Search

The photophysics of N-(4-cyanophenyl)carbazole (NP4CN) was investigated by using absorption and fluorescence spectra, picosecond fluorescence decays, and femtosecond transient absorption. In the nonpolar n-hexane as well as in the polar solvent acetonitrile (MeCN), a locally excited (LE) state is detected, as a precursor for the intramolecular charge transfer (ICT) state. A LE → ICT reaction time τ(2) at 22 °C of 0.95 ps in ethyl cyanide (EtCN) and 0.32 ps in MeCN is determined from the decay of the LE excited state absorption (ESA) maximum around 620 nm. In the ESA spectrum of NP4CN in n-hexane at a pump-probe delay time of 100 ps, an important contribution of the LE band remains alongside the ICT band, in contrast to what is observed in EtCN and MeCN. This shows that a LE ⇄ ICT equilibrium is established in this solvent and the ICT reaction time of 0.5 ps is equal to the reciprocal of the sum of the forward and backward ICT rate constants 1/(k(a) + k(d)). In the photostationary S(0) → S(n) absorption spectrum of NP4CN in n-hexane and MeCN, an additional CT absorption band appears, absent in the sum of the spectra of its electron donor (D) and acceptor (A) subgroups carbazole and benzonitrile. This CT band is located at an energy of ∼4000 cm(-1) lower than for N-phenylcarbazole (NPC), due to the larger electron affinity of the benzonitrile moiety of NP4CN than the phenyl subunit of NPC. The fluorescence spectrum of NP4CN in n-hexane at 25 °C mainly consists of a structured LE emission, with a small ICT admixture, indicating that a LE → ICT reaction just starts to occur under these conditions. In di-n-pentyl ether (DPeE) and di-n-butyl ether (DBE), a LE emission is found upon cooling at the high-energy edge of the ICT fluorescence band, caused by the onset of dielectric solvent relaxation. This is not the case in more polar solvents, such as diethyl ether (DEE) and MeCN, in which a structureless ICT emission band fully overlaps the strongly quenched LE fluorescence. For the series of D/A molecules NPC, N-(4-fluorophenyl)carbazole (NP4F), N-[4-(trifluoromethyl)phenyl]carbazole (NP4CF), and NP4CN, with increasing electron affinity of their phenyl subgroup, an ICT emission in n-hexane 25 °C only is present for NP4CN, whereas in MeCN an ICT fluorescence is observed with NP4CF and NP4CN. The ICT fluorescence appears when for the energies E(ICT) of the ICT state and E(S(1)) of the lowest excited singlet state the condition E(ICT) ≤ E(S(1)) holds. E(ICT) is calculated from the difference E(D/D(+)) - E(A(-)/A) of the redox potentials of the D and A subgroups of the N-phenylcarbazoles. From solvatochromic measurements with NP4CN an ICT dipole moment μ(e)(ICT) = 19 D is obtained, somewhat larger than the literature values of 10-16 D, because of a different Onsager radius ρ. The carbazole/phenyl twist angle θ = 45° of NP4CN in the S(0) ground state, determined from X-ray crystal analysis, has become smaller for its ICT state, in analogy with similar conclusions for related N-phenylcarbazoles and other D/A molecules in the literature.

The effects of the nature of solvent, temperature and complex formation with alkali and alkaline-earth metal cations, as well as protonation, on the efficiency and the kinetics of fluorescence of 3-azacrowned 7-diethylaminocoumarins have been studied. For the crown-ethers under investigation, the ratio of a dipole moment to the radius of Onsager cavity delta micro/rho is a constant value, and a macrocycle does not affect delta micro, and rho. The fluorescence of coumarin 1 in acetonitrile is quenched by an electron donor, triethylamine, with the Stern-Volmer constant being equal to (0.474+/-0.009) M(-1). The decrease in coumarin 1 fluorescence quantum yield upon the introduction of N-alkylazacrown moiety into position 3 is caused by an intramolecular photoinduced electron transfer from the nitrogen atom of macroheterocycle to the coumarin moiety, where the excitation is localized. The fluorescence quenching has an activation energy 2.32+/-0.05 kcal/mol in various hydrocarbons, and does not depend on the solvent viscosity. The fluorescence kinetics of free crowned coumarins in methanol is not monoexponential because of the existence of macrocycle conformers, or because of the hydrogen bond complex formation between the solvent and the nitrogen atom of macrocycle, in which the efficiency of intramolecular electron transfer is different. Upon complex formation with alkali and alkaline-earth metal cations and upon protonation, the fluorescence quantum yield increases and fluorescence decay becomes monoexponential.

The spectroscopic investigation of the binding properties of berberine‐type 9‐aryl‐substituted isoquinolinium derivatives with G‐quadruplex DNA (G4‐DNA) are presented. Photometric titrations show that these ligands bind with high affinity to the telomeric G4‐DNA form 22AG (K=1.0–44×105 M−2). Furthermore, fluorimetric analysis of thermal DNA denaturation (FRET melting) reveals a significant thermal stabilization of G4‐DNA 22AG upon association with the methoxy‐substituted derivatives. As an analytically useful property, the derivatives with a phenyl substituent or with additional electron‐donating groups show a very weak fluorescence intensity, which increased significantly upon G4‐DNA complexation (fluorescence light‐up effect). Additional time‐resolved fluorescence spectroscopy indicated increased fluorescence lifetimes of the DNA‐bound 9‐(4‐methoxyphenyl)‐substituted derivative, when interacting with the quadruplex‐forming strand 22AG. Notably, the changes of the steady‐state and time‐resolved emission properties of the ligand are more pronounced with G4‐DNA than with duplex DNA so that the combination of these complementary methods may be used for the selective G4‐DNA detection. [ABSTRACT FROM AUTHOR]

This study reports on the characterization of reporter dye-loaded block copolymer vesicles (polymersomes) of PS115- b-PAA15 (polystyrene-block-poly(acrylic acid)) and PEG114- b-PLA167 (poly(ethylene glycol)-block-poly(lactic acid)) and their drying behavior by fluorescence lifetime imaging microscopy (FLIM). The characteristic changes of the fluorescence decay components of the dye calcein incorporated in the three different local nanoenvironments, namely, the solvated dye, dye associated with the vesicle wall, and dried agglomerated dye, are observed by FLIM during the drying of vesicles. The amplitude ratio R1/2 of the components attributed to the solvated dye in the vesicle interior with a lifetime τ1 ≈ 3.9 ns to the one attributed to calcein associated with the wall with a lifetime τ2 ≈ 1.5 ns is found to decrease exponentially with drying time. The time constants, which are found to depend linearly on the radius of the vesicle, yield by extrapolation to zero vesicle diameter an estimate of the polymersome wall thickness. For PS115- b-PAA15 and PEG114- b-PLA167 vesicle wall thicknesses r0 of 16 ± 5 nm and 28 ± 4 nm, respectively, are observed, which are in favorable agreement with transmission electron and atomic force microscopy data. [ABSTRACT FROM AUTHOR]

The effects of solvent and crown-ether moiety on spectral properties of pyridinium styryl dye were studied by steady-state absorption and fluorescent spectroscopy. Analysis of viscosity and polarity effects on fluorescence quantum yield and Stokes shift permitted us to suggest that there is a two stage process of excited state relaxation. The macrocyclic moiety has a little influence on the first stage of relaxation, which manifests itself in a magnitude of Stokes shift, but suppresses considerably the second stage, which manifests itself in a magnitude of fluorescence quantum yield. The metal complex shows an additional stage of excited state relaxation, namely, photorecoordination of metal cation within the macrocyclic cavity. [ABSTRACT FROM AUTHOR]

Intramolecular charge transfer (ICT)with crystal violet lactone(CVL) in the excited singlet state takes place in solvents more polarthan n-hexane, such as ethyl acetate, tetrahydrofuran,and acetonitrile (MeCN). In these solvents, the fluorescence spectrumof CVL consists of two emission bands, from a locally excited (LE)and an ICT state. The dominant deactivation channel of the lowestexcited singlet state is internal conversion, as the quantum yieldsof fluorescence (0.007) and intersystem crossing (0.015) in MeCN at25 °C are very small. CVL is a weakly coupled electron donor/acceptor(D/A) molecule, similar to an exciplex 1(A–D+). A solvatochromic treatment of the LE and ICT emissionmaxima results in the dipole moments μe(LE) = 17D and μe(ICT) = 33 D, much larger than those previouslyreported. This discrepancy is attributed to different Onsager radiiand spectral fluorimeter calibration. The LE and ICT fluorescencedecays of CVL in MeCN are double exponential. As determined by globalanalysis, the LE and ICT decays at 25 °C have the times τ2= 9.2 ps and τ1= 1180 ps, with an amplituderatio of 35.3 for LE. From these parameters, the rate constants ka= 106 × 109s–1and kd= 3.0 × 109s–1of the forward and backward reaction in the LE ⇄ICT equilibrium are calculated, resulting in a free enthalpy differenceΔGof −8.9 kJ/mol. The amplitude ratioof the ICT fluorescence decay equals −1.0, which signifiesthat the ICT state is not prepared by light absorption in the S0ground state, but originates exclusively from the directlyexcited LE precursor. From the temperature dependence of the fluorescencedecays of CVL in MeCN (−45 to 75 °C), activation energies Ea= 3.9 kJ/mol (LE → ICT) and Ed= 23.6 kJ/mol (ICT → LE) are obtained,giving an enthalpy difference ΔH(= Ea– Ed) of−19.7 kJ/mol, and an entropy difference ΔS= −35.5 J mol–1K–1. These data show that the ICT reaction of CVL in MeCN is not barrierless.The ICT reaction time of 9.2 ps is much longer than the mean solventrelaxation time of MeCN (0.26 ps), indicating, in contrast with earlierreports in the literature, that the reaction is not solvent controlled.This conclusion is supported by the observation of double exponentialLE and ICT fluorescence with the same decay times. [ABSTRACT FROM AUTHOR]

Calculations of molecular structures in the electronic ground state S0 and of excited state and fluorescence energies generally refer to the gas phase. This complicates a comparison with experimental data, which often are only available for molecules in solution. Therefore, experimental absorption and fluorescence spectra in the vapor phase are presented for 1 -tert-butyl-6-cyano-1 ,2,3,4-tetrahydroquinoline (NTC6), 1 -methyl-6-cyano-1 ,2,3,4-tetrahydroquinoline (NMC6), 4-(dimethylamino)benzonitrile (DMABN), and 4-(diisopropylamino)benzonitrile (DIABN). NTC6 and DIABN show a dual fluorescence in the gas phase, with emissions from an intramolecular charge transfer (ICT) and a locally excited (LE) state, whereas with NMC6 and DMABN only LE emission is observed. For a comparison of the experimental molecular structure in S0 with the results of recent computations, X-ray crystal structures of NTC6, NMC6, and several analogues are presented. For DMABN, NMC6, and NTC6, LE/ICT energy diagrams are constructed, in which the experimental energies of the Franck-Condon singlet excited states S1 and S2, and the LE and ICT states together with their emissions, are compared with the calculations. The LE and CT dipole moments are also discussed. This comparison reveals substantial differences, in particular for the ICT energies, but even for the structure of the S0 ground states. It is concluded that the computed ICT states of NTC6 and DMABN, in which the full conjugation of the phenyl ring is interrupted, is different from the CT states measured in the experiments. [ABSTRACT FROM AUTHOR]

Fast and efficient intramolecular charge transfer (ICT) and dual fluorescence is observed with the planarized aminobenzonitrile 1-tert-butyl-6-cyano-1,2,3,4-tetrahydroquinoline (NTC6) in a series of solvents from n-hexane to acetonitrile and methanol. Such a reaction does not take place for the related molecules with 1-isopropyl (NIC6) and 1-methyl (NMC6) groups, nor with the 1-alkyl-5-cyanoindolines with methyl (NMC5), isopropyl (NIC5), or tert-butyl (NTC5) substituents. For these molecules, a single fluorescence band from a locally excited (LE) state is found. The charge transfer reaction of NTC6 is favored by its relatively small energy gap ΔE(S1,S2), in accordance with the PICT model for ICT in aminobenzonitriles. For the ICT state of NTC6, a dipole moment of around 19 D is obtained from solvatochromic measurements, similar to μe(ICT) = 17 D of 4-(dimethylamino)benzonitrile (DMABN). For NMC5, NIC5, NTC5, NMC6, and NIC6, a dipole moment of around 10 D is determined by solvatochromic analysis, the same as that of the LE state of DMABN. For NTC6 in diethyl ether at -70 °C, the forward ICT rate constant (1.3 × 1011s-1) is much smaller than that of the back reaction (5.9 × 109s-1), showing that the equilibrium is on the ICT side. The results presented here make clear that ICT can very well take place with a planarized molecule such as NTC6, when ΔE(S1,S2) is sufficiently small, indicating that a perpendicular twist of the amino group relative to the rest of the molecule is not necessary for reaching an ICT state with a large dipole moment. The six-membered alicyclic ring in NMC6, for example, prevents ICT by increasing ΔE(S1,S2) relative to that of DMABN. [ABSTRACT FROM AUTHOR]

In this study, a pH-sensitive fluorescent probe was created through the host-guest assembly of an amphiphilic hydroxyphenyl benzotriazole derivative (HBT-11) and cucurbit[7]uril (CB7). The fluorescence of HBT-11 is highly dependent on the molar ratio (r) of CB7 to HBT-11. If r < 1, the peak positions remain unchanged while the intensity increases with r. If r > 1, the emission peaks shift to longer wavelengths. The complexation also results in a shift in the p K a of HBT-11. The 1:1 mixture of CB7 and HBT-11 has a p K a close to neutral pH. The supramolecular probe allows for discrimination of malignant A549 and benign HEK 293T cells owing to a pronounced 8-fold light-up effect in malignant A549 cells attributed to the pH-responsive fluorescence. Given that the selective light-up mechanism is based on the pH-sensitive fluorescence of the supramolecular probe, this approach is expected to be applicable to other cancer cells. [Display omitted] • A pH-sensitive supramolecular fluorescence probe was constructed. • The fluorescent intensity of the probe was greatly enhanced as pH < p K a. • The probe was applied to discriminate malignant and benign cells. [ABSTRACT FROM AUTHOR]