A Case-Study on the Photophysics of Chalcogen-Substituted Zinc(II) Phthalocyanines

Singlet dioxygen has been widely applied in different disciplines such as medicine (photodynamic therapy or blood sterilization), remediation (wastewater treatment) or industrial processes (fine chemicals synthesis). Particularly, it can be conveniently generated by energy transfer between a photosensitizer's triplet state and triplet dioxygen upon irradiation with visible light. Among the best photosensitizers, substituted zinc(II) phthalocyanines are prominent due to their excellent photophysical properties, which can be tuned by structural modifications, such as halogen- and chalcogen-atom substitution. These patterns allow for the enhancement of spin-orbit coupling, commonly attributed to the heavy atom effect, which correlates with the atomic number ( Z ${Z}$ ) and the spin-orbit coupling constant ( ζ ${\zeta }$ ) of the introduced heteroatom. Herein, a fully systematic analysis of the effect exerted by chalcogen atoms on the photophysical characteristics (absorption and fluorescence properties, lifetimes and singlet dioxygen photogeneration), involving 30 custom-made β-tetrasubstituted chalcogen-bearing zinc(II) phthalocyanines is described and evaluated regarding the heavy atom effect. Besides, the intersystem crossing rate constants are estimated by several independent methods and a quantitative profile of the heavy atom is provided by using linear correlations between relative intersystem crossing rates and relative atomic numbers. Good linear trends for both intersystem crossing rates (S1-T1 and T1-S0) were obtained, with a dependency on the atomic number and the spin-orbit coupling constant scaling asZ 0 . 4 ${{Z}^{0.4}}$ andζ 0 . 2 ${{\zeta }^{0.2}}$ , respectively The trend shows to be independent of the solvent and temperature.

π-Extended Pyrrole-Fused Heteropine: Synthesis, Properties, and Application in Organic Field-Effect Transistors

Sulfur-embedded polycyclic aromatic compounds have been used as building blocks for numerous organic semiconductors over the past few decades. While the success is based on thiophene-containing compounds, aromatic compounds that contain thiepine, a sulfur-containing seven-membered-ring arene, has been less well investigated. Here we report the synthesis and properties of π-extended pyrrole-fused heteropine compounds such as thiepine and oxepine. A π-extended pyrrole-fused thiepine exhibited a "pitched π-stacking" structure in the crystal, and exhibited a high charge carrier mobility of up to 1.0 cm²  V^-1  s^-1 in single-crystal field-effect transistors.

Excited states and spin–orbit coupling in chalcogen substituted perylene diimides and their radical anions

A novel series of chalcogen bay-substituted perylene diimides show increasing SOC, which was investigated in detail via (time-resolved) optical spectroscopy, (spectro)electrochemistry, EPR spectroscopy and TD-DFT calculations.

Revisiting Dehydrothiopheno[12]annulenes: Synthesis, Electronic Properties, and Aromaticity

The aromaticity and electronic properties of acetylene-bridged hexadehydrotristhiopheno[12]annulenes (HDTAs) were revisited using a combined experimental and theoretical approach. Moreover, we attempted the synthesis of the butadiyne-bridged octadehydrobisthiopheno[12]annulenes (ODTAs). While the formation of ODTAs was indicated by NMR spectroscopy, mass spectrometry, and UV-vis absorption measurements, our attempts to isolate ODTAs were unsuccessful on account of its instability. Instead, their structure and energetic properties were predicted using DFT calculations. HDTA isomers in which the position where the thiophene rings are fused to the 12-membered ring differs (b- vs c-position) show distinct differences in their HOMO-LUMO energy gaps (E_Gap). ODTAs also show large E_Gap differences depending on the fusion position of the thiophene rings. The diene character of the thiophene ring significantly changes the electronic properties; i.e., E_Gap differences of >1 eV were observed between the isomers of both HDTAs and ODTAs. A theoretical evaluation of HDTAs and ODTAs revealed significant variation in the local aromaticity/antiaromaticity between the b- and c-isomers. The antiaromatic character of the 12-membered ring is attenuated for the b-isomers, whereas it is decreased substantially for the c-isomers. The results of this study are useful for a detailed understanding of the fundamental aspects of dehydrothiopheno[12]annulenes.

Investigations of Thienoacene Molecules for Classical and Entangled Two-Photon Absorption

Investigations of the optical effects in thienoacene chromophores with different central atoms were performed. These chromophores provide a basis for the comparison of the linear, two-photon, and entangled two-photon properties in organic molecules with varying degrees of dipolar or quadrupolar character. Linear absorption and emission as well as time-dependent density functional theory calculations were performed for the chromophores investigated. Measurements of the classical two-photon absorption (TPA), entangled two-photon absorption (ETPA), as well as entangled two-photon fluorescence were experimentally performed for the four chromophores. Electronic structure calculations were utilized to provide estimates of the classical two-photon absorption coefficients. The results of the measured entangled two-photon cross sections were compared with theoretical estimates for the molecules investigated. It is found that the dipole (transition or permanent) pathway can have an effect on the trends in the entangled photon absorption process in dipolar organic chromophores. This study helps predict the properties of the entangled two-photon effect in chromophores with different dipolar and quadrupolar character.

Tuning the Structure and Photophysics of a Fluorous Phthalocyanine Platform

Phthalocyanines are an important class of industrial dyes with potential commercial applications ranging from photovoltaics to biomedical imaging and therapeutics. We previously demonstrated the versatility of the commercially available zinc(II) hexadecafluorophthalocyanine (ZnF16Pc) as a platform for rapidly developing functional materials for these applications and more. Because this core-platform approach to dye development is increasingly common, it is important to understand the photophysical and structural consequences of the substitution chemistry involved. We present a fundamental study of a series of ZnF16Pc derivatives in which the aromatic fluorine atoms are progressively substituted with thioalkanes. Clear spectroscopic trends are observed as the substituents change from electron-withdrawing to electron-releasing groups. Additionally, there is evidence for significant structural distortion of the normally planar heterocycle, with important ramifications for the photophysics. These results are also correlated to DFT calculations, which show that the orbital energies and symmetries are both important factors for explaining the excited-state dynamics.

Heterocyclic Nanographenes and Other Polycyclic Heteroaromatic Compounds: Synthetic Routes, Properties, and Applications

Two-dimensionally extended, polycyclic heteroaromatic molecules (heterocyclic nanographenes) are a highly versatile class of organic materials, applicable as functional chromophores and organic semiconductors. In this Review, we discuss the rich chemistry of large heteroaromatics, focusing on their synthesis, electronic properties, and applications in materials science. This Review summarizes the historical development and current state of the art in this rapidly expanding field of research, which has become one of the key exploration areas of modern heterocyclic chemistry.

The internal heavy-atom effect on 3-phenylselanyl and 3-phenyltellanyl BODIPY derivatives studied by transient absorption spectroscopy

Three monosubstituted 3-phenylselanyl and 3-phenyltellanyl BODIPY derivatives were synthesized and their spectroscopic properties were characterized and compared to those of iodine and chlorine-atoms containing analogues as well as an unsubstituted BODIPY derivative. The fluorescence quantum yields were found to decrease, whereas the intersystem crossing quantum yields (ΦISC), determined by transient spectroscopy, increased in the order of the H → Cl → Se/I → Te substitution. The maximum ΦISC, found for the 3-phenyltellanyl derivative, was 59%. The results are interpreted in terms of the internal heavy-atom effect of the substituents.

Synthesis of 4-chalcogeno-1-aza-1,3-butadiene derivatives by intramolecular cyano-Diels-Alder reaction and borane-coordination-induced fluorescence enhancement

4-Thio-1-aza-1,3-butadiene derivatives 3 incorporating a rigid dibenzobarrelene skeleton were synthesized by the intramolecular cyano-Diels-Alder reaction of 9-anthryl cyanoethenyl sulfides 6. The thermal reaction of 6 afforded an equilibrium mixture of 3 and 6, but the cyclization was effectively promoted by the addition of BF3·Et2O or B(C6F5)3 to yield imine-borane adducts 8 and 9. The imine-borane adducts emit intense blue fluorescence both in solution and in the crystalline state. This is in stark contrast to free imines 3, which are weakly fluorescent. Selenium analogue 4 and N-oxide 12 of 3a were synthesized, along with their B(C6F5)3 adducts.

Singlet oxygen quantum yield of sulfur and selenium analogs of psoralen

The quantum yield of singlet oxygen production by eight newly synthesized sulfur and selenium analogs of psoralen irradiated with UV-A (366 nm) has been determined in CCl4 with the help of the steady state luminescence technique. The new psoralen derivatives are generally better singlet oxygen producers than psoralen itself. In particular, the replacement of selenophene for furan and/or of thiopyrone for pyrone induces an important enhancement of the singlet oxygen quantum yield.