Lowest Unoccupied Molecular Orbital Energy Research Articles

High‐Performance Organic Electrochemical Transistors and Neuromorphic Devices Comprising Naphthalenediimide‐Dialkoxybithiazole Copolymers Bearing Glycol Ether Pendant Groups

AbstractOrganic electrochemical transistors (OECTs) have emerged as building blocks for low power circuits, biosensors, and neuromorphic computing. While p‐type polymer materials for OECTs are well developed, the choice of high‐performance n‐type polymers is limited, despite being essential for cation and metabolite biosensors, and crucial for constructing complementary circuits. N‐type conjugated polymers that have efficient ion‐to‐electron transduction are highly desired for electrochemical applications. In this contribution, three non‐fused, planar naphthalenediimide (NDI)‐dialkoxybithiazole (2Tz) copolymers, which systematically increase the amount of polar tri(ethylene glycol) (TEG) side chains: PNDI2OD‐2Tz (0 TEG), PNDIODTEG‐2Tz (1 TEG), PNDI2TEG‐2Tz (2 TEG), are reported. It is demonstrated that the OECT performance increases with the number of TEG side chains resulting from the progressively higher hydrophilicity and larger electron affinities. Benefiting from the high electron mobility, excellent ion conduction capability, efficient ion‐to‐electron transduction, and low‐lying lowest unoccupied molecular orbital energy level, the 2 TEG polymer achieves close to 105 on‐off ratio, fast switching, 1000 stable operation cycles in aqueous electrolyte, and has a long shelf life. Moreover, the higher number TEG chain substituted polymer exhibits good conductance state retention over two orders of magnitudes in electrochemical resistive random‐access memory devices, highlighting its potential for neuromorphic computing.

Solvatochromism and ZINDO-IEFPCM solvation study on NHS ester activated AF514 and AF532 dyes: Evaluation of the dipole moments

In this study, the solvatochromic effect on the photophysical properties of Alexa Fluor 514 (AF514) and Alexa Fluor 532 (AF532) fluorescent dyes is examined experimentally and computationally. To explore the solvatochromism and dipole moments, the steady-state absorption and fluorescence spectra of the dyes were measured in a series of organic solvents. Various solvent correlation models, like Bilot-Kawski, Lippert-Mataga, Bakhshiev, Kawski-Chamma-Viallet, and Reichardt microscopic solvent polarity parameters, were adapted to determine the dipole moments in their ground and excited states. For the computational investigation, the ground and excited-state geometries are optimized using density functional theory (DFT) and time-dependent density functional theory (TD-DFT), respectively, in vacuum. Furthermore, semiempirical ZINDO with the IEF-PCM model is used to evaluate the absorption transition energies of these dyes, which are comparatively studied in various solvent polarity along with experimental data. Additionally, the highest occupied molecular orbital energies (HOMO) and lowest unoccupied molecular orbital energies (LUMO), chemical softness, chemical hardness, energy gap, chemical potential, electronegativity, and molecular electrostatic potential (MEP) were estimated using DFT calculations at the CAM-B3LYP/6-311G(d,p) level, in gas phase. The experimental and computational results reveal that the singlet excited state dipole moment is greater than that of the ground state for the molecules considered. The angle between ground- and singlet excited-state dipole moments are found to be 0.50 and 0.49° making them almost parallel to each other. The natural bond orbital analysis (NBO) has been employed to investigate the stability of the molecule, inter- and intra-hyper-conjugative interactions and charge delocalization within the molecule.

Small molecules based on strongly electron-deficient aza-isatinylidene malononitrile for solution-processed n-type field-effect transistors

In this paper, three donor–acceptor (D/A) small molecules (M1, M2, and M3) based on aza-isatinylidene malononitrile (AIMN) and diketopyrrolopyrrole (DPP) containing different N-alkyl chains in AIMN unit were synthesized and characterized for application in solution-processable organic field-effect transistors (OFETs). The D/A small molecules with strongly electron-withdrawing AIMN and DPP units exhibited broad absorption spectra, low bandgaps, and deep lowest unoccupied molecular orbital energy levels (< – 4.0 eV). Consequently, OFET devices displayed unipolar electron transport characteristics with the highest electron mobilities of 0.26, 0.22, and 0.0079 cm2V–1s–1 for M1, M2, and M3, respectively. The small molecules with linear side chains showed much higher field-effect performances than that of branched side chains- based small molecule, due to the strong π–π stacking and appropriate morphology. This work indicated that the strongly electron-deficient AIMN should be a potential unit for constructing the n-type solution-processable small molecular semiconductors.

Enhancing the Intermolecular Interactions of Ladder-Type Heteroheptacene-Based Nonfullerene Acceptors for Efficient Polymer Solar Cells by Incorporating Asymmetric Side Chains

Enhancing the Intermolecular Interactions of Ladder-Type Heteroheptacene-Based Nonfullerene Acceptors for Efficient Polymer Solar Cells by Incorporating Asymmetric Side Chains

Isogenous Asymmetric–Symmetric Acceptors Enable Efficient Ternary Organic Solar Cells with Thin and 300 nm Thick Active Layers Simultaneously

AbstractIntegrating desirable light absorption, energy levels, and morphology in one matrix is always the aspiration to construct high‐performance organic solar cells (OSCs). Herein, an asymmetric acceptor Y6‐1O is incorporated into the binary blends of acceptor Y7‐BO and donor PM6 to prepare ternary OSCs. Two isogenous asymmetric–symmetric acceptors with similar chemical skeletons tend to form alloy‐like state in blends due to their good compatibility, which contributes to optimizing the morphology for efficient charge generation and extraction. The complementary absorption of two acceptors helps to improve the photon harvesting of ternary blends, and the higher lowest unoccupied molecular orbital (LUMO) energy level of Y6‐1O offers the chance to uplift the mixed LUMO energy levels of acceptors. Combining the aforesaid benefits, the ternary OSCs with 10 wt% Y6‐1O produce a top‐ranked power conversion efficiency (PCE) of 18.11% with simultaneously elevated short‐circuit current density, open‐circuit voltage, and fill factor in comparison to Y7‐BO‐based binary devices. Furthermore, the optimized ternary OSCs with ≈300 nm active layers obtain a champion PCE of 16.61%, which is the highest value for thick‐film devices reported so far. This work puts forward an avenue for further boosting the performance of OSCs with two isogenous acceptors but different asymmetric structures.

Nonfused Ring Electron Acceptors for Efficient Organic Solar Cells Enabled by Multiple Intramolecular Conformational Locks

Nonfused ring electron acceptors (NFREAs) have become a research hotspot of organic solar cells (OSCs) due to their facile synthesis. However, efficient NFREAs not only need to maintain the advantages of FREAs but also need to optimize the molecular structure of the conjugate backbone to achieve good planarity. Therefore, choosing the appropriate building blocks is a prerequisite for achieving efficient OSCs. Here, two simple NFREAs 2T2CSi-4F and 4T2CSi-4F, based on diester-thieno[3,2-b]thiophene (2T2C) as the central core unit and 4,4-di-2-ethylhexyl-dithieno[3,2-b:2′,3′-d]silole (DTSi) or thieno[3,2-b]thiophene (TT) as the conjugated linking unit, were designed and synthesized. The density function theory results manifest that the oxygen atom of the thiophene ester group can form O···S interaction with the sulfur atom. Introducing noncovalent interactions can form multiple intramolecular conformational locks, which greatly enhance the molecular planarity. In addition, 2T2CSi-4F with a symmetrical structure exhibits red-shifted absorption, shallower lowest unoccupied molecular orbital energy levels, and stronger crystallinity than 4T2CSi-4F. Therefore, the PBDB-T:2T2CSi-4F device with favorable molecular packing and morphology achieves a higher power conversion efficiency of 10.04%. Our work demonstrates that the 2T2C unit and multimolecular conformational lock strategy are conducive to the development of efficient NFREAs.

Single Excitation Energies Obtained from the Ensemble “HOMO–LUMO Gap”: Exact Results and Approximations

In calculations based on density functional theory, the "HOMO-LUMO gap" (difference between the highest occupied and lowest unoccupied molecular orbital energies) is often used as a low-cost, ad hoc approximation for the lowest excitation energy. Here we show that a simple correction based on rigorous ensemble density functional theory makes the HOMO-LUMO gap exact in principle and significantly more accurate in practice. The introduced perturbative ensemble density functional theory approach predicts different and useful values for singlet-singlet and singlet-triplet excitations, using semilocal and hybrid approximations. Excitation energies are similar in quality to time-dependent density functional theory, especially at high fractions of exact exchange. The approach therefore offers an easy-to-implement and low-cost route to robust prediction of molecular excitation energies.

Molecular Optimization on Polymer Acceptor Enables Efficient All-Polymer Solar Cell with High Open-Circuit Voltage of 1.10V.

Currently, rational design of polymer acceptors is desirable but there is still a challenge to develop high-performance all-polymer solar cells (all-PSCs). In this work, brominated thienyl-fused malononitrile-based monomer is employed to copolymerize with indacenodithiophene (IDT) and benzodithiophene (BDT)-based linking units to develop two polymerized small molecule acceptors (PSMAs) PIDT and PBDT, respectively, for all-PSCs. The two PSMAs show similar absorption edges, while PBDT shows a slightly higher lowest unoccupied molecular orbital (LUMO) energy level than PIDT. Benefitted from the relatively high LUMO levels of the two polymer acceptors, notable open-circuit voltage (Voc ) values over 1.0V are achieved when using them as acceptor to blend with PTQ10 as polymer donor. Particularly, the all-PSC based on PTQ10:PIDT demonstrates a power conversion efficiency of 10.19%, with an outstanding Voc of 1.10V benefitted from the higher LUMO energy level of PIDT acceptor. The results demonstrate a feasible strategy to design PSMAs by selecting appropriate linking units for increasing the Voc and improving the efficiency of all-PSCs.

Sulfidation extent of nanoscale zerovalent iron controls selectivity and reactivity with mixed chlorinated hydrocarbons in natural groundwater

Sulfidated nanoscale zerovalent iron (S-nZVI) exhibits low anoxic oxidation and high reactivity towards many chlorinated hydrocarbons (CHCs). However, nothing is known about S-nZVI reactivity once exposed to complex CHC mixtures, a common feature of CHC plumes in the environment. Here, three S-nZVI materials with varying iron sulfide (mackinawite, FeSm) shell thickness and crystallinity were exposed to groundwater containing a complex mixture of chlorinated ethenes, ethanes, and methanes. CHC removal trends yielded pseudo-first order rate constants (kobs) that decreased in the order: trichloroethene > trans-dicloroethene > 1,1-dichlorethene > trichloromethane > tetrachloroethene > cis-dichloroethene > 1,1,2-trichloroethane, for all S-nZVI materials. These kobs trends showed no correlation with CHC reduction potentials based on their lowest unoccupied molecular orbital energies (ELUMO) but absolute values were affected by the FeSm shell thickness and crystallinity. In comparison, nZVI reacted with the same CHCs groundwater, yielded kobs that linearly correlated with CHC ELUMO values (R2 = 0.94) and that were lower than S-nZVI kobs. The CHC selectivity induced by sulfidation treatment is explained by FeSm surface sites having specific binding affinities towards some CHCs, while others require access to the metallic iron core. These new insights help advance S-nZVI synthesis strategies to fit specific CHC treatment scenarios.

Thieno[3,4‐c]pyrrole‐4,6‐dione‐Based Conjugated Polymers for Nonfullerene Organic Solar Cells

AbstractDonor–acceptor (D–A) conjugated polymers have gotten significant attention in the past decades. Wide application area, ease of production, and cost‐effectiveness have made the D–A conjugated polymers a pivotal class of organic electronics. Organic solar cells (OSCs) became one of the most critical application areas of conjugated polymers among many. Up to the last decade, intensive research has been conducted on fullerene‐based OSCs. Drawbacks of fullerene derivatives like high cost, production obstacles, and difficulties in modifying molecular structure made the academia seek new alternatives. Emerging nonfullerene acceptors (NFAs) draw a new path for academia to tune OSCs' power conversion efficiency (PCE). With the ease of synthesis and purification, modification of molecular structure via various functional groups, NFAs start outperforming fullerene‐based acceptors. Thieno[3,4‐c]pyrrole‐4,6‐dione (TPD) is widely used in D–A conjugated polymers for high‐performance OSCs. As a strong electron‐withdrawing group, TPD provides deepened lowest unoccupied molecular orbital energy level and wide bandgaps when introduced into polymer backbones, enhancing optoelectronic properties photovoltaic performance. Herein, an overview of the applications of TPD‐based donor and acceptor type conjugated polymers for nonfullerene and all‐polymer solar cells in the last 5 years is provided.

Carbon nanotubes mediated chemical and biological decolorization of azo dye: Understanding the structure-activity relationship

Chemical structure of azo dyes molecules showed significant influence on their decolorization rate, while the structure-activity relationship between chemical structure and their reduction decolorization rate is not fully understand. In this study, we found that azo dye molecule with closer position for electron-withdrawing substituent to azo bond resulted in faster chemical and biotic reduction rate with or without presence of carbon nanotubes (CNTs), while electron-repulsive substituent closer to azo bond leading to slower azo dye chemical and biotic reduction rate no matter with or without presence of CNTs. Additionally, galvanic cell experiments implied that electron transfer process may play important roles for both chemical and biological reduction decolorization of azo dyes, and CV results indicated that the higher (azo bond breakage) reduction wave potential corresponding to a faster azo dye chemical decolorization reaction. Finally, the results of Lowest Unoccupied Molecular Orbital (LUMO) energy established that lower LUMO energy for azo dye corresponding to a faster chemical decolorization reaction. This study not only offer systematized relationships between structure property of azo dye and their decolorization rate, but also provide a universal and propagable reduction rules.

Investigation of Syzygium Aromaticum and Nicotiana Tabacum Extracts as Corrosion Inhibitor

In the present work, corrosion inhibition of Syzygium aromaticum and Nicotiana tabacum extract was studied by DFT method. Frontier molecular orbitals (FMO) plots, quantum chemical descriptors such as highest occupied molecular orbital energy (EHOMO), lowest unoccupied molecular orbital energy (ELUMO), energy gap (ΔE), dipole moment (l) and partial charge analysis were calculated. The results obtained show that the eugenol as extract of Syzygium aromaticum and nicotine as extract of Nicotiana tabacum acts as an effective green corrosion inhibitor. In general, nicotine has a better potential as an green corrosion inhibitor than eugenol based on chemical reactivity. However, further studies are needed to gain insight into the mechanism of corrosion inhibition through the study of the adsorption of eugenol and nicotine molecules on metal surfaces.

Highly stable bulk heterojunction organic solar cells based on asymmetric benzoselenadiazole‐oriented organic chromophores

A new asymmetrically designed D-A-D organic chromophore 4-(7-(5′-hexyl-[2,2′-bithiophen]-5-yl)benzo[c][1,2,5]selenadiazol-4-yl)-N,N-diphenylaniline (RSeT) with benzoselenadiazole acceptor unit has been synthesized using two different electron donor materials of triphenylamine (TPA) and n-hexyl bithiophene unit. Attachment of nonplanar electron-donating TPA unit subsequently heightened the optoelectronic properties and altered the solubility in organic solvents. RSeT of the optical bandgap of ~1.95 eV with the highest occupied molecular orbital and lowest unoccupied molecular orbital energy levels of −5.31 and −3.36 eV was applied as an electron donor material for stable bulk heterojunction organic solar cells. In this work, two different buffer layers of poly (3, 4-ethylenedioxythiophene):polystyrene sulfonate acid (PEDOT:PSS) and compact titanium oxide (c-TiO2) were used and their influences on the device performance were investigated. Buffer layer of c-TiO2 with RSeT:PC61BM (1:3, w/w) active layer achieved a high open-circuit voltage (Voc) of ~0.985 V, short-circuit current density (Jsc) of ~11.26 mA/cm2 and improved fill factor of ~0.62 which resulted in a high power conversion efficiency (PCE) of ~6.88%. After 30 days, interestingly, RSeT:PC61BM (1:3, w/w) devices with a c-TiO2 buffer layer demonstrated good stability with the PCE value remaining at 85% of its initial value.

Quantitative structure–activity relationship(QSAR) models for color and COD removal for some dyes subjected to electrochemical oxidation

ABSTRACT Electrochemical oxidation is an efficient method for the destruction of dyes in wastewater streams. The experimental conditions during electrochemical oxidation (EO) and molecular structure of a dye greatly influence the extent of degradation. The extent of degradation for a variety of dyes by EO can be predicted conveniently by the use of Quantitative structure–activity Relationship (QSAR) models. An abundant amount of published data on dye degradation by EO using highly variable experimental conditions lies unutilized to prepare QSAR models. In this study, an effort is made to use published experimental data on EO of aqueous dyes after applying an easy method of normalization, to prepare QSAR models for percent color and COD removal. Normalized color and COD removal were obtained by multiplying the reported removal by volume of reactor and concentration of dye; and divided by total current passed and the time of electrolysis. More than 15 molecular descriptors were computed using Schrodinger-suit 2018-3. The multiple linear regression (MLR) approach was used to develop normalized color and COD removal models. The quantum chemical descriptors: highest occupied molecular orbital energy (HOMO) and lowest unoccupied molecular orbital energy (LUMO), polar surface area (PSA), hydrogen bond donor count (HBD), and number of atoms were found significant. The statistical indices: goodness-of-fit, R2 > 0.75, and internal and external validations, Q2 LOOCV and Q2 ext, > 0.5, satisfied the criteria for predictive models and indicated that the method of normalization used in this study is adequate. Developed QSAR models are quite simple, interpretable, and transparent.

Recent Developments of n-Type Organic Thermoelectric Materials: Influence of Structure Modification on Molecule Arrangement and Solution Processing.

Organic semiconductor (OSCs) thermoelectric materials have been studied widely due to their low thermal conductivity and solution processing characteristics. Currently, the high conductivity (up to 1000 s cm-1 ) has boosted the performance of p-type organic thermoelectric materials substantially. In contrast, the development of n-type organic thermoelectric materials is still limited by their low mobility, inferior air stability, and poor doping efficiency, which is relevant to the molecule structure and dopant dispersion. Herein, the recent development of n-type organic thermoelectric materials was reviewed with an emphasis on molecule structure modification and solution processing. Methods for optimizing conjugate structure were summarized from the effects of conjugated backbone modification and side chains diversification on molecular stacking. The primary n-type dopants were also summarized briefly. Especially, the role of solution aggregation controlling on film preparation and properties was given special attention. Additionally, the emergence of organic diradicals with low lowest unoccupied molecular orbital energy level and no doping was introduced, which shows great potential in n-type organic thermoelectric materials. All these endeavors have led to the development of n-type OSCs materials. This Review is aimed at illustrating the state-of-the-art progress and providing some guideline for the design of organic thermoelectric materials in the future.

Synthesis, spectroscopic investigation and quantum chemical computation of 2-(2-arylamino-4-aminothiazol-5-oyl) naphthalene derivatives

The aim of the work is to synthesis and examines novel 2-(2-arylamino-4-aminothiazol-5-oyl) naphthalene derivatives by experimental and theoretical spectroscopic investigations and to identify its structural and biological properties. The vibrational behavior of the title molecule was extracted by using Fourier transform infrared analysis. The experimental FT-IR (400–4000 cm−1) spectra of 2-(2-arylamino-4-aminothiazol-5-oyl) naphthalene derivatives have been recorded in the solid phase. The UV–visible spectrum recorded and analyzed using ethanol solvent, reveals the electronic transition properties. Hydrogen (1H) nuclear magnetic resonance spectra were also recorded. The geometry optimization has been performed to analyze the 2-(2-arylamino-4-aminothiazol-5-oyl) naphthalene active site with bonding features. Natural bond orbital (NBO) analysis showed proton transfer within the selected donor-acceptor depicting large energy of stabilization for the molecules under study. The chemical stability of the molecule was identified with the help of highest occupied and lowest unoccupied molecular orbital energies. The reactive sites of the title molecule were discussed by Fukui and MEP analysis. The RDG analysis was used to predict the weak interaction of the molecule with the help of electron density. Docking studies have been performed for title molecules using the molecular docking software.

Synthesis and Properties of V-Shaped Xanthene Dyes with Tunable and Predictable Absorption and Emission Wavelengths.

V-shaped xanthene dyes capable of predicting absorption and emission wavelengths are described. These dyes were synthesized by bridging a xanthene ring and an aryl moiety of fluorescein through ether covalent bonds. These dyes showed longer absorption and emission wavelengths than those of the parent fluorescein. Furthermore, substituents introduced on the aryl moiety mainly affected the lowest unoccupied molecular orbital energy level of the molecule. Therefore, the Hammett substituent constants could be used to predict the absorption and emission wavelengths of the compound.

Corrosion Inhibition of Mild Steel in Acidic Media by Expired Carbimazole Drug

ABSTRACT &#x0D; The efficiency of the anti-thyroid, Carbimazole, expired drug, (CD), as corrosion inhibitor for mild steel in 0.5MHCl was investigated by weight loss method. Data obtained from weight loss measurement were analysed, and the results show that, the used Carbimazole drug was a good inhibitor. The inhibition efficiency increases with increasing the concentration of CD inhibitor and reached a maximum value of 84.95% at 30 OC and 100 ppm of the used inhibitor. The adsorption isotherm of CD obeyed Langmuir adsorption isotherm. Quantum chemical calculations based on density function theory, DFT, were performed on CD drug. Calculated parameters such as the highest occupied molecular orbital energy (EHOMO), the lowest unoccupied molecular orbital energy (ELUMO), energy gap (ΔE), dipole moment (µ), electronegativity (), electron affinity (A), global hardness (η), softness (σ) and ionization potential (I), were calculated and correlated to the experimental data.&#x0D;

P-Tetrafluorophenylene Divinylene-Bridged Nonfullerene Acceptors as Binary Components or Additives for High-Efficiency Organic Solar Cells

In this study, we designed, synthesized, and characterized an A-D-A'-D-A-type indacenodithienothiophene (IDTT)-based molecular acceptor that exhibited a broader absorption range and higher lowest unoccupied molecular orbital energy level with a nearly comparable band gap compared to a well-known electron acceptor IT-M. The designed electron-deficient molecular acceptor FB-2IDTT-4Cl with a fluorinated benzene tether (FB), that is, p-tetrafluorophenylene divinylene, demonstrated long-wavelength absorption and high hole and electron charge mobility in the thin films blended with the electron donor PBDB-T for an inverted organic photovoltaic (OPV) binary device, resulting in a maximum power conversion efficiency (PCE) of 11.4%. Such a performance is comparably as high as that of the device with PBDB-T:IT-M, and particularly, it was 18.8% higher than that of the devices with ITIC-4Cl as the acceptor (PCE 9.1% ± 0.5%) and 24.9% higher than that of the devices with the thiophene-flanked benzothiadiazole-bridged acceptor CNDTBT-IDTT-FINCN (PCE 9.01% ± 0.13%). Furthermore, varying the illumination intensity from 200 to 2000 lux increased the Jsc and Voc values as well as the FF values, thus leading to increased PCE levels. In addition, the best PCE of the PM6:Y6 device with 1% FB-2IDTT-4Cl as additives was 16.9%. Our stability test showed that the PM6:Y6 standard device efficiency downgraded very soon either at room temperature or under thermal-annealing conditions. However, with the addition of 1% FB-2IDTT-4Cl as additives, the device efficiency still can be maintained at 90-95% in 500 h at room temperature and 95% at 20 h and 85-95% in 45 h at an annealing temperature of 80 °C. These findings demonstrate FB-2IDTT-4Cl to be a promising candidate as an electron acceptor with a fluorinated π-bridging fused-ring design for OPV applications.

Computational Design of Rhenium(I) Carbonyl Complexes for Anticancer Photodynamic Therapy.

New Re(I) carbonyl complexes are proposed as candidates for photodynamic therapy after investigating the effects of the pyridocarbazole-type ligand conjugation, addition of substituents to this ligand, and replacement of one CO by phosphines in [Re(pyridocarbazole)(CO)3(pyridine)] complexes by means of the density functional theory (DFT) and time-dependent DFT. We have found, first, that increasing the conjugation in the bidentate ligand reduces the highest occupied molecular orbital (HOMO)–lowest unoccupied molecular orbital (LUMO) energy gap of the complex, so its absorption wavelength red-shifts. When the enlargement of this ligand is carried out by merging the electron-withdrawing 1H-pyrrole-2,5-dione heterocycle, it enhances even more the stabilization of the LUMO due to its electron-acceptor character. Second, the analysis of the shape and composition of the orbitals involved in the band of interest indicates which substituents of the bidentate ligand and which positions are optimal for reducing the HOMO–LUMO energy gap. The introduction of electron-withdrawing substituents into the pyridine ring of the pyridocarbazole ligand mainly stabilizes the LUMO, whereas the HOMO energy increases primarily when electron-donating substituents are introduced into its indole moiety. Each type of substituents results in a bathochromic shift of the lowest-lying absorption band, which is even larger if they are combined in the same complex. Finally, the removal of the π-backbonding interaction between Re and the CO trans to the monodentate pyridine when it is replaced by phosphines PMe3, 1,4-diacetyl-1,3,7-triaza-5-phosphabicyclo[3.3.1]nonane (DAPTA), and 1,4,7-triaza-9-phosphatricyclo[5.3.2.1]tridecane (CAP) causes another extra bathochromic shift due to the destabilization of the HOMO, which is low with DAPTA, moderate with PMe3, but especially large with CAP. Through the combination of the PMe3 or CAP ligands with adequate electron-withdrawing and/or electron-donating substituents at the pyridocarbazole ligand, we have found several complexes with significant absorption at the therapeutic window. In addition, according to our results on the singlet–triplet energy gap, all of them should be able to produce cytotoxic singlet oxygen.