Abstract
In the present study, the influence of the hydrogen bonding for the one- and two-photon absorption of the prototypical squaraine dye is investigated with quantum chemistry tools. The central squaraine unit is bound by strong hydrogen bonds with 4-substituted N,N′-diphenylurea and, alternatively, N,N′-diphenylthiourea molecules, which affects to a high extend the properties of the squaraine electron accepting moiety, thus shifting its maximum absorption wavelength and enhancing the TPA cross section. The replacement of oxygen by sulfur atoms in the squaraine central ring, known to affect its photophysical behavior, is considered here as the way of modifying the strength and nature of the intermolecular contacts. Additionally, the influence of the oxygen-by-sulfur replacement is also considered in the N,N′-diphenylurea moiety, as the factor affecting the acidity of the N–H protons. The introduction of the sequence of the substituents of varying electron-donating or electron-withdrawing characters in the position 4 of N,N′-diphenyl(thio)urea subsystems allows to finely tune the hydrogen bonding with the central squaraine unit by further modification of the N–H bond characteristics. All of these structural modifications lead to the controlled adjustment of the electron density distribution, and thus, the properties affected such as transition moments and absorption intensity. Ab initio calculations provide strong support for this way of tailoring of one- or two-photon absorption due to the obtained strong hypsochromic shift of the maximum one-photon absorption wavelength observed particularly for thiosquaraine complexes and an increase in the TPA wavelength together with the increase in the TPA cross section. Moreover, the source of the strong modification of the thiosquaraine OPA in contrast to the pristine oxosquaraine upon N,N′-diphenyl(thio)urea substitution is determined. Furthermore, for the first time, the linear dependence of the non-additivity in the interaction energy on the Hammett substituent constant is reported. The stronger the electron-donating character of the substituent, the larger the three-body non-additive components and the larger their percentage to the total interaction energy.
Highlights
The growing interest in the applications of chromophores and fluorophores in the fields of biomedical techniques such as bioimaging or photodynamic therapy or in photovoltaic devices requires the rational design of the photoactive systems with respect to both their photophysical characteristics and solubility, stability, non-toxicity, and availability
The present study is devoted to the development of the controlled modifications of photophysical properties of model squaraine dye
The investigated features such as one-photon absorption, two-photon absorption, or singlet–triplet energy gap affecting the intersystem crossing efficiency are shown to be prone to the finetuning by O-by-S replacement in the squaric ring of the chromophore by symmetric hydrogen bonding, by N,N′diphenyl(thio)urea, and even by systematic modification of the character of substituents present in this N,N′-diphenyl-(thio)urea units
Summary
The growing interest in the applications of chromophores and fluorophores in the fields of biomedical techniques such as bioimaging or photodynamic therapy or in photovoltaic devices requires the rational design of the photoactive systems with respect to both their photophysical characteristics and solubility, stability, non-toxicity, and availability. Squaraine dyes belong to the class of quadrupolar molecules of high interests in material chemistry, bioimaging, non-linear optics, or photonics. Their peculiar photooptical properties arise from their unique structure: an electron-deficient fourmembered squaric acid ring (denoted further by A as acceptor) is placed in between two electron-rich donating groups (denoted by D as donors). This D–A–D structure results in a specific strong and sharp one-photon absorption (OPA) in the visible or near-IR region, exceptional brightness, and unique non-linear properties. The extended π-electron scaffold of squaraines promotes the stacking intermolecular interactions in polar solvents, causing the fluorescence quenching Arunkumar et al (2007)
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