Enhanced fluorescence of phthalimide compounds induced by the incorporation of electron-donating alicyclic amino groups

Abstract

Due to their high thermal and environmental stability, polyimides (PIs) are one of the most attractive candidates for novel highly fluorescent polymers, though photophysical studies of PIs are challenging owing to their poor solubility in common solvents. To overcome these problems, we have synthesized and examined a series of low molecular weight model imide compounds: substituted N-cyclohexylphthalimides with alicyclic amino groups at the 3 or 4-positions of the benzene rings (x-NHPIs). Their photophysical properties were systematically investigated by steady-state UV/Visible absorption, fluorescence, and time-resolved fluorescence techniques. In solution, unsubstituted N-cyclohexylphthalimide (NHPI) showed almost no emission, while x-NHPIs exhibited enhanced fluorescence emission depending on the solvent polarity. Analysis of the solvatochromism of the x-NHPIs via Lippert-Mataga plots indicated the generation of large dipole moments in the excited singlet states originating from the intramolecular charge-transfer (ICT) states. The significant difference in the fluorescence quantum yields (Φ) between the 3-substituted (3Pi and 3Pyr) and 4-substituted NHPIs (4Pi and 4Pyr) strongly suggests that the former form a twisted ICT (TICT) state, whereas the latter form a planar ICT (PICT) state when excited. 4-Substituted NHPIs also show high fluorescence yields in the crystalline state. A particularly large Φ value was obtained for the 4Pi crystal, which we explain by the large intermolecular distances and the arrangement of molecules minimizing intermolecular interactions as well as the small non-radiative deactivation rate. These facts clearly demonstrate that the introduction of an alicyclic amino group into NHPI at the 4-position enhances the fluorescence quantum yields significantly, which suggests a new pathway for the development of novel, highly fluorescent PIs.

Intra‐ and Intermolecular Fluorescence Quenching of Alkylthio‐Substituted Phthalimides by Photoinduced Electron Transfer: Distance, Position and Conformational Dependence

“…Oxidation of other thioethers to the corresponding sulfoxides restores the strong fluorescence of undistorted DMPht but still with a moderate quenching by the sulfoxide group for several examples (2 S=O , 3 S=O ). Methionine derivatives (10)(11)(12) are in general less fluorescent than the corresponding cysteine derivatives (13)(14), an unexpected behavior from the comparison of the model compounds 3 and 2, respectively. The decay dynamics of the regioisomeric 3-/4-methylthiophthalimides 8 and 9 strongly differs (quantum yields for fluorescence 0.04 vs. 0.56) which can be explained by an additional decay channel only available for the geometrically more constrained compound 8.…”

“…Coupling patterns were designated as follows: s, singlet; d, doublet; dq, doublet of quartets; t, triplet; q, quartet; quin, quintet; m, multiplet. 13 C-NMR spectra were recorded either on a Bruker Avance II 300 spectrometer instrument operating at 75 MHz or on a Bruker Avance II 600 spectrometer instrument operating at 126 MHz. Melting points were measured on a MP50 Melting Point System by Mettler Toledo.…”

“…Alkoxy and amino (or amido) groups directly coupled to the chromophore are the optimal groups for fluorescence increase and absorption redshift. [12][13][14] An especially successful approach was the 4,5dialkoxyphthalimide that shows strong fluorescence with quantum yields near to 100 %. [15] Strongly fluorescent phthalimides that also constitute very strong reductants in the excited singlet states have been developed as excellent sensors for anions such as fluoride, [16,17] or cyanide.…”

Photophysical relaxation mechanism of excited phtalimide compounds

“…Previously, it has been discovered that the introduction of a strong electron-donating amino group into phtalimide, a model compound for PI, significantly enhanced its quantum yield of fluorescence [9,10]. Therefore, a number of phtalimide compounds were synthesized containing tertiary amino groups at the 3-or 4-position [11]. It was discovered that while compounds with alicyclic amino groups at the 4-position had fluorescence quantum yields that exceeded 50% in the majority of the solvents, the compounds with exactly the same amino groups at the 3-position had 10% quantum yield of fluorescence at most.…”

“…It was discovered that while compounds with alicyclic amino groups at the 4-position had fluorescence quantum yields that exceeded 50% in the majority of the solvents, the compounds with exactly the same amino groups at the 3-position had 10% quantum yield of fluorescence at most. The explanation for such observation was that poorly fluorescent phtalimide derivatives formed a 'dark' twisted intramolecular charge transfer (TICT) state instead of fluorescing [11]. It was hypothesized that molecules relax from the TICT state back to the ground state non-radiatively and, as a result, an alternative relaxation channel to fluorescence is formed.…”