution. This distinct behavior can be rationalized in terms of the inversion of the lowest-lying excited state of the usual observed MLCTRe→NN to the unusual MLCTRe→et-isonic. On the other hand, the contribution of ligand-based excited state in the deactivation process cannot be ruled out. Additionally, as a triplet emitter, a high quantum yield for the singlet oxygen generation (0.50 ± 0.03) can be associated with either, 3MLCT or a 3LC state. The photophysics ally to the higher singlet oxygen generation reported herein provides new fundamental insights into the understanding of substituent groups on the polypyridyl ligands that are relevant to the practical development of the scientific field.