Fast Nonradiative Deactivation Research Articles

Ultralong-lived room temperature triplet excitons: molecular persistent room temperature phosphorescence and nonlinear optical characteristics with continuous irradiation

Heavy atom-free molecular materials with ultralong-lived room-temperature triplet excitons display nonlinear absorption using ordinary light and persistent room-temperature emission characteristics.

Photoinduced Electron Transfer in Asymmetrical Perylene Diimide: Understanding the Photophysical Processes of Light-Absorbing Nonfullerene Acceptors

In bulk-heterojunction-based organic photovoltaics, strong light-absorbing ability is one of the most important advantages of nonfullerene acceptors prior to fullerene derivatives. Herein, a series of electron-donating units was asymmetrically connected at the imide positions of perylene diimide (PDI), which is a classic-electron acceptor building block. The photoinduced electron-transfer behaviors were compared in different substitutions by steady/time-resolved spectroscopy and theoretical simulation. The electron-donating ability as well as the donor–acceptor distance has a significant impact on the photoinduced electron transition from electron-donating units to the PDI core. Because of the process of charge transfer at excited states, a fast nonradiative deactivation is observed for the PDIs with relatively strong electron substituent. These results would help us to understand the electron-transfer processes from donor to acceptor in the case of the excited electron-acceptor molecule.

Ultrafast Deactivation of Excitation Energy in Rutin and Quercetin via Electron and Proton Transfers

Using femtosecond spectroscopy and steady-state luminescence methods, the mechanisms of very fast non-radiative deactivation (knr ~ 5 ? 1011 ?-1) of the electronic excitation energy at room temperature in organic and buffer solutions of well-known natural antioxidants rutin and quercetin have been studied.

Modulation of the fluorescence properties of diketopyrrolopyrroles via various electron-rich substituents

Electron-donating functionalities allow modulation of the emission properties of diketopyrrolopyrroles.

Roles of octabutoxy substitution and J-aggregation in stabilization of the excited state in nickel phthalocyanine.

Nickel phthalocyanine (NiPc) complexes are known to show a rapid nonradiative deactivation of the photoexcited state through the internal conversion. This could be exploited in practical applications, such as photoprotection and photodynamic therapy. The butoxy substitution of NiPc plays an important role for drug delivery but also greatly influences its photophysics. We prepared novel peripherally substituted 2,3,9,10,16,17,23,24-octabutoxy nickel(II) phthalocyanine and characterized the deactivation pathway of its photoexcited state in solution by femtosecond transient absorption spectroscopy and quantum chemical calculations. We bring experimental evidence for the kinetic model, in which the photoexcitation evolves in two independent branches. In the first branch, assigned to the monomer, it undergoes ultrafast intersystem crossing to a triplet state, which subsequently decays to the ground state through a pathway involving lower-lying triplet states, with a ground-state recovery lifetime of 814 ps. It is about three-times longer than the lifetime published for unsubstituted NiPc. In the second branch, the photoexcitation decayed to a triplet state with an orders of magnitude longer lifetime, with the quantum yield of about 4%. This state showed spectral features of J-aggregates. These findings are important for the applications that rely on singlet oxygen formation or fast nonradiative deactivation of the excited state.

Radiationless deactivation of 6-aminocoumarin from the S1-ICT state in nonspecifically interacting solvents

This paper presents the results of a spectral (absorption and emission) and photophysical study of 6-aminocoumarin (6AC) in the solvents with which this molecule interacts only nonspecifically (n-alkanes, tetrachloromethane and 1-chloro-n-alkanes) and in nitriles. The strong effects of the solvents on the emission spectra, fluorescence quantum yield and lifetime of 6AC were observed. The results of the steady-state and time-resolved photophysical study suggest the presence of very fast nonradiative deactivation processes. It is concluded that besides fluorescence, the efficient S(1)-ICT → S(0) internal conversion in nonpolar aprotic solvents arises from vibronic interactions between close-lying S(1)-ICT(π,π*) and S(2)(n,π*) states. Moreover, unexpectedly efficient triplet state formation occurs. In nitriles the intermolecular hydrogen-bonding interactions with solvent molecules also facilitate the nonradiative decay process involving the S(1)-exciplex.

Development of an innovative method of fluoro-electrode for simultaneous detection of electrical activity and fluorescence spectroscopy

We report investigations of resonance energy transfer in the green fluorescent protein and calmodulin-based fluorescent indicator constructs for Ca2+ called cameleons using steady-state and time-resolved spectroscopy of the full construct and of the component green fluorescent protein mutants, namely ECFP (donor) and EYFP (acceptor). EYFP displays a complicated photophysical behavior including protonated and deprotonated species involved in an excited-state proton transfer. When EYFP is excited in the absorption band of the protonated species, a fast nonradiative deactivation occurs involving almost 97% of the excited protonated population and leading to a low efficiency of excited-state proton transfer to the deprotonated species. ECFP displays a multiexponential fluorescence decay with a major contributing component of 3.2 ns. The time-resolved fluorescence data obtained upon excitation at 420 nm of Ca2+-free and Ca2+-bound YC3.1 cameleon constructs point to the existence of different conformations of calmodulin dependent on Ca2+ binding. Whereas steady-state data show only an increase in the efficiency of energy transfer upon Ca2+ binding, the time-resolved data demonstrate the existence of three distinct conformations/populations within the investigated sample. Although the mechanism of the interconversion between the different conformations and the extent of interconversion are still unclear, the time-resolved fluorescence data offer an estimation of the rate constants, of the efficiency of the energy transfer, and of the donor-acceptor distances in the Ca2+-free and Ca2+-bound YC3.1 samples.

Photophysical study of bay substituted perylenediimides

A detailed investigation of the photophysical properties of a series of perylenediimide systems bearing three different types of bay substituents is presented. Single photon timing and femtosecond transient absorption experiments reveal that the dynamics of interconversion between two conformational arrangements of these substituents occurs with a time constant of 550 ps. In addition, charge transfer from the electron-rich units attached to the bay area of the electron-poor perylenediimide core is observed. This process leads to a fast non-radiative deactivation of the locally excited state of the perylenediimide in polar solvents. When the experimental results of the investigated systems are compared we observe a shift from a conformational dynamics towards competitive excited state processes involving charge transfer in the -meta and -para substituted perylenediimide chromophore.

Ab Initio Studies on the Photophysics of Guanine Tautomers: Out-of-Plane Deformation and NH Dissociation Pathways to Conical Intersections

The radiationless decay mechanisms of the S1 excited states of the 7H-keto-amino, 7H-enol-amino, and 7H-keto-imino tautomers of guanine have been investigated with the CASPT2//CASSCF method. Out-of-plane deformation of the six-membered ring or the imino group as well as dissociation of NH bonds have been considered as photochemical pathways leading to conical intersections with the electronic ground state. It has been found that all three tautomers can reach S0-S1 conical intersections by out-of-plane deformation. However, only in the 7H-keto-amino tautomer the reaction path leading to the conical intersection is barrierless. This tautomer also has the lowest energy barrier for hydrogen detachment via the (1)pi sigma* state, whose potential energy surface intersects that of the (1)pi pi* state as well as that of the ground state. The other tautomers of guanine exhibit substantial energy barriers on their S1 potential energy surfaces with respect to both reaction mechanisms. These findings suggest that the 7H-keto-amino tautomer exhibits the shortest excited-state lifetime of the three tautomers due to particularly fast nonradiative deactivation processes through S0-S1 conical intersections. The computational results explain the remarkable observation that the energetically most stable 7H-keto-amino tautomer is missing in the resonant two-photon ionization spectrum of guanine in a supersonic jet. The results also explain that the energetically less stable 7H-enol-amino and 7H-keto-imino tautomers have longer excited-state lifetimes and are thus detectable by resonant two-photon ionization.

Intensity of singlet-triplet transitions in C60 fullerene calculated on the basis of the time-dependent density functional theory and taking into account the quadratic response

The singlet-singlet and triplet-triplet absorption spectra of C60 fullerene are calculated using the density functional method and taking into account the theory of linear and quadratic responses. The B3LYP density functional and the 6–31G and 3–21G atomic basis sets are used. The calculations are performed using the D2h and D5d symmetry groups, although the real symmetry of the ground state is described by the Ih symmetry group. The matrix elements of the operator of the spin-orbit coupling are calculated and the probabilities of some singlet-triplet transitions are estimated. Taking into account the data in the literature on vibronic interactions of vibrations of the t1u, t2u, gu, and hu symmetry species, the radiative lifetime of the 13T2g → 11Ag phosphorescence was estimated to be 45 s. The fact that this time proved to be considerably greater than the experimentally observed total lifetime of the triplet testifies to a fast nonradiative deactivation of the lowest triplet state of C60 fullerene and agrees with a low phosphorescence intensity. The zero-field splitting of some triplets and the intensities of magnetic dipole transitions are discussed.

Interaction of Pheophorbide a molecules covalently linked to DAB dendrimers

The photophysical properties of DAB–dendrimers from first to fourth generation as well as Diaminohexane – all substituted with the in maximum achievable quantity of Pheophorbide a (Pheo) molecules are compared with those of Pheo in ethanolic solution. The high local concentration of the linked dye molecules on the dendrimer surface yields in interactions between the chromophores within each complex. The investigation of these interactions is the aim of this paper. It was found, that a larger number of chromophores in one dendrimer–dye-complex increases the stacking probability of Pheo moieties accompanied by excitonic interaction leading to a fast non-radiative deactivation after excitation. The efficiency of deactivation depends on the relative position of the chromophores one to each other. Since the Förster radius for energy transfer between Pheo molecules is bigger than the diameter of the whole complex, energy is jumping very rapidly between the chromophores within the complex. Just one pair of excitonic interacting chromophores can act as energy trap for the whole complex.

Resonance Energy Transfer in a Calcium Concentration-Dependent Cameleon Protein

We report investigations of resonance energy transfer in the green fluorescent protein and calmodulin-based fluorescent indicator constructs for Ca 2+ called cameleons using steady-state and time-resolved spectroscopy of the full construct and of the component green fluorescent protein mutants, namely ECFP (donor) and EYFP (acceptor). EYFP displays a complicated photophysical behavior including protonated and deprotonated species involved in an excited-state proton transfer. When EYFP is excited in the absorption band of the protonated species, a fast nonradiative deactivation occurs involving almost 97% of the excited protonated population and leading to a low efficiency of excited-state proton transfer to the deprotonated species. ECFP displays a multiexponential fluorescence decay with a major contributing component of 3.2 ns. The time-resolved fluorescence data obtained upon excitation at 420 nm of Ca 2+-free and Ca 2+-bound YC3.1 cameleon constructs point to the existence of different conformations of calmodulin dependent on Ca 2+ binding. Whereas steady-state data show only an increase in the efficiency of energy transfer upon Ca 2+ binding, the time-resolved data demonstrate the existence of three distinct conformations/populations within the investigated sample. Although the mechanism of the interconversion between the different conformations and the extent of interconversion are still unclear, the time-resolved fluorescence data offer an estimation of the rate constants, of the efficiency of the energy transfer, and of the donor-acceptor distances in the Ca 2+-free and Ca 2+-bound YC3.1 samples.

Internal conversion of o-aminoacetophenone in solution

The photophysical properties of o-aminoacetophenone (o-AAP) in solution have been studied by using a femtosecond laser–single photon counting system and time-resolved thermal lensing (TRTL) method. The fluorescence quantum yield (Φf) and lifetime (τf) of o-AAP depend strongly on the nature of the solvent. In nonpolar solvents, o-AAP gives very small Φf values (Φf = 2.4 × 10−4 in n-hexane) and remarkably short fluorescence lifetimes (τf = 9.4 ps in n-hexane), suggesting the presence of very fast nonradiative deactivation processes. The measurements of the quantum yield (Φisc) of intersystem crossing based on the energy transfer and TRTL methods clearly show that the fast radiationless processes in nonpolar solvents are due to internal conversion. In aprotic solvents, the rate (kic) of internal conversion for o-AAP decreases significantly with increasing solvent polarity (kic = 1.0 × 1011 s−1 in n-hexane, kic = 2.4 × 109 s−1 in acetonitrile). In protic solvents, the kic value tends to increase with an increase of hydrogen-bonding donor ability of the solvent. The internal conversion rate in aprotic solvents is scarcely affected by deuterium substitution of the NH2 group in o-AAP, while a large isotope effect is found for o-AAP in deuteriated protic solvents. It is concluded that the efficient S1 → S0 internal conversion in nonpolar aprotic solvents arises from vibronic interactions between close-lying 1(π,π*) and 1(n,π*) states (the proximity effect), and in protic solvents intermolecular hydrogen-bonding interactions with solvent molecules also facilitate the nonradiative process.

Unusual Photophysical Properties of Coumarin-151

The photophysical properties of 7-amino-4-trifluoromethyl-1,2-benzopyrone (coumarin-151; C151) in nonpolar (NP) solvents are drastically different than those in other solvents of moderate to higher polarities. Thus, the absorption and fluorescence maxima are largely blue-shifted and the Stokes' shifts unusually low in NP solvents compared to those in the other solvents. The fluorescence quantum yields (Φf) in NP solvents are also exceptionally lower. While in all other solvents the fluorescence decays of C151 are single-exponential, in NP solvents the decays show non-single-exponential behavior. The average fluorescence lifetimes (τf) in NP solvents are also substantially shorter. Low Φf and τf values in NP solvents indicate very fast nonradiative deactivation for the S1 state of C151. Investigating the triplet characteristics of C151 by using picosecond laser flash photolysis and pulse radiolysis techniques, it has been established that the fast nonradiative deactivation in NP solvents is not due to an e...

Trans-4-Dimethylamino-4′-(1-oxobutyl)stilbene: a new fluorescent probe of the bilayer lipid membrane

Trans-4-dimethylamino-4′-(1-oxobutyl)stilbene is a water-insoluble fluorescent probe which was used to measure the dielectric constant, micro-viscosity and phase behaviour of various lipid bilayer membranes. It emits in the blue to red spectral range depending on environmental polarity, and the quantum yield is sensitive to matrix rigidity. The quantum yield is governed by the freedom of rotation about the central ethylenic bond, and the very low value of quantum yield in non-viscous solvents is attributed to a fast non-radiative deactivation of an excited singlet state associated with trancis isomerization. The results for dipalmitoyl phosphatidyl choline (DPPC) indicate an average dieletric constant of 5.5, a phase transition of 39°C and an average microviscosity of 105 - 2.5 cP over a temperature range of 10 – 60°C. These values are consistent with numerous other experimental techniques, but are in apparent contradiction with a number of previous fluorescence experiments.

Influence of restricted intramolecular motions on the fluorescence quantum yield of fluorophores

Fluorescence spectra, fluorescence quantum yields and decay times were measured for fluorophores capable of either conformational or rotational mobility in dilute solutions of benzene, methyl methacrylate (MMA) and in the rigid poly(methyl methacrylate) (PMMA) matrix at room temperature. When passing from fluid solutions of MMA to the polymeric matrix of PMMA a dramatic increase in the fluorescence quantum yield from zero to values of about 0.60 is obtained for structurally related molecules of 1,3-diphenyl-2-pyrazoline such as 1-phenyl-3-mesityl-2-pyrazoline, 1,3-diphenyl-1,4,5,6-tetrahydro-1,2-diazine and benzaldehyde phenylhydrazone, all containing the same arylCNNaryl chromophore. The rigidity of the PMMA matrix slows down the rate of intramolecular motions such as ring inversion of rotational mobility within the fluorophore and lengthens the time interval between the electronic excitation and achievement of those conformations that give rise to fast non-radiative deactivation processes. In contrast with these compounds the fluorescence quantum yield of the nitrogen hetero aromatic compound 1,3-diphenylindazole decreases when going from MMA to PMMA at the expense of phosphorescence. The practical consequences of the effect of restricted intramolecular motions on the fluorescence quantum yield of fluorescent dyes in polymers are briefly discussed.