Photochemistry of Far Red Responsive Tetrahydroquinoxaline-Based Squaraine Dyes

Abstract

The photochemical and redox properties of two newly synthesized tetrahydroquinoxaline-based squaraine dyes (SQ) are investigated using femto- and nanosecond laser flash photolysis, pulse radiolysis, and cyclic voltammetry. In acetonitrile and dichloromethane, these squaraines exist as monomers in the zwitterionic form (lambda(max) approximately 715 nm, epsilon(max) approximately 1.66 x 10(5) M(-1) cm(-1) in acetonitrile). Their excited singlet states ((1)SQ*) exhibit a broad absorption band at 480 nm, with singlet lifetimes of 44 and 123 ps for the two dyes. Both squaraines exhibit poor intersystem crossing efficiency (Phi(ISC) < 0.001). Their excited triplet states ((3)SQ*), however, are efficiently generated by triplet-triplet energy transfer using triplet excited 9,10-dibromoanthracene. The excited triplet states of the squaraine dyes exhibit a broad absorption band at ca. 560 nm (epsilon(triplet) approximately 4.2 x 10(4) M(-1) cm(-1)) and undergo deactivation via triplet-triplet annihilation and ground-state quenching processes. The oxidized forms of the investigated squaraines (SQ(*+)) exhibit absorption maxima at 510 and 610 nm.

Superfluorescent Squaraine with Efficient Two‐Photon Absorption and High Photostability

“…Most symmetrical squaraines possess efficient two‐photon absorption (2PA) properties, which are typically exhibited in the two‐photon‐allowed short‐wavelength absorption bands under double‐resonance excitation conditions, whereby the effect of intermediate state resonance enhancement is realized 10. The intensive, linear absorption bands (typically the maximum of the extinction coefficient, ε max ≈1–4×10 5 M −1 cm −1 ;6b, 10b, 11) and large 2PA cross sections [ δ 2PA ≈ (1–3)×10 4 GM,12] are suitable for absorption in the biological transparency window and the emission spectral range;13 high fluorescence quantum yields— Φ fl of approximately 0.5 – 1.04b,c, 11—and high photostability5b, 14 strongly suggest that squaraine derivatives have good potential for applications in bioimaging,15 which includes two‐photon fluorescence microscopy (2PFM) 12c. In contrast to conventional (one‐photon) excited fluorescent labels, 2PA squaraine‐based fluorescent molecules have received less attention for two‐photon NIR excitation and need further research.…”

Synchronized Energy and Electron Transfer Processes in Covalently Linked CdSe–Squaraine Dye–TiO2 Light Harvesting Assembly

“…A careful design of a linker molecule with absorption in the near-IR region should enable capture of the photons over a wider spectral region. We have now employed a bifunctional squaraine dye as a linker since these classes of dyes are widely used to capture photons in the NIR region of dye-sensitized solar cells. − The squaraine dye (SQSH) thus serves dual functionality. First, the linker dye captures the NIR region of sunlight and sensitizes the mesoscopic TiO 2 film, and second, it provides a link between CdSe QDs and TiO 2 moieties.…”

Squarylium‐Triazine Dyad as a Highly Sensitive Photoradical Generator for Red Light

“…The positive band before 500 nm is ascribed to the transient absorbance of the emissive excited state of the chromophore in S‐2‐T 27. 28 The negative band with a maximum at 630 nm is assigned to the (S‐2‐T) bleaching and stimulated emissions of the (S‐2‐T) emissive excited state. All bands decay quite homothetically in the first hundred femtoseconds.…”