One- and two-photon absorption properties of quadrupolar thiophene-based dyes with acceptors of varying strengths

Multifunctional Photoelectroactive Materials for Optoelectronic Applications Based on Thieno[3,4-b]pyrazines and Thieno[1,2,5]thiadiazoles

In this study, we introduce a novel family of symmetrical thiophene-based small molecules with a Donor-Acceptor-Donor structure. These compounds feature three different acceptor units: benzo[c][1,2,5]thiadiazole (Bz), thieno[3,4-b]pyrazine (Pz), and thieno[1,2,5]thiadiazole (Tz), coupled with electron donor units based on a carbazole-thiophene derivative. Using Density Functional Theory (DFT), we investigate how the molecular geometry and strength of the central acceptor unit impact the redox and spectroscopic properties. Notably, the incorporation of Pz and Tz moieties induces a significant redshift in the absorption and emission spectra, which extend into the near-infrared (NIR) region, simultaneously reducing their energy gaps (~1.4-1.6 eV). This shift is attributed to the increased coplanarity of the oligomeric inner core, both in the ground (S0 ) and excited (S1 ) states, due to the enhanced quinoidal character as supported by bond-length alternation (BLA) analysis. These structural changes promote better π-electron delocalization and facilitate photoinduced charge transfer processes in optoelectronic devices. Notably, we show that Pz- and Tz-containing molecules exhibit NIR electrochromic behavior and present ambivalent character in bulk heterojunction (BHJ) solar cells. Finally, theoretical calculations suggest that these molecules could serve as effective two-photon absorption (2PA) probes, further expanding their potential in optoelectronic applications.

Effect of tunable π bridge on two-photon absorption property and intramolecular charge transfer process of polycyclic aromatic hydrocarbons

The influences of the conjugation effect on the charge transfer and nonlinear optical (NLO) properties of polycyclic aromatic hydrocarbons (PAHs) are comprehensively investigated at the microscopic molecular level. We found that the conjugation effect of π bridge is negatively correlated with molecular planarity, excitation energy, two-photon absorption (TPA) cross-section, and the second hyperpolarizability. For the first time, the charge transfer matrix (CTM) is applied to the molecular two-photon transition process. Combining the charge difference density (CDD) diagram with CTM heat map to visually quantitative investigate the characteristics of excited states, the charge transfer path and transfer amount between atoms. During the two-photon transition of all molecules, the electronic excited state is locally excited. Compared with the first process, the range of intramolecular charge transfer in the second process of the two-photon transition is expanded. Comprehensive results prove that the π bridge with large conjugation effect distorts the molecular structure, which is not conducive to the intramolecular charge transfer. Therefore, the molecule DBP-1 with a carbon-carbon double bond as the π bridge has the largest transition dipole moments, TPA cross-section, and second static hyperpolarizability. Our research method can provide effective guidance for the design and optimization of nonlinear organic conjugated molecular materials.

Human Serum Albumin-Oligothiophene Bioconjugate: A Phototheranostic Platform for Localized Killing of Cancer Cells by Precise Light Activation

The electronic, optical, and redox properties of thiophene-based materials have made them pivotal in nanoscience and nanotechnology. However, the exploitation of oligothiophenes in photodynamic therapy is hindered by their intrinsic hydrophobicity that lowers their biocompatibility and availability in water environments. Here, we developed human serum albumin (HSA)-oligothiophene bioconjugates that afford the use of insoluble oligothiophenes in physiological environments. UV-vis and electrophoresis proved the conjugation of the oligothiophene sensitizers to the protein. The bioconjugate is water-soluble and biocompatible, does not have any "dark toxicity", and preserves HSA in the physiological monomeric form, as confirmed by dynamic light scattering and circular dichroism measurements. In contrast, upon irradiation with ultralow light doses, the bioconjugate efficiently produces reactive oxygen species (ROS) and leads to the complete eradication of cancer cells. Real-time monitoring of the photokilling activity of the HSA-oligothiophene bioconjugate shows that living cells "explode" upon irradiation. Photodependent and dose-dependent apoptosis is one of the primary mechanisms of cell death activated by bioconjugate irradiation. The bioconjugate is a novel theranostic platform able to generate ROS intracellularly and provide imaging through the fluorescence of the oligothiophene. It is also a real-time self-reporting system able to monitor the apoptotic process. The induced phototoxicity is strongly confined to the irradiated region, showing localized killing of cancer cells by precise light activation of the bioconjugate.

Revealing the biradicaloid nature inherited in the derivatives of thieno[3,4-c][1,2,5]thiadiazole: a computational study

Computational studies were performed on non-classical thieno[3,4-c][1,2,5] thiadiazole and its pi donor derivatives (TT dyes) so as to delineate the factors responsible for their near-infrared (NIR) absorption. For all dyes except the unsubstituted bare dye, adiabatic singlet-triplet energy gaps (estimated through the ΔSCF procedure using the B3LYP and M062X DFT methods and SFTDDFT with the 5050 functional) were less than 1eV. Percentage calculations of the biradicaloid character suggested a moderate biradicaloid nature in all derivatives. There was a resemblance between the frontier molecular orbital (MO) picture of the TT bicyclic ring and the degenerate non-bonding molecular orbitals of Trimethyleneethane (TME, a known biradical). Inter-fragment charge transfer analysis revealed not only a considerable donation of charge to the central ring (Acceptor, TT part) but also substantial charge redistribution within the ring itself. From these results, it was inferred that NIR absorption, in these dyes, was due to: (1) a reduced HOMO-LUMO gap (HLG) as a TME biradical substructure forms its chromophoric part; and (2) charge transfer from the donor substituents. The non-bonding nature of the S atom, in the bare dye, with its neighbouring N/C atom (of the highest occupied π-MOs), led to an examination of its electronic structure using the ab initio valence bond method. The relatively large weight and energetic stability of the biradicaloid VB structures compared to those of the ylidic structures clearly disclosed the importance of biradicaloid structures in the overall resonance of the bare dye. Their utility as singlet fission materials was screened using singlet and triplet energy-based molecular structure activity criteria. The results were encouraging, demanding experiments to reaffirm the materials' usefulness.

Pillar[5]arene-Based Solid-State Supramolecular Polymers with Suppressed Aggregation-Caused Quenching Effects and Two-Photon Excited Emission

Herein we develop a new pillar[5]arene-mediated supramolecular polymerization strategy to control the assembly of dyes 4,7-di-2-thienyl-2,1,3-benzothiadiazole and 4,7-di-2-thienyl-2,1,3-benzoselenadiazole in the solid state. The resulting supramolecular polymeric structures drive the dye aggregates in the solid state from parallel alignments to highly ordered "head-to-tail" structures, which effectively suppresses excimer formation and thus aggregation-caused quenching effects and enhances the solid-state emission efficiency by almost 1 order of magnitude. This, together with two-photon excited emission, endows these solid-state polymers with exciting potential in optoelectronic, photonic, and bioimaging applications.

One- and two-photon absorption properties of quadrupolar A-π-D-π-A dyes with donors of varying strengths

In this work, the one- and two-photon absorption properties of two quadrupolar A-π-D-π-A dyes, featuring carbazole and triphenylamine as donors of varying strengths, are calculated using density functional theory. The characteristics of two-photon excitation are analysed by using the three-state model closest to the real physical process. By taking the intermediate state as a bridge, the charge transfer characteristics in the two-photon excitation process are studied in detail by using various visualisation methods. Comparing the intramolecular charge transfer (ICT) characteristics shows that increasing the ability of the push-pull electrons of the central substituents, thus increasing the intramolecular charge transfer, is a more efficient method to enhance the molecular σ_tp. Our study promotes a deeper understanding of the design of A-π-D-π-A molecules with large cross-sections in two-photon absorption.

Tuning of hyperpolarizability, and one- and two-photon absorption of donor-acceptor and donor-acceptor-acceptor-type intramolecular charge transfer-based sensors

The present work aims to study the effect of solvent as well as arrangement of donor-acceptor groups on linear and non-linear optical (NLO) response properties of two experimentally studied intramolecular charge-transfer (ICT)-based fluorescent sensors. One of them (molecule 1) is a donor-acceptor (D-A) system with hemicyanine and dimethylanilino as electron withdrawing and donating groups, respectively, while the other one (molecule 3) is molecule 1 fused with a boron-dipyrromethene (BODIPY) moiety. BODIPY acts as the electron acceptor group of molecule 2 that as well consists of dimethylanilino as the electron donor. Density functional theory (DFT) as well as time-dependent DFT has been employed to optimize the geometry of the molecules, followed by computation of dipole moment (μ), static first hyperpolarizability (β_total), and one- and two-photon absorption (TPA) strengths. The results reveal that dipole moment as well as total static first hyperpolarizability (β_total) of the studied molecules is dominated by the respective components in the direction of charge transfer. The ratio of vector component of first hyperpolarizability (β_vec) to β_total also supports the unidirectional charge transfer in the studied systems. In molecule 3, which is a donor-acceptor-acceptor (D-A-A)-type system, the BODIPY moiety is found to play a major role in controlling the NLO response over the other acceptor group. Solvents are also found to play an important role in controlling the linear as well as NLO response of the studied systems. A significant increase in the first hyperpolarizability as well as TPA cross-section of the studied molecules is predicted due to an increase in the dielectric constant of the medium. The results presented are expected to provide a clue in tuning the NLO response of many ICT-based chromophores, especially those with D-A-A arrangements.

Impact of the Subunit Arrangement on the Nonlinear Absorption Properties of Organometallic Complexes with Ruthenium(II) σ-Acetylide and Benzothiadiazole as Building Units §

In this paper, the nonlinear absorption properties of two complexes consisting of Ru(C≡CPh)(C≡C)(dppe)2 (dppe = Ph2PCH2CH2PPh2) as electron donor (D) and 4,7-di(2-thienyl)benzo[c][1,2,5]thiadiazole as electron acceptor (A) units in two different arrangement, i.e., A–D–A and D–A–D, are presented. They were measured in solution by the femtosecond open-aperture Z-scan method. The complexes show moderate two-photon absorption cross-sections σ(2) of several hundred to one thousand GM (here 1 GM = 10−50 cm4 s molecule−1 photon−1). Although they are formed by the same building units, it was found that the two-photon absorption values of the D–A–D arrangement are six times higher than that of the A–D–A one. This difference can be explained by the number of metal cores (one or two ruthenium centers), the geometrical configurations of the complexes (more or less planar), and the resonance enhancement by lowering the intermediate state.