Fluorescence Properties of Phenol-Modified Zinc Phthalocyanine that Tuned by Photoinduced Intra-Molecular Electron Transfer and pH Values

Tetra[α-(4-hydroxyphenoxy)] zinc phthalocyanine, ZnPc(α-OPhOH)₄, was synthesized and its photophysics was found to be sharply pH dependent. Dual fluorescence emission around 700 nm was observed when it is dissolved in basic solution. The fluorescence of the phthalocyanine can be sharply switched off at pH 9.1 due to the intramolecular photoinduced electron transfer (PET) in ZnPc(α-OPhONa)₄, formed by the deprotonation of ZnPc(α-OPhOH)₄. The photophysics of both ZnPc(α-OPhOH)₄ and ZnPc(α-OPhONa)₄ were studied in detail by UV-vis absorption, steady state and time-resolved fluorescence and transient absorption (TA) to reveal the fluorescence quenching mechanism. Intra-molecular PET in ZnPc(α-OPhONa)₄ from the donor, PhONa subunits, to the acceptor, ZnPc moiety, was characterized by the much smaller fluorescence quantum yield (0.003) and lifetime (<0.20 ns). PET was further evidenced by the occurrence of charge separation state (CSS) in TA spectra, i.e. the bands due to anion radical of ZnPc and phenol radical. The lifetime of the charge separation state is ca. 3 ns, the efficiency of PET is ca. 99% and the rate constant of PET is 2.3 × 10¹⁰ s⁻¹.     

Absorption and fluorescence spectroscopy of 3-hydroxy-3-phenyl-1-o-carboxyphenyltriazene and its copper (II), nickel (II) and zinc (II) complexes: a novel fluorescence sensor

Absorption and fluorescence spectroscopic properties of 3-hydroxy-3-phenyl-1-o-carboxyphenyltriazene (HT) are studied. The mechanism of photo-induced electron transfer (PET) followed by energy transfer process of the ligand and the Cu (II), Ni (II) and Zn (II) metal complexes have been investigated. The excited state photo induced intramolecular hydrogen transfer from N-OH to triazene 1-nitrogen atom is explained. The effect of pH, solvent and concentration on the absorption and fluorescence of the ligand is studied and it has been found that the absorption and fluorescence of HT is highly pH, solvent and concentration dependent. Participation of the N-OH proton of HT in the solvent assisted O to N-proton transfer has also been proposed. The fluorescence band shift and changes in intensity is modulated by protonation and complexation with metal ions. This fluorophore can thus be used as a pH dependent and M⁽ⁿ⁺¹⁾⁺/Mⁿ⁺ redox on/off switchable molecular sensor.     

Photoinduced intramolecular electron-transfer reactions in carbazole-fullerene and phenothiazine-fullerene linked compounds in benzene and benzonitrile as studied by fluorescence, transient absorption, time-resolved EPR, and magnetic field effects

Photoinduced intramolecular electron-transfer reactions in carbazole (Cz)-fullerene (C₆₀) (Cz(8)C₆₀) and phenothiazine (Ph)-C₆₀ (Ph(n)C₆₀ (n=8, 10, 12)) linked compounds have been investigated in benzene and benzonitrile by fluorescence, transient absorption, and time-resolved electron paramagnetic resonance measurements, and by magnetic field effects on the decay rate constants of the photogenerated biradicals. In benzonitrile, photoinduced intramolecular electron transfer from Cz to the singlet excited state of C₆₀ (¹C₆₀ ^*) occurred in Cz(8)C₆₀, but not to the triplet excited state (³C₆₀ ^*), while the intramolecular electron-transfer to both¹C₆₀ ^* and³C₆₀ ^* occurred in Ph(n)C₆₀ (n=8, 10, 12). In benzene, on the other hand, no electron transfer to both¹C₆₀ ^* and³C₆₀ ^* took place in all linked compounds. These results were interpreted in terms of the different Gibbs free energy changes in the two solvents.     

Photophysical studies of PET based acridinedione dyes with globular protein: Bovine serum albumin

Interaction of acridinedione dyes with model transport proteins, bovine serum albumin (BSA) in aqueous solution were investigated by fluorescence spectral studies. A fluorescence enhancement was observed on the addition of BSA to photoinduced electron transfer (PET) based acridinedione dyes, which posses C₆H₄(p-OCH₃) in the 9th position of the basic acridinedione ring. On the contrary, the addition of BSA to non-PET based acridinedione dyes with methyl or phenyl substitution in the 9th position does not result in any fluorescence enhancement. The enhancement in the fluorescence intensity is attributed to the suppression of PET process through space between -OCH₃ group and the acridinedione moiety is elucidated by steady state fluorescence measurements. The fluorescence anisotropy value (r) of 0.40 reveals that the motion of the dye molecule is highly constrained and is largely confined to the rigid microenvironment of the protein molecule. The binding constant (K) was found to be in the order of 6.0×10³ [M]⁻¹, which implies the existence of hydrophobic interaction between the PET based dye and BSA. Time resolved fluorescence lifetime measurements reveal that the PET based acridinedione dye preferably binds in the hydrophobic interior of BSA.     

Density functional theory study on a fluorescent chemosensor device of aza‐crown ether

Three derivatives of alkyl anthracene covalently bonded to aza‐18‐crown‐6 at the nitrogen position, anthracene(CH₂)_n, (n = 1–3) which act as an on–off fluorogenic photoswitch have been theoretically studied using a computational strategy based on density functional theory at B3LYP/6‐31 + G(d,p) method. The fully optimized geometries have been performed with real frequencies which indicate the minima states. The binding energies, enthalpies and Gibbs free energies have been calculated for aza‐18‐crown‐6 ( L ) and their metal complexes. The natural bond orbital analysis is used to explore the interaction of host–guest molecules. The absorption spectra differences between L and their metal ligands, the excitation energies and absorption wavelength for their excited states have been studied by time‐dependent density functional theory with the basis set 6‐31 + G(d,p). These fluorescent sensors and switchers based on photoinduced electron transfer mechanism have been investigated. The PET process from aza‐crown ether to fluorophore can be suppressed or completely blocked by the entry of alkali metal cations into the aza‐crown ether‐based receptor. Such a suppression of the PET process means that fluorescence intensity is enhanced. The binding selectivity studies of the aza‐crown ether part of L indicate that the presence of the alkali metal cations Li⁺, Na⁺ and K⁺ play an important role in determining the internal charge transfer and the fluorescence properties of the complexes. In addition, the solvent effect has been investigated. Copyright © 2014 John Wiley & Sons, Ltd.     

Dual excimer and CO2 laser etching studies of polyethylene terephthalate

Polyethylene terephthalate (PET) films preheated with a pulsed CO₂ laser have been ablatively etched with an XeCl laser. The observed reduction in ablation threshold, from 170 to 140 mJ cm^–2, is consistent with a thermal mechanism for XeCl laser ablation of PET. Transient changes in the UV absorption coefficient of PET caused by heating with pulsed CO₂ laser radiation have also been studied and a significant increase in absorption observed at 308 nm. Permanent changes in the ultraviolet absorption of PET following exposure to low fluence XeCl laser radiation are also reported.     

Exclusive attributes of undoped poly (ethylene terephthalate) for alpha particle detection

Poly (ethylene terephthalate) (PET) is available worldwide and has a broad range of applications. However, its basic properties as a scintillation material that is undoped with fluorescent guest molecules are not completely known. Here, we optically characterise undoped PET for use in radiation detection. Light absorption is primarily below 350 nm, with an emission maximum at 385 nm. An effective refractive index, determined from the emission spectrum and the wavelength dependence of the refractive index, is 1.62, which is greater than that for the sodium D line (N_D = 1.57). The density of PET is 1.33 g/cm³, and its stopping power for 1-MeV electrons is 1.72 MeV cm²/g. Distinct peaks generated by alpha particles from ²¹⁰Pb and ²⁴¹Am radioactive sources appear in PET light-yield distributions. The PET response to 5-6-MeV alpha particles is approximately one-eighth that for electrons. These results demonstrate that undoped PET has special attributes for alpha particle detection. This knowledge will enable better performance of radiation equipment based on PET and its blends with other aromatic ring polymers.     

Origin of athermal refractive index changes observed in Yb doped YAG and KGW

Time-resolved excited-state absorption and fluorescence measurements were performed in the Yb³⁺ doped YAG and KGW laser crystals. The results indicate that the athermal refractive index changes observed in previous transient interferometric and diffraction grating experiments can be related to polarisability changes caused by the existence of strong UV charge transfer absorption bands.     

Ratiometric Recognition of Protons by a Multiple Tagged Designer Fluorescent Chemosensor

Molecular architecture with different fluorophoric units can offer improved and effective recognition of biologically important analytes. We present here a new strategy for the design of ratiometric chemosensors that operate by photoinduced electron transfer (PET). This ratiometric sensor endowed with tryptophan and anthracene exhibits high sensitivity, excellent selectivity and remarkable reversibility towards recognition of H⁺ in methanol. This “Turn-On” type behaviour is crafted into the molecule by incorporation of bispidine entity. Effective quenching of the fluorescence of the anthracene by the adjacent amine groups of the bispidine results in negligible fluorescence from the anthracene group leading to highly sensitive recognition of protons by the compound as H⁺ protonate the amine functionalities giving rise to the emergence of the fluorescence from the anthracene group. This, combined with the reduction in the fluorescence from the Trp group by H⁺, results in highly sensitive ratiometric nature of the response especially at low [H⁺].

     

Effect of nonlinear absorption in estimation of order of nonlinear optical process by fluorescence intensity

We report the effect of nonlinear absorption on the fluorescence signal. Attenuation of the incident beam intensity (I) in the medium due to nonlinear absorption reduces the fluorescence signal. We show that fluorescence signal proportional to I² is not always indicative of two-photon absorption process but such a dependence can be obtained for three or four-photon absorption process due to attenuation of the pump beam by nonlinear absorption. This reduction in the fluorescence signal due to nonlinear absorption is found to be more sensitive in the higher order nonlinear optical processes.     

Dye Laser for Absorption Trace Analysis of Sodium

The advent of organic dye lasers made possible many experiments in physics, chemistry and biology. Most of these applications are based on the possibility to tune dye laser emission almost at any wavelength between 3400 Å and 12000 Å.This quality combined with high light power made feasible multiphotone absorption, selective absorption and fluorescence experiments. Recently a group of authors published the papers: “Enhancement of absorption spectra by dye laser quenching I and II”^1,2 which start a new field of applications of dye lasers. Namely if a weak absorber is placed inside the broad band laser cavity the absorption is enhanced and this results in the same absorption bands or lines observable in the spectrally resolved laser output. This effect can be used for detection of the traces of elements as a complementary method to the classical absorption spectroscopy but with much higher sensitivity. Same method could be also very useful for detection of transient species with a very short lifetimes.     

Complexation of bis‐crown stilbene with alkali and alkaline‐earth metal cations. Ultrafast excited state dynamics of the stilbene‐viologen analogue charge transfer complex

The complex formation of bis(18‐crown‐6)stilbene ( 1 ) and its supramolecular donor‐acceptor complex with N,N′‐bis(ammonioethyl) 1,2‐di(4‐pyridyl)ethylene derivative ( 2 ) with alkali and alkaline‐earth metal perchlorates has been studied using absorption, steady‐state fluorescence, and femtosecond transient absorption spectroscopy. The formation of 1 ?Mⁿ⁺ and 1 ?(Mⁿ⁺)₂ complexes in acetonitrile was demonstrated. The weak long‐wavelength charge‐transfer absorption band of 1 · 2 completely vanishes upon complexation with metal cations because of disruption of the pseudocyclic structure. The spectroscopic and luminescence parameters, stability constants, and 2‐stage dissociation constants were calculated. The initial stage of a recoordination process was found in the excited complexes 1 ?M⁺ and 1 ?(M⁺)₂ (M = Li, Na). The pronounced fluorescence quenching of 1 · 2 is explained by very fast back electron transfer (τ_et = 0.397 ps). The structure of complex 1 · 2 was studied by X‐ray diffraction; stacked ( 1 · 2 )_m polymer in which the components were connected by hydrogen bonding and stacking was found in the crystal. These compounds can be considered as novel optical molecular sensors for alkali and alkaline‐earth metal cations.     

Comparative charge transfer studies in nonmetallated and metallated porphyrin fullerene dyads

Single material organic solar cells become an interesting area of research to overcome the challenges with efficient charge separation efficiencies in conventional organic solar cells. In this article, we have synthesized nonmetallated and metallated porphyrin‐fullerene dyad materials (H2P‐C60 and ZnP‐C60, respectively) with simple structure, comprehensively studied their charge transfer mechanism, and established a proof of concept that nonmetallated porphyrin‐fullerene dyads are better candidates to be used in organic solar cells compared with metallated dyads. Absorption and electrochemical analysis revealed the ground state electronic interactions between donor‐acceptor moieties in both types of dyads. Driving force (?ΔG^o_ET) for intramolecular electron transfer process was calculated by first oxidation and reduction potentials of dyads. The excited state electronic interactions were characterized by time‐resolved fluorescence and pump‐probe transient absorption experiments. Strong fluorescence quenching of porphyrin along with reduced lifetimes in dyads due to deactivation of singlet excited states by photoinduced charge transfer process between porphyrin/Zn‐porphyrin core and fullerene in different polarity solvents was observed. Transient absorption spectroscopy was also applied to identify the transient spectral features, ie, cationic (H2P⁺/ZnP⁺) and anionic (C₆₀^?) radicals formed because of the charge separation in both types of dyads. Finally, organic solar cell device was also fabricated using the dyads. We obtained higher V_oc, J_sc, and fill factor in single material organic solar cell using H2P‐C60 compared to previous reports.     

Fluorescence Studies of Melanin by Stepwise Two-Photon Femtosecond Laser Excitation

Fluorescence of synthetic melanin in the solvents H₂O, KOH, ethylene glycol monomethyl ether, and dimethyl sulfoxide has been excited by two-photon absorption at 800 nm, using 120-fs pulses with photon flux densities of 10²⁷ cm^–2.S^–1. Compared to the one-photon (400-nm)-induced fluorescence of melanin, the overall spectral shape is red-shifted and shows a strong environment sensitivity. The decay of the two-photon-induced fluorescence (TPF) of melanin is three-exponential, with a shortest main component of about 200 ps. The results of the TPF studies in line with the unique light absorption property of melanin of a monotonously decreasing absorption spectrum between the near UV-region and the near infrared region indicate that the TPF is realized via stepwise absorption of two 800-nm photons. In comparison to the simultaneous absorption of two photons, the stepwise process needs lower photon flux densities to get a sufficient population of the fluorescent level. This stepwise process offers new possibilities of selective excitation of melanin in skin tissue in a spectral region where there is no overlap with any absorption of another fluorescent tissue component. The first results with different samples of excised human skin tissue (healthy, nevus cell nevi, malignant melanoma) suggest that fluorescence excited in this way yields information on malignant transformation.     

Bis(Naphthalimide-Piperazine)-Based Off-On Fluorescent Probe for Acids

A bis(naphthalimide-piperazine) derivative (1) was synthesized as a pH-sensitive Off-On fluorescent probe. Operation mechanism of 1 is based on photo-induced electron transfer (PET) and its pH-dependent optical changes were investigated by using absorption and fluorescence spectroscopy. In the pH range of 11–4.5, this probe undergoes PET process from the piperazine to the naphthalimide moiety, leading to a fluorescence quenching. However, in the pH range of 4.5–1, the PET is inhibited to give a fluorescence enhancement. Moreover, the fluorescence ‘turn-on’ response of 1 is highly selective for protons (H⁺) over other metal cations, biomolecules and it shows a good reversibility between acidic and basic conditions.

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Graphical Abstract ?

     

Fluorescence quenching of a rhodamine derivative: Selectively sensing Cu2+ in acidic aqueous media

A new rhodamine derivative (RhB-Im) was synthesized as an “on-off” chemosensor for Cu²⁺ in an acidic aqueous solution. RhB-Im exhibited chemically reversible and highly selective and sensitive fluorescence response toward Cu²⁺ in aqueous acetate-buffer/DMF solution (pH 3.6) over other competitive metal ions. Upon addition of Cu²⁺, RhB-Im displayed remarkable fluorescence quenching accompanied by a clear color change from pink to red. Based on the analysis of Stern–Volmer plots, a static quenching mode was proposed to be primarily responsible for the fluorescence quenching event when the concentration of Cu²⁺ was low, but, the energy- and electron-transfer processes cannot be ruled out.     

Photophysical properties of tris-methoxycoumarin derivative of a cryptand

The paper reports the photophysical studies of a macrobicyclic cryptand where the three secondary amino nitrogen have been derivatized with methoxy coumarin in various solvents at room temperature and at 77 K. The system having (F-S)₃-R configuration (where F is fluorophore, S is CH₂ spacer and R is receptor unit) shows intramolecular PET from the receptor nitrogen atoms of the cryptand moiety to the coumarin units reducing the fluorescence quantum yield and lifetime of coumarin in the ligand. However, the fluorescence intensity is enhanced in presence of protons and Zn⁺² ion in dry THF medium. Metal ions like Cu(II) and Co(II) quench the fluorescence. Presence of protons induces a greater enhancement of emission intensity implying all the nitrogen lone pairs responsible for PET are protonated. Another interesting feature of this ligand is that it exhibits room temperature phosphorescence (RTP) apart from exhibiting PET. The observation of RTP has not been reported earlier for any other coumarin derivative.     

Size effect of nanostructured formations on scattering of optical phonons in poly(ethylene terephthalate)

The temperature dependence of the half-width of the infrared (IR) absorption band at a frequency of 971.5 cm^?1 in the spectra of poly(ethylene terephthalate) (PET) is investigated for crystallized and amorphous PET samples in which the lengths of trans sequences are approximately equal to 4–7 and 2–3 nm, respectively. The observed increase in the half-width with increasing temperature is explained by inelastic scattering of phonons of stretching vibrations of the macromolecular skeleton by other phonons. The half-width of the band at 971.5 cm^?1 in the IR spectra of the amorphous polymer is approximately 1.5 times larger than that in the spectra of the crystallized polymer. This is associated with the violation of the wave-vector selection rules due to a small length of the trans sequences in the amorphous sample.     

Charge Transfer Photophysics of Tetra(α-amino) Zinc Phthalocyanine

The absorption, fluorescence, and transient absorption spectra of Tetra(α-amino) zinc phthalocyanine, ZnPc(α-NH₂)₄, have been measured in polar solvents and compared with that of ZnPc(α-R)₄ (R?=?H, NO₂, OCH(CH₃)₂). While the latter three showed the typical photophysics of phthalocyanines, ZnPc(α-NH₂)₄ exhibits distinct spectral properties, a very low fluorescence quantum yield and a relatively long fluorescence lifetime. These observations are explained by the substantial charge transfer characters in the absorption and fluorescence spectra of ZnPc(α-NH₂)₄. NMR indicates that intramolecular H-bonding makes atoms in NH₂ actually coplanar with other elements of ZnPc(α-NH₂)₄. The local excited state is non emissive and the weak emission is assigned to its charge transfer state. The transient absorption bands from laser flash photolysis located at 630 nm, 645 nm is assigned to the mono-charge transfer state, while that at 545 nm is assigned to the di-charge transfer state.     

Femtosecond time‐resolved spectroscopy on biological photoreceptor chromophores

This paper reviews our results on femtosecond time‐resolved spectroscopy (transient absorption, transient‐grating and fluorescence spectroscopy) to study the photophysics and photochemistry of the two very important biological photoreceptor chromophores phycocyanobilin (PCB) and protochlorophyllide a (PChla). The compound PCB serves as a model chromophore for the photoreceptor phytochrome. By means of transient‐grating spectroscopy where the excitation wavelength was varied ove r the spectral region of the S₀→ S₁‐absorption the ultrafast processes were studied upon excitation with varying excess energy delivered to the system. On the basis of the results obtained, both the rate of the photoreaction in PCB and the rate of the decay of different excited‐state species via different decay channels depend on the excitation wavelength. Furthermore, transient absorption experiments illuminating the excited‐state dynamics of PChla, a porphyrin‐like compound and, as substrate of the NADPH/protochlorophyllide oxidoreductase (POR), a precursor of the chlorophyll biosynthesis are presented. In addition to pump‐energy‐dependent measurements performed with PChla dissolved in methanol, the excited‐state dynamics of PChla was interrogated in different solvents that were chosen to mimic different environmental conditions. In addition to the femtosecond time‐resolved absorption experiments the picosecond time‐resolved fluorescence of the system was studied. The transient absorption and tim e‐resolved fluorescence data allow suggesting a detailed model for the excited‐state relaxation of PChla describing the excited‐state processes in terms of a branching of the initially excited state population into a reactive and nonreactive path. Thus, the excited‐state potential energy surface exhibits two distinct S₁ and S_x minima separated from the Franck–Condon region along two most likely orthogonal reaction coordinates. Finally, the model derived is related to models suggested to acco unt for the reduction of PChla to chlorophyllide a within the natural enzymatic environment of POR.