Photophysical Properties of Squaraine Derivatives:  Evidence for Charge Separation

Abstract

The polarity dependent fluorescence quantum yields and lifetimes at room temperature have been measured for two squaraine dyes, SQ-DMA and its crown ether derivative SQ-CR. The unstructured fluorescence bands and the polarity independent k f values indicate fluorescence from one single species. The nonradiative decay rates increase strongly with solvent polarity, indicating a polarity-induced intramolecular fluorescence quenching process. The latter could be due to the formation of a nonemissive twisted intramolecular charge transfer (TICT) state. Low-temperature measurements and the comparison of SQ-DMA and SQ-CR support this view.

Squaraines as Reporter Units: Insights into their Photophysics, Protonation, and Metal‐Ion Coordination Behaviour

“…Our present experimental and theoretical results, in combination with the lack of a pronounced solvatochromism as found by Cornelissen-Gude et al [25] and the negligible influence of intersystem crossing, [46] suggest that the states of the squaraines involved in the optical transitions have a more polymethinic than intramolecular charge-transfer (ICT) character, often associated with donor-acceptorA C H T U N G T R E N N U N G (-donor)-substituted dyes. In contrast, the moderate fluorescence quantum yields in acetonitrile observed for all the dyes except SJuOH and the strong "positive solvatokinetic behaviour" (a decrease in emission yield with increasing solvent polarity) reported for such compounds previously [25] indicate that an intramolecular charge separation process in the excited state, involving large amplitude motions around b1 and b1', is nonetheless important.…”

“…In contrast, the moderate fluorescence quantum yields in acetonitrile observed for all the dyes except SJuOH and the strong "positive solvatokinetic behaviour" (a decrease in emission yield with increasing solvent polarity) reported for such compounds previously [25] indicate that an intramolecular charge separation process in the excited state, involving large amplitude motions around b1 and b1', is nonetheless important. However, this process does not produce another emissive species, but leads to the population of an entirely non-emissive species, most likely connected to the concept of twisted intramolecular chargetransfer (TICT) states.…”

“…Despite the large number of application-oriented publications (vide infra), squaraines have also received considerable treatment in fundamental studies that make use of NMR spectroscopy, [21] X-ray crystallography, [22] quantum chemical calculations [23] and in various photo-physical studies as a function of solvent nature, [24,25] chemical structure [26] or temperature. [25] However, several contradictory interpretations that have appeared throughout the years in the literature within the context of the use of squaraines as functional dyes or signalling units in molecular probes and other supramolecular ensembles suggest that further insight into the photophysical performance of squaraine reporters is urgently required, especially when the use of such chromophores is extended to areas such as molecular logics, [27,28] multitopic or allosteric and multichannel chemosensors.…”

“…[25] However, several contradictory interpretations that have appeared throughout the years in the literature within the context of the use of squaraines as functional dyes or signalling units in molecular probes and other supramolecular ensembles suggest that further insight into the photophysical performance of squaraine reporters is urgently required, especially when the use of such chromophores is extended to areas such as molecular logics, [27,28] multitopic or allosteric and multichannel chemosensors. [29,30] We report here the photophysical properties, and a rationalisation of the colour modulations observed upon interaction with protons and metal cations, of a family of squaraine-based receptors.…”

Fabrication of Squaraine Dye Sensitized Spherical Zinc Oxide Nanocomposites and Their Visible-Light Induced Photocatalytic Activity

“…23 They exhibit sharp and intense absorption bands in the visible and near-IR regions, possess appropriate amount of°u-orescence quantum yield, good photoconductivity, high molar extinction coe±cient and nontoxic, squaraine dye has been applied to various di®erent elds including employed as sensitizers in the electrography and optical data storage applications, Dye-Sensitized Solar Cells (DSSCs), ion microprobe and long wavelength°uorescence probe. [24][25][26][27][28] Many studies about ZnO nanostructures were focused on the construction of di®erent morphologies and their application in the¯eld of DSSCs. [29][30][31][32] However, to the best of our knowledge, no research has been reported on the photocatalytic activity of ZnO nanoparticles sensitized by squarylium dye under visible-light irradiation.…”