Squaraine chemistry: effects of structural changes on the absorption and multiple fluorescence emission of bis[4-(dimethylamino)phenyl]squaraine and its derivatives

Abstract

The absorption and the steady-state fluorescence emission of a class of donor-acceptor-donor (D-A-D) molecules, bis-[4-(dimethylamino)phenyl]squaraine and its derivatives (1-19), have been studied. Squaraines generally exhibit intense solution absorption in the red (e ~3 X 105 cm"1 M"1). All substituents studied in this work exert a bathochromic effect on the absorption. The effect is small and is attributable to the minor involvement of the donor group in the S0 -*• S, excitation. In conjunction with emission data, we are able to show that the bathochromic shift originates from the solute-solvent complex and that the complexation constant increases as the D-A-D charge-transfer character of squaraine increases. Electronic and steric factors affecting the D-A-D character and subsequently the absorption maximum are discussed in terms of the solute-solvent complex model. Multiple-emission bands are observed in the fluorescence spectra of 1-19. Three bands, a, ß, and y, are identified from their typical Stokes shifts. Controlled experiments showed that the multiple-emission bands are intrinsic emissive properties of squaraines. Results from mixed-solvent experiments, solvent-effect studies, and temperature-effect studies show that squaraines form solute-solvent complexes in organic solvents. A photophysical model to account for the multiple emission is proposed. Excitation of squaraine in solution results in two excited states, namely, the excited state of free squaraine and the excited state of the solute-solvent complex. These two excited states emit photons to give the «-emission and the /3-emission. Rotational relaxation(s) (around the C-C bond between the phenyl ring and the four-membered ring of squaraine) is (are) shown to be the major radiationless decay process(es) of these two excited states. As a result, a twisted relaxed excited state is generated. This relaxed excited state can undergo a rotational relaxation to the ground state or emit a photon to give the -emission. The effect of structural changes on the multiple emission is discussed.

Squaraine‐Doped Functional Nanoprobes: Lipophilically Protected Near‐Infrared Fluorescence for Bioimaging

“…As representatively shown with Sq-St in THF solution ( Figure 1 ), all the obtained Sq-X exhibited sharp spectral bands, with the same absorption and fl uorescence peak wavelengths ( λ max.abs = 629 nm and λ max.fl = 644 nm) regardless of lipophilicity. Similar to common squaraine dyes, [ 11 ] they have intense absorption with a molar extinction coeffi cient Scheme 1 . Chemical structure of PMAO and synthetic procedures for lipophilized squaraines (Sq-X ).…”

“…Accordingly, the increase in the Φ f value with increasing molecular size is attributable to the reduced rate of non-radiative relaxation in the excited state because the torsional motion is expected to slow down as the size of the rotating group increases. [ 11 ] To apply the attractive spectral properties of squaraines to the physiological condition, lipophilized Sq-X were encapsulated in waterdispersed PMAO nanoparticles. Simultaneous nanoparticle formation and dye encapsulation were performed by co-precipitating the homogeneous mixture of PMAO and Sq-X (0.5 wt%) in a THF solution into water under sonication and subsequent removal of THF by vacuum evaporation.…”

New Trends in Near-Infrared Fluorophores for Bioimaging

“…14), which have one electron-accepting group (hydroxycyclobutenone) in the center and two electron-donating groups on the edge of the fluorophore, have been widely investigated. 127,128 Their donor-acceptor-donor sandwich structure leads to a large degree of charge-transfer, resulting in intense NIR absorption/fluorescence (650 -700 nm) with high extinction coefficients (around 200000 M -1 cm -1 ), and polarity-sensitive quantum yields (high in low-polar solvents, but very low in high-polar solvent). 127,129 Recently, several NIR squaraines with emission maxima above 800 nm were reported, and herein some of them are reviewed.…”

Squaraine chemistry. A new approach to symmetrical and unsymmetrical photoconductive squaraines. Characterization and solid state properties of these materials

“…In the case of unsymmetrical squaraines lb-le, all exhibit intense and sharp absorption bands in dilute chloroform solutions with A, , , ranging from 625 to 633 nm. Their molar extinction coefficients are typical of squaraines, in the order of 10' cm-I M-' (32,33). Substituents are known to have an effect on the Am.…”

“…Substituents are known to have an effect on the Am. In the case of lb-le, it is interesting to point out that the substituent effect on their A, , , is smaller than on the A, , , of their symmetrical, disubstituted analogs (33 in the positions adjacent to the central C40, unit, the A , , , is at 635.9 nm (33). The A , , , of l b is at 626.1 nm and is midway between those of l a and the dihydroxy compound.…”