Comprehensive Model of the Photophysics of N-Phenylnaphthalimides: The Role of Solvent and Rotational Relaxation

A comparative study into two dual fluorescent mechanisms via positional isomers of N-hydroxyarene-1,8-naphthalimides

Three isomers of hydroxy substituted N-aryl-1, 8-naphthalimides based on N-aryl naphthalic anhydride fluorophore have been synthesized. The decrease in fluorescence intensity from ortho to para substitution of hydroxy group on N-aryl reveals that para substituted isomer undergoes ESEC (Excited State with Extended Conjugation) mechanism which is proved by low quantum yield and appearance of dual emission. The ortho isomer, however, has high quantum yield and no tautomer emission, indicating ESIPT (Excited State Intramolecular Proton Transfer) mechanism is not operating. Similarly, all these isomers show strong fluorescence quenching in presence of strong H-bonding solvents like DMSO and pyridine, but there was neither the shift of emission bands nor the appearance of new bands for proton transfer to these solvents. Thus, it also indicates the absence of excited state proton transfer mechanism. Both the ortho isomer, and to a greater degree the meta isomer, showed larger quenching constants (Kapp) with pyridine than DMSO. This trend opposes the hydrogen-bond affinity for these solvents with phenol and points to a 2-point recognition interaction. In addition, a naphthalimide derivative using 2-aminoimidazole was prepared and examined for optimal positioning of a six-membered ring hydrogen bond pattern. No dual fluorescence was observed for this compound either.

Photophysics and Photochemical Studies of 1,4-Dihydropyridine Derivatives

The absorption and fluorescence properties of nifedipine (NPDHP), felodipine (CPDHP) and a series of structurally related 1,4-dihydropyridines were studied in aqueous solution and organic solvents of different properties. The absorption and fluorescence spectra were found to depend on the chemical nature of the substituents at the position 4 of the 1,4-dihydropyridine ring (DHP) and on solvent properties. In aqueous solution, the fluorescence spectra of 4-phenyl substituted compounds are blue-shifted with respect to the alkyl substituted compounds. The more fluorescent compound is CPDHP. Nifedipine is not fluorescent. All compounds, with the exception of CPDHP, present monoexponential fluorescence decay with very short lifetime (0.2-0.4 ns). CPDHP showed a biexponential emission decay with a long-lived component of 1.7 ns; this behavior is explained in terms of different conformers because of the hindered rotation of the phenyl group by the ortho-substitution. Analysis of the solvent effect on the maximum of the absorption spectrum by using the linear solvent-energy relation solvato-chromic equation indicates the redshifts are influenced by the polarizability, hydrogen bonding ability and the hydrogen bond acceptance of the solvent. Whereas, the fluorescence characteristics (spectra, quantum yields and lifetimes) are sensitive to the polarizabilty and hydrogen bond ability of the solvents. Photo-decomposition of nifedipine is dependent on the solvent properties. Faster decomposition rates were obtained in nonprotic solvents. The 4-carboxylic derivative goes to decarboxylation. Under similar conditions, the other DHP compounds did not show appreciable photodecomposition.

9-Donor-Substituted Acridizinium Salts: Versatile Environment-Sensitive Fluorophores for the Detection of Biomacromolecules

The absorption and steady-state emission properties of a series of N-alkyl- and N-aryl-9-aminoacridizinium derivatives and two 9-sulfanyl-substituted acridizinium derivatives were investigated. The N-alkyl derivatives and the 9-methylsulfanylacridizinium have an intense intrinsic fluorescence (phi(f) = 0.2-0.6), whereas the N-aryl-substituted compounds are virtually nonfluorescent in liquid solutions (phi(f) < or = 0.01). The emission intensity of the latter compounds significantly increases with increasing viscosity of the medium. It is demonstrated that the excited-state deactivation of the N-aryl-9-aminoacridizinium derivatives is due to two nonradiative processes: (i) torsional relaxation by rotation about the N-aryl bond and (ii) an electron-transfer process from an electron-donor substituted phenyl ring to the photoexcited acridizinium chromophore. The binding of several representative acridizinium derivatives to double-stranded DNA was studied by the spectrophotometric titrations and linear dichroism spectroscopy. The results give evidence that the prevailing binding mode is intercalation with binding constants in the range (0.5-5.0) x 10(5) M(-1) (in base pairs). Notably, the binding of most of the N-aryl-9-aminoacridizinium derivatives leads to a fluorescence enhancement by a factor of up to 50 upon binding to the biomacromolecules. Moreover, the addition of selected proteins, namely albumins, to N-(halogenophenyl)-9-aminoacridizinium ions in the presence of an anionic surfactant (sodium dodecyl sulfate) results in a 20-fold fluorescence enhancement. In each case, the emission enhancement is supposed to result from the hindrance of the torsional relaxation in the corresponding binding site of the biomacromolecule, which in turn suppresses the excited-state deactivation pathway.

Toward intrinsically fluorescent proteomimetics: fluorescent probe response to alpha helix structure of poly-gamma-benzyl-L-glutamate

A fluorescent probe (1), developed for recognition of alpha helical secondary structure, shows a large fluorescence change upon titration with the synthetic protein PBLG. Compared to fluorophores of similar size and shape, 1 displayed the smallest dissociation constant (K(D)=80microM) when titrated with PBLG. These preliminary studies are directed toward developing small molecule proteomimetics that have intrinsic fluorescence and are specific for helical-protein binding-sites.

Matrix Screening of Substituted N-Aryl-1,8-naphthalimides Reveals New Dual Fluorescent Dyes and Unusually Bright Pyridine Derivatives

A 3 x 14 matrix of substituted N-aryl-1,8-naphthalimides was synthesized for the evaluation and discovery of dual fluorescence. Because of their unique photophysical properties, these dual fluorescent systems represent an exception to the widely studied TICT (Twisted Internal Charge Transfer) fluorescent dyes or tautomeric benzofluorescein class of two-color dyes. The matrix library was designed to investigate the effects of heterocycles, particularly pi-excessive and pi-deficient systems. Of the 42 compounds surveyed, five displayed well-resolved two-color emission in solvents as nonpolar as hexane. Based on the observed trends in fluorescence lambda(max) and quantum yield, a new model is proposed that predicts LW and SW emission for these systems. In addition, this model provides potential design features for the synthesis of new dual fluorescent species.

Hydrogen-Bond Formation between Isoindolo[2,1-a]indol-6-one and Aliphatic Alcohols in n-Hexane

The spectroscopic, kinetic, and equilibrium properties of isoindolo[2,1-a]indol-6-one (I) were studied in n-hexane in the presence and absence of alcohols (X). Hydrogen-bonded-complex formation was found to occur between the alcohol and the ground state as well as the excited state of the I molecule. The spectra of I and its singly complexed derivative (IX) are similar; however, that of IX is red shifted. The extent of red shift increases with the hydrogen-bonding ability of the alcohol. Equilibrium constant measurements were made to determine the hydrogen-bond basicity (beta(2)(H)) for I and the singlet excited (1)I. The beta(2)(H) value for (1)I is found to be about twice that of the ground-state I. Time-resolved fluorescence decay measurements indicate that the reaction of singlet excited I with fluorinated alcohols is diffusion controlled, while the rate of complexation with nonfluorinated (weaker hydrogen bonding) aliphatic alcohols depends on the Gibbs energy change in the complexation reaction. The quantitative correlation between the rate coefficient of complexation of (1)I with alcohols and the Gibbs energy change in the complexation process allowed us to estimate the rate coefficient for the complexation of the ground-state I with alcohols. The formation of the singlet excited hydrogen-bonded complex is irreversible; (1)IX disappears in a first order and an alcohol induced second order reaction. The first order decay is predominantly due to internal conversion to the ground state, the rate of which depends on the ionization energy of the complexing alcohol.

Copper-Promoted N-Arylations of Cyclic Imides within Six-Membered Rings: A Facile Route to Arylene-Based Organic Materials

[reaction: see text] Cyclic imides within six-membered rings are shown to undergo efficient N-arylation using various arylboronic esters mediated by copper(II) acetate in the presence of an amine base and oxygen atmosphere with gentle heating. Until now, the synthesis of N-arylated cyclic imides having six-membered rings was restricted largely to strongly heating anilines in the presence of anhydrides. This reaction is applicable to the synthesis of new organic materials based on arylene imide and bis(imide) dyes, such as perylene-3,4:9,10-bis(dicarboximide)s.

Substituent Effects on Monoboronic Acid Sensors for Saccharides Based on N-Phenyl-1,8-naphthalenedicarboximides

In the course of our investigations on new monoboronic acid saccharide sensors with C(0) spacers, a series of probes 1-6 based on 1,8-naphthalenedicarboximide were synthesized. Sensor 1 displays features typical of PET monoboronic acid sensors and shows high selectivity to fructose. Sensor 2 exhibits a novel dual emission and remarkable sensitivity for glucose relative to fructose and galactose through subtle changes in pH. Sensor 3 displays significantly enhanced fluorescence in the presence of galactose at low pH. Although probes 4-6 exhibit unique properties such as high quantum yield (Phi(F) = 0.407) and excellent solubility in water, they did not show significant change in fluorescence intensity in the presence of monosaccharides. The effects of substituent on all six probes lend support to the proposed photoelectrochemical model.

Effect of protonation and hydrogen bonding on the fluorescent properties and exciplex formation of N-(4-pyridyl)-1,2-naphthalimide

The effects of fluorinated hydroxy compounds and naphthalene on the fluorescence of N-(4-pyridyl)-1,2-naphthalimide (PyNI) have been studied in toluene. The interaction of the pyridyl moiety of PyNI with hexafluoro-2-propanol (HFIP) gave rise to a hydrogen-bonded complex, whereas a more stable, hydrogen-bonded ion pair was formed with trifluoroacetic acid (TFA). Time-resolved fluorescence measurements demonstrate that hydrogen bonding with HFIP is a reversible process, even in the excited state, and revealed the rate constants of the various energy dissipation processes. The fluorescence yield enhancement of about one order of magnitude upon the 1 : 1 binding of PyNI to HFIP or TFA is primarily attributed to the deceleration of the internal conversion, and the fluorescence proved to be the dominant deactivation pathway of the singlet excited complexes. Both PyNI and its TFA complex produced fluorescent exciplexes with naphthalene. Protonation of PyNI markedly decreases the energy of the exciplex, leading to faster radiationless energy dissipation as well as to slow dissociation into an excited PyNI-TFA complex and ground-state naphthalene.

Pressure dependence of the dual luminescence of twisting molecules. The case of substituted 2,3-naphthalimides

The effect of high pressure (up to 5 kbar) has been studied for triacetin solutions of 2-phenyl-2,3-dihydro-1H-benzo[f]isoindole-1,3dione 1 (N-phenyl-2,3-naphthalimide) and its 3-fluorophenyl- (2), 4-carbethoxyphenyl(4) and 4-methoxyphenyl (5) derivatives which all show dual fluorescence. When the N-phenyl group is unsubstituted (compound 1) or substituted with electron-attracting groups (2 and 4), the increase of pressure over the solution decreases slightly the emission at the long-wavelengths (LW) and increases dramatically the intensity of the short-wavelength (SW) fluorescence. Plotting the logarithm of the SW/LW fluorescence quantum yield ratio for compounds, 1,2 and 4 versus the logarithm of the viscosity of the medium shows a substantial increase of this ratio which corresponds mainly to the increase of the SW emission intensity, the effect on the LW emission being only moderate. As the pressure is increased, the rotation of the N-phenyl group of compound 1 is progressively hindered and the prevailing emission comes from a state which has the same geometry as the ground state (in which the planes of the two moieties of the molecule form an angle close to 60 degree). The effect is different when an electron-donating methoxy group is attached in the para position to the N-phenyl ring, compound 5, as mainly the LW fluorescence intensity increases with pressure. For this molecule which has an electron-donating p-substituent on the N-phenyl ring, the two moieties of the ground state molecule have a more planar geometry (43 degree angle) and the LW fluorescence appears to originate from an intramolecular charge transfer state the fluorescence of which increases with pressure. A three-level reaction scheme is proposed to account for the observed kinetics. In all cases, the viscosity of the medium is found to be the main factor which induces the changes in the fluorescence spectra, and the deceleration of the non-radiative deactivation from the SW* excited state is responsible for these modifications whether a reversible process between the two emitting SW* and LW* states is observed (as for compounds 2 and 4) or not (as for compound 1).

Dual luminescence properties of differently benzo-fused N-phenylphenanthridinones

The photophysics of some newly prepared N-arylphenanthridinone derivatives have been investigated. It has been demonstrated how the luminescence properties are influenced by the size of the aromatic ring system. It has been shown that the replacement of the phenyl group in N-phenyphenanthridinone (PP) by an alpha-naphthyl or beta-naphthyl group (alphaNP and betaNP, respectively), influences the fluorescence spectra very differently. For alphaNP, the long-wavelength (LW) emission, which is well observable in case of PP, disappears, while for betaNP, the intensity of LW emission increases compared to the short-wavelength (SW) fluorescence. The rotation of the alpha-naphthyl group to the coplanar geometry, which is a requirement of the formation of the LW state, is strongly hindered, resulting in the lack of LW emission. In respect of steric hindrance, the beta-napthyl group is similar to phenyl, however, it decreases the energy of the LW state more as a consequence of its better electron donating character and the more extended conjugation of the coplanar system. This causes the increase of the LW/SW fluorescence ratio. The benzo-fusing on the phenanthridinone moiety results in a 6-7 kcal mol(-1) decrease in the SW singlet energy, however, surprisingly the LW state energy also decreases in almost the same manner. The phenomenon shows that the entire benzo-phenanthridinone group is strongly involved in both transitions. As a consequence, the benzo-fused N-aryl derivatives also show dual luminescence. The dipole moments of the LW state of betaNP and betaNBiP (6-naphthalen-2-yl-6H-benzo[i]phenanthridin-5-one) proved to be bigger by 30 and 50%, than that of the SW state, respectively. MO calculation indicates that in the SW --> LW reaction not only the size but the direction of the excited state dipole also changes significantly. In apolar solvents, the dominant deactivation process of the examined molecules is intersystem crossing. In polar solvents, where the LW emission energy is smaller, internal conversion becomes more significant than the other processes, resulting in low fluorescence yield.

1,8-Naphthalimides in phosphorescent organic LEDs: the interplay between dopant, exciplex, and host emission

Four different 1,8-naphthalimide derivatives were examined in phosphorescent organic light emitting diodes (OLEDs), i.e., 1,8-naphthalimide, N-phenyl-1,8-naphthalimide, N-2,6-dibromophenyl-1,8-naphthalimide (niBr), and bis-N,N-1,8-naphthalimide. Photoluminescence from all four naphthalimides have violet-blue fluorescence and phosphorescent bands between 550 and 650 nm (visible at 77 K). While all four compounds gave good glassy films when doped with a phosphorescent dopant, only the niBr films remained glassy for extended periods. OLED studies focused on niBr, with two different architectures. One OLED structure (type 1) had the niBr layer as a doped luminescent layer and an undoped niBr layer to act as a hole-blocking layer. The alternate structure (type 2) utilizes a doped CBP layer as the luminescent layer and the niBr layer is used as a hole-blocking layer only (CBP = 4,4'-N,N'-dicarbazolylbiphenyl). Type 1 and 2 OLEDs were prepared with green, yellow, and red emissive phosphorescent dopants (Irppy, btIr, and btpIr, respectively). The dopants were organometallic Ir complexes, previously shown to give highly efficient OLEDs. Of the three dopants, the btpIr-based OLEDs showed the best device performance in both structures (peak efficiencies for type 2: 3.2% and 2.3 lum/W at 6.3 V; type 1: 1.7% and 1.3 lm/W at 6.1 V). The green and yellow dopants gave very similar performance in both type 1 and 2 devices (peak efficiencies are 0.2-0.3%), which were significantly poorer than the btpIr-based OLEDs. The emission spectrum of the btIr- and btpIr-based devices (type 1 and 2) are the same as the solution photoluminescence spectrum of the dopant alone, while the Irppy device gives a broad red emission line (lambda(max) = 640 nm). The red Irppy.niBr emission line is assigned to an Irppy.niBr exciplex. The type 2 Irppy-based device gave a voltage-dependent spectrum, with the red emission observed at low bias (4-8 V), switching over to strong green emission as the bias was raised. All other devices showed bias-independent spectra. Estimates of HOMO, LUMO, and excited-state energies (dopant, niBr, and exciplex) were used to explain the observed spectral properties of these devices. btpIr-based devices emit efficiently from isolated dopant states (external efficiencies = 3.2 %, 2.3 lum/W). Irppy-based devices emit only from exciplex states, with low efficiency (external efficiency = 0.3%). btIr.niBr films have very similar energies for the dopant, exciplex, and niBr triplet states, such that relaxation can go through any of these states, leading to low device efficiency (external efficiency = 0.4%). High device efficiency is achieved only when dopant emission is the dominant pathway for relaxation, since exciplex and niBr triplet states give either weak or no electroluminescence.

1,8-Naphthalimides as Stereochemical Probes for Chiral Amines: A Study of Electronic Transitions and Exciton Coupling

The absorption and magnetic circular dichroism (MCD) spectra of 1,8-naphthalimide are analyzed in terms of the number of contributing electronic transitions and their polarization. The experimental results are supported by semiempirical INDO/S calculations. It is demonstrated that the strongly allowed, pi --> pi naphthalene transition of the 1,8-naphthalimide chromophore located at 231 nm is perfectly suited for absolute configuration assignments of amine derivatives on the basis of the bichromophoric exciton coupling. Both degenerate (bis-1,8-naphthalimide) and nondegenerate (1,8-naphthalimide-phthalimide, 1,8-naphthalimide-phenyl, or 1,8-naphthalimide-benzoate) couplings were studied. In the latter case, the sign of the exciton Cotton effect was opposite to the sign of the degenerate exciton Cotton effect for the same absolute configuration. An extension of the application of the exciton coupling to the 264 nm pi --> pi transition of 1,9-anthraimide is also shown.