Luminescent Fluorene-Based Bis-Pyrazolyl Aniline Ligand for Aluminum Detection

The design, synthesis, and photophysical properties of a new fluorene-based fluorescent chemosensor, 4-((E)-2-(2-(benzo[d]thiazol-2-yl)-9,9-diethyl-9H-fluoren-7-yl)vinyl)-N,N-bis((3,5-dimethyl-1H-pyrazol-1-yl)methyl)benzenamine (AXF-Al), is described for the detection of Al³⁺. AXF-Al exhibited absorption at 382 nm and strong fluorescence emission at 542 nm (fluorescence quantum yield, Φ _F, of 0.80). The capture of Al³⁺ by the pyrazolyl aniline receptor resulted in nominal change in the linear absorption (372 nm) but a large hypsochromic shift of 161 nm in the fluorescence spectrum (542 to 433 nm, Φ _F?=?0.88), from which Al³⁺ was detected both ratiometrically and colorimetrically. The addition of other metal ions, namely Mg²⁺, Ca²⁺, Mn²⁺, Fe²⁺, Co²⁺, Ni²⁺, Cu²⁺, Zn²⁺, Cd²⁺, Hg ²⁺ and Pb²⁺, produced only minimal changes in the optical properties of this probe. The emission band of this probe was also accessed by two-photon excitation in the near-IR, as two-photon absorption (2PA) is important for potential applications in two-photon fluorescence microscopy (2PFM) imaging. The 2PA cross section of the free fluorenyl ligand AXF-Al was 220 GM at 810 nm and 235 GM at 810 nm for the Al-ligand complex, practically useful properties for 2PFM.     

Deoxyribonucleoside‐Modified Squaraines as Near‐IR Viscosity Sensors

Deoxyribonucleoside‐modified squaraines were synthesized by Sonogashira coupling reactions using an unsymmetrical, terminal alkynylated benzothiazolium squaraine dye. These non‐natural nucleosides exhibited fluorescent ‘turn‐on’ properties in viscous conditions with an enhancement of >300‐fold. The viscosity‐dependent fluorescence enhancement was attributed to a combination of hampering both molecular aggregation and intramolecular bond rotation of the squaraine probe. Fluorescence microscopy allowed visualization of highly viscous regions during various stages of cellular mitosis.     

Structural identification of a novel axially chiral binaphthyl fluorene based salen ligand in solution using electronic circular dichroism: a theoretical-experimental analysis

A novel axially chiral binaphthyl fluorene based salen ligand, AFX-155 [2,2'-(1E,1'E)-(R)-1,1'-binaphthyl-2,2'-diylbis(azan-1-yl-1-ylidene)bis(methan-1-yl-1-ylidene)bis(4-((7-(diphenylamino)-9,9-dihexyl-9H-fluoren-2-l)ethynyl)phenol)], with potential applications in homogeneous catalysis, biophotonics, and sensing was synthesized. A full comparative theoretical-experimental analysis of the UV-vis and electronic circular dichroism (ECD) spectra of the 10 primary isomers, comprising stereoisomers and optical isomers, revealed the presence of the unique structure in tetrahydrofuran (THF) solution, the trans-R-intra//trans-R-extra. A proposed route of attack of the (R)-(+)-2,2'-diamino-1,1'-binapthalene onto a salicaldehyde 5-(2-(2-(diphenylamino)-9,9-dihexyl-9H-fluoren-7-yl)ethynyl)-2-hydroxybenzaldehyde followed by a consecutive attack of the resulting species onto another salicaldehyde, both via Burgi:Dunitz trajectory, validates the unambiguous formation of the established isomer. Steric hindrances seem to be the determinant factor that defines the 3D structural conformation of this particular stereoisomer of AFX-155 with triple axial chirality. The determination of every optimal structure and the dominant conformers of AFX-155 were calculated evaluating, in CONFLEX, their steric energies using force fields at MMFF94S (2006-11-24HGTEMP) level in gas phase. The geometry of the conformers was optimized in THF (using PCM) using Gaussian 09 at the DFT/B3LYP level of theory and 6-31G* basis set. The first 100 electronic excited states were calculated using the same level of theory and basis set.     

Excited State Intramolecular Proton Transfer and Photophysics of a New Fluorenyl Two‐Photon Fluorescent Probe

The steady‐state photophysical, NMR, and two‐photon absorption (2PA) properties of a new fluorene derivative ( 1 ) containing the 2‐(2′‐hydroxyphenyl)benzothiazole (HBT) terminal construct is investigated for use as a fluorescence probe in bioimaging. A comprehensive analysis of the linear spectral properties reveals inter‐ and intramolecular hydrogen bonding and excited state intramolecular proton transfer (ESIPT) processes in the HBT substituent. A specific electronic model with a double minimum potential energy surface is consistent with the observed spectral properties. The 2PA spectra are obtained using a standard two‐photon induced fluorescence method with a femtosecond kHz laser system, affording a maximum 2PA cross section of ～600 GM, a sufficiently high value for two‐photon fluorescence imaging. No dependence of two‐photon absorption efficiency on solvent properties and hydrogen bonding in the HBT substituent is observed. The potential use of this fluorenyl probe in bioimaging is demonstrated via one‐ and two‐photon fluorescence imaging of COS‐7 cells.