Solvent and pH Dependent Fluorescent Properties of a Dimethylaminostyryl Borondipyrromethene Dye in Solution

Abstract

Steady-state and time-resolved fluorescence techniques have been used to study the photophysical properties of the fluorescent BODIPY-derived dye 3-{2-[4-(dimethylamino)phenyl]ethenyl}-4,4-difluoro-8-(4-methoxyphenyl)-1,5,7-trimethyl-3a,4a-diaza-4-bora-s-indacene. This compound has been synthesized via a microwaveassisted condensation of p-N,N-dimethylaminobenzaldehyde with the appropriate 1,3,5,7-tetramethyl substituted borondipyrromethene unit. The fluorescence properties of the dye are strongly solvent dependent: increasing the solvent polarity leads to lower fluorescence quantum yields and lifetimes, and the wavelength of maximum fluorescence emission shifts to the red. The Catalán solvent scales are found to be the most suitable for describing the solvatochromic shifts of the fluorescence emission. These are dominated by polarity/polarizability effects, as confirmed by quantum-chemical calculations performed in the dielectric continuum approximation. Fluorescence decay profiles of the dye can be described by a single-exponential fit in most solvents investigated, while two decay times are found in alcohols. The dye undergoes a reversible protonation-deprotonation reaction in the acidic pH range with a pK a of 2.25 in acetonitrile solution. Fluorimetric titrations as a function of pH produce fluorescence emission enhancements at lower pH. The fluorescence excitation spectra show a hypsochromic shift from 600 nm for the neutral amine to 553 nm for the ammonium form, so that ratiometric measurements can be used to determine pK a .

Original Design of Fluorescent Ligands by Fusing BODIPY and Melatonin Neurohormone

“…[17][18][19]27,28 Such a π extension can be performed by introducing an aryl or a hetaryl group at the 3 and/or 5 positions of the pyrrole moiety using metal catalyzed reactions, 29−32 the Knoevenagel reaction, 33,34 vicarious nucleophilic substitution (VNS), 35 or by modifying the substituents linked to the boron atom. 36 For our purpose, aryl and styryl groups were envisaged and pyrroles 3 and 4 were first synthesized.…”

Quantitative Structure‐Fluorescence Property Relationship Analysis of a Large BODIPY Library

“…4-aminostyryl BODIPYs are known to observe strongly solvent and pH dependent fluorescence emission. [60,62,63] As elaborated by Baruah et al [60] their emission maximum shifts to longer wavelengths in going from nonpolar to polar solvents while the absorption maximum is mostly unaffected by different solvents. This results in strongly solvent dependent Stokes shifts.…”

“…This results in a difference in the dipole moment between the ground and excited states, such that polar, non-protic solvents stabilize the excited state and lead to a bathochromic shift of emission. [60] ICT has been discussed with BODIPY-linked phenol and anisole, as well, where the oxygen is the electron donating group. [61,[64][65][66] However, the emission maximum of these compounds was found to be solvent independent.…”

“…The spectral properties of molecules of the external validation set were experimentally determined in the solvents ethanol and methanol in which ICT is known to occur to a decreased extent than in DMSO. [60] …”

“…In fact, a similar hypsochromic shift of l em in polar, protic solvents such as methanol and ethanol, as compared to polar, aprotic solvents such as DMSO has been observed by others and could be a consequence of ground state stabilization due to hydrogen bonding with the solvent. [60,61] Scatter plots of calculated vs. experimental wavelengths based on the non-linear models of the Gap + Mold2 descriptor set are shown in Figure 4. The scatter plot of the non-linear model of l abs (Figure 4a) shows considerably less variation as compared to the corresponding plot of the linear model (Figure 3a) and is in line with its good validation statistics.…”