Singlet and Triplet Excited-State Dynamics of 3,7-Bis(arylethynyl)phenothiazines: Intramolecular Charge Transfer and Reverse Intersystem Crossing

Abstract

A series of 3,7-bis(arylethynyl)-substituted phenothiazine-based fluorophores (bearing benzene, naphthalene, methoxynaphthalene, anthracene, phenanthrene, and pyrene) were designed and synthesized via a Pd-catalyzed Sonogashira cross-coupling reaction. These molecules show in solution large Stokes shifts (generally 5500−6500 cm −1 ) and high quantum yields (40−82% depending on the aryl groups), as a consequence of the planarization of their butterfly-like structure occurring in the excited state. Moreover, significant two-photon absorption was revealed for these phenothiazine derivatives, whose cross section increases upon enhancing the molecular conjugation. The remarkable Stokes shifts, fluorescence quantum yields, and twophoton absorption make these molecules appealing as fluorescent probes for bioimaging applications. Significant emission was also detected in the solid state where anthracene-, phenanthrene-, and pyrene-substituted phenothiazines show red-shifted emission maxima compared to the solution, indicating considerable π−π staking. The anthracene-substituted phenothiazine arouses particular interest, as it exhibits photoinduced intramolecular charge transfer (ICT) from the phenothiazine to the anthracene, revealed by its important fluorosolvatochromism and through ultrafast transient absorption experiments. The spectroscopic results are in line with the time-dependent density functional theory (TD-DFT) calculations and cyclic voltammetry measurements. The anthracene-substituted molecule also features a remarkable triplet production and hints of a delayed fluorescence behavior, allowed in nonpolar solvents by a fairly competitive reverse intersystem crossing pathway. These findings make the investigated molecules interesting as new organic active materials for optoelectronic devices.

Symmetric and Asymmetric Push–Pull Conjugates: Significance of Pull Group Strength on Charge Transfer and Separation

“…Kinetic parameters derived in the present study are summarized in Table . As pointed out earlier, due to the close proximity of the donor and acceptor entities in this type of molecular systems, − ,− both CT and CS processes occur rapidly. Expectedly, increasing solvent polarity further accelerated these processes, wherein formation of charge-separated states was possible to observe in polar solvents.…”

“…Synthesis and characterization of the studied multimodular push–pull conjugates and their control compounds is given elsewhere. , Before performing spectral and photochemical studies, the compounds were purified over column chromatography, and the purity was checked by thin-layer chromatography.…”

Tuning the Fluorescence and the Intramolecular Charge Transfer of Phenothiazine Dipolar and Quadrupolar Derivatives by Oxygen Functionalization

“…The PTZ unit is an electron-rich tricyclic heteroarene with nonplanar butterfly structure, characterized by the presence of powerful electron-donor sulfur and nitrogen atoms . In the literature, the photophysical properties and the HOMO–LUMO energy levels of PTZ derivatives have been easily modulated by substitutions at the nitrogen and the 3,7-positions of the phenothiazine unit. , Our group has functionalized the 3,7-positions of the phenothiazine unit by using polycyclic aromatic hydrocarbons of increasing complexity or strong acceptors such as benzothiadiazole or tetracyanobutadiene. , In the literature, push–pull phenothiazine–naphthalimide systems have been successfully employed in some optoelectronic applications. − However, the research on varying the oxidation state of the sulfur atom (sulfides, sulfoxides, and sulfones) in the thiazine ring of phenothiazine is still very limited. − In this study, we have synthesized new naphthalimide and phenothiazine-based systems, in which we have changed the oxidation state of the sulfur atom on the thiazine ring to investigate its effect on the photonic properties of the obtained materials.…”

“…21 In the literature, the photophysical properties and the HOMO− LUMO energy levels of PTZ derivatives have been easily modulated by substitutions at the nitrogen and the 3,7positions of the phenothiazine unit. 22,23 Our group has functionalized the 3,7-positions of the phenothiazine unit by using polycyclic aromatic hydrocarbons of increasing complexity 24 or strong acceptors such as benzothiadiazole 25 or tetracyanobutadiene. 26,27 In the literature, push−pull phenothiazine−naphthalimide systems have been successfully employed in some optoelectronic applications.…”

Excited-State Modulation of Aggregation-Induced Emission Molecules for High-Efficiency Triplet Exciton Generation

“…The fluorescence spectra of both TPA-P-C and CBZ-P-C were significantly red-shifted and broadened by increasing the solvent polarity, suggesting that photoinduced ICT may be operative in these D−A compounds. 39,40 The Stokes shift values obtained for the investigated molecules were large, ranging from 4500 cm −1 in nonpolar solvents to 10 500 cm −1 in polar media (Table S1). This suggests that a significant relaxation dynamic occurs for these dyes after photoexcitation, particularly in polar solvents.…”