Aggregation-Induced Linear and Non-Linear Optical Properties of Four Hydroxy-Substituted Analogues of 2,4-Bis[4-(dibutylamino)phenyl]squaraine

Abstract

Langmuir films of the title compound, 2,4-bis[4-(dibutylamino)phenyl]squaraine, Sq0, and its substituted analogues, Sq1 (monohydroxy) to Sq4 (tetrahydroxy), exhibit three well defined phases at the air–water interface: λmax 525–550 nm (H-aggregate); 660–670 nm (intermediate); 760–770 nm (J-aggregate). The monolayer spectra are influenced by the surface pressure and the number of hydroxy substituents and, when the monolayers are transferred to a solid substrate, there is a correlation between the linear and non-linear optical behaviour of the Langmuir-Blodgett films. Second-harmonic generation has been observed even though the unit molecular structures of two analogues, Sq0 and Sq4, are centric. The behaviour is aggregation-induced and there is evidence that the dyes associate in dilute solution. The electrospray ionization mass spectra exhibit peaks with high mass/charge (m/z) ratios; the data conform to the dimeric aggregate and, for solutions of two or more squaraines, corroborate the existence of heteromolecular associates.

Laser welding of polymers using unsymmetrical squaraine dyes

“…Squarine absorbers have been reported in other contexts, where electronic properties have been achieved by carefully selecting the substituents on the squaric acid motif . The use of 3,5‐dimethoxy‐ N , N ‐dialkylanilines ( 1 ) has been chosen in the present work as a convenient way of introducing various alkyl‐substituents and the methodology of condensing 3,5‐dimethoxy‐ N , N ‐dialkylaniline with squaric acid has not previously been described.…”

Diminished Second-Harmonic Generation from Oligomeric Squaraine Derivatives of N-Octadecylpyrrole Relative to the Intensity from Films of the Centric Anilinosquaraines

“…530 nm (H aggregate); ,, (b) an SHG-active phase 5,7,13 with optimum intensity arising from films which have a broad absorption at 695 nm; (c) a red-shifted phase with a sharp absorption band at ca. 770 nm (J aggregate). ,, …”

“…Furthermore, solid solutions of the dye in poly(vinyl acetate) exhibit SHG, and thus to satisfy the structural requirement, the behavior is attributed to an association of the squaraine molecules even in dilute solution. Evidence has been provided by electrospray ionization mass spectrometry, which shows m / z values which conform to the dimeric species. , The molecules must adopt an acentric SHG-active arrangement, and the persistent aggregation infers intermolecular interaction between the terminal donor (anilino group) and central acceptor (C 4 O 2 ) moieties. Thus, within the dimeric aggregate, there is most likely a T arrangement, with the second-order properties being dependent upon the extent of the charge-transfer interaction. , This concept has been verified by the theoretical analysis of Brédas and Brouyère and independently corroborated by Honeybourne; their work provides a theoretical basis for the second-order activity of the centric molecules and clearly demonstrates that strong second-order coefficients can result from a dimeric T arrangement.…”

Photoelectrochemical properties of Langmuir–Blodgett films of two squaraine derivatives

“…the blue-shifted H-aggregate phase at 530 nm and the previously reported SHG-active intermediate phase at 660 nm (ref. 15). These may have di †erent contributions to the photocurrent generation but, as the two LB phases are present in Ðlms deposited at both low (5 mN m~1) and high (20 mN m~1) pressures, any interpretation would be ambiguous.…”

“…Interestingly, the intermediate phase exhibits strong second-harmonic generation (SHG),12h15 even though the molecule is centric, and the anomalous behaviour has been attributed to a noncentrosymmetric aggregate structure.12,13 Solid solutions of the dye are also SHG-active indicating that the molecules associate in solution and, for a variety of analogues, this has been conÐrmed by electrospray ionisation mass spectrometry. 15…”