Singlet Excited State Deactivation Research Articles

Protective action of graphene oxide against singlet oxygen generation by cosmetic dye methylene blue

Nanohybrid methylene blue/graphene oxide (MB/GO) was prepared via non-covalent functionalization with the focus on spectroscopic characterization of MB molecules adsorbed on GO. Fluorescence was not detected for the MB/GO hybrid which indicates that other competitive fast singlet excited state deactivation process must take place. It was demonstrated by measuring singlet oxygen (1O2) phosphorescence at 1270 nm that MB functionalized with GO does not generate 1O2. Nanosecond transient absorption spectroscopy experiments pointed to a lack of population of triplet state by MB/GO hybrid material. Femtosecond pump–probe experiments revealed much faster dynamics of the singlet excited state of MB/GO in comparison to free MB, which was attributed to the photoinduced electron transfer from MB to GO. Suppressing of the singlet oxygen quantum efficiency by MB upon functionalization to GO might be useful for designing graphene-based dye additives to color cosmetics with a reduced risk of the 1O2 generation on the skin.

Photoinduced charge transfer in push-pull pyrazoline-based chromophores – Relationship between molecular structure and photophysical, photovoltaic properties

The manuscript describes the effect of molecular structure on the photophysical and photovoltaic properties of the pyrazoline-based donor-branched-π-system-acceptor compounds decorated with two end groups: phenyl or thiophene. Although the absorption to the first singlet excited state is strongly allowed, the emission quantum yield is low in all studied solvents. This behaviour was explained by the existence of two non-radiative deactivation channels: the back electron transfer process, especially operated in polar solvents, and internal conversion realized as the rotation of flexible rotors (cyano, keto phenyl or thiophene). The feasibility of the photoinduced electron transfer process was corroborated by electrochemical, spectroelectrochemical measurements as well as DFT calculations. DFT calculations also support the existence of multiple conformations in the ground state, which differ from one another in terms of charge distribution and the values of ground state dipole moment. Finally, the mechanism of the singlet excited state deactivation of the studied compounds was determined by ultrafast pump-probe measurements. Our studies revealed that charge/electron transfer process may undergo over carbonyl bridge, included in branched π-system. Moreover, the thiophene decorated pyrazoline is characterized by a better photovoltaic power conversion efficiency, while the phenyl-ended pyrazoline can be applied as a viscosity sensor.

Triphenylamine/Tetracyanobutadiene-Based π-Conjugated Push-Pull Molecules End-Capped with Arene Platforms: Synthesis, Photophysics, and Photovoltaic Response.

π-Conjugated push-pull molecules based on triphenylamine and 1,1,4,4-tetracyanobuta-1,3-diene (TCBD) have been functionalized with different terminal arene units. In solution, these highly TCBD-twisted systems showed a strong internal charge transfer band in the visible spectrum and no detectable photoluminescence (PL). Photophysical and theoretical investigations revealed very short singlet excited state deactivation time of ≈10 ps resulting from significant conformational changes of the TCBD-arene moiety upon photoexcitation, opening a pathway for non-radiative decay. The PL was recovered in vacuum-processed films or when the molecules were dispersed in a PMMA matrix leading to a significant increase of the excited state deactivation time. As shown by cyclic voltammetry, these molecules can act as electron donors compared to C60 . Hence, vacuum-processed planar heterojunction organic solar cells were fabricated leading to a maximum power conversion efficiency of ca. 1.9 % which decreases with the increase of the arene size.

Comparative charge transfer studies in nonmetallated and metallated porphyrin fullerene dyads

AbstractSingle 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 (−ΔGoET) 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 Voc, Jsc, and fill factor in single material organic solar cell using H2P‐C60 compared to previous reports.

Selective quenching of triplet excited states of pteridines.

Steady-state and time-resolved studies on quenching of excited states of pterin (Ptr) and lumazine (Lum) in the presence of iodide in aqueous solution have been performed. In contrast to the typical iodide enhancement in the triplet state population, iodide promotes a fast non-radiative T1→ S0 transition for both Ptr and Lum. In this work, we present evidence for the effective iodide-induced deactivation of singlet and triplet excited states, with rate constants close to the diffusion-controlled limit (between 3 × 10(9) M(-1) s(-1) and 1 × 10(10) M(-1) s(-1)). The longer lifetimes of the triplet excited states over the singlet excited states increase the probability of deactivation (k(T)(q)τ(0)(T)≫k(S)(q)τ(0)(S)). Therefore, at micromolar concentrations of iodide, where the deactivation of the singlet excited state is negligible, an efficient deactivation of the triplet excited states is observed. This selective deactivation of the excited triplet state is an analytical tool for the study of photosensitized reactions where pteridines are involved.

Solvatochromic investigation of highly fluorescent 2-aminobithiophene derivatives

The solvatochromic and electrochemical properties of electronic push-pull 2-aminobithiophenes consisting of an aldehyde and nitro withdrawing groups were examined. With the use of an integrating sphere, the absolute quantum yields of the bithiophenes were measured. They were found to be highly fluorescent (Φfl > 70%), provided the nitro group was not located in the 4'-position. High fluorescence yields were observed regardless of solvent, except for alcohols, notably methanol and ethanol. Cryofluorescence was used to probe the bithiophene temperature dependent excited state deactivation modes. The singlet excited state deactivation mode other than fluorescence was found to be internal conversion involving rotation around the thiophene-thiophene bond. Deactivation by intersystem crossing to the triplet state occurred in ca. 40% only for the unsubstituted 2-aminobithiophene. In contrast, the fluorescence was quenched by photoinduced intramolecular electron transfer when the nitro group was located in the 4'-position of the bithiophene. Both the absorbance and fluorescence of the bithiophenes were found to be solvatochromic with more pronounced solvent dependent shifts being observed with the fluorescence. In fact, both the fluorescence and Stokes shifts were linearly dependent on the ET(30) solvent parameter. Deviations from the linear trend of the Stokes shift with ET(30) were observed in ethanol and methanol as a result of intermolecular hydrogen abstraction from the solvent and by the excited nitro group. The oxidation potential of the bithiophenes was also highly dependent on the type and number of the electron withdrawing substituents, with values ranging between 0.8 and 1.2 V vs. SCE.

Photophysical, Electrochemical and Crystallographic Investigations of the Fluorophore 2,5-Bis(5-tert-butyl-benzoxazol-2-yl)thiophene

The photophysics of 2,5-bis(5-tert-butyl-benzoxazol-2-yl)thiophene (BBT) were investigated for assessing its limitations for use as a universal fluorophore and as a viable sensor for both polymeric and solution studies. This is of importance given the limitations of currently used materials. BBT's steady-state and time-resolved fluorescence were additionally investigated to correlate its solid-state features, observed by fluorescence spectroscopy when mixed in poly(1,4-butylene succinate) (PBS) films, with its single crystal characteristics. The conjugated fluorophore was found to be highly fluorescent, with absolute quantum yields of (Φ(fl)) ≥ 0.60. The Φ(fl) values were high, regardless of solvent polarity and proticity and whether alone or in polymeric films. The major competitive fluorescence quenching pathway was found to occur by intersystem crossing to the triplet state. This was confirmed by laser flash photolysis in which the BBT triplet absorbed at 500 nm. The triplet transient was confirmed by quenching studies with 1,3-cyclohexadiene. Meanwhile, nonradiative deactivation of BBT's singlet excited state by internal conversion was found to be negligible. In solution and especially when distributed in semicrystalline PBS, BBT exhibits spectral changes and a bathochromic shift as a function of concentration due to aggregation of ground state molecules, which is present even at low BBT concentrations. Consistent monoexponential lifetimes on the order of ∼2 ns were observed regardless of solvent and independent of both the excitation wavelength and concentration. The constant excited state kinetics confirm the absence of a singlet excited state deactivation by excimer formation. The electrochemistry of BBT demonstrated that it is irreversibly oxidized and the resulting radical cation is unstable. Conversely, the cathodic process, resulting in the radical anion, is reversible, confirming its n-doping character. Crystallographic studies revealed that the planes described by the benzoxazolyl moieties are twisted from the plane described by the central thiophene. Several weak C-H···π and π-π intermolecular interactions were also observed. BBT's high solubility in common solvents combined with its measured enhanced optoelectronic properties make it a candidate as a universal fluorophore reference and smart material for both polymeric and solution studies.

Revisiting Fluorenone Photophysics via Dipolar Fluorenone Derivatives

The nonradiative decay of four dipolar fluorenone derivatives (FODs) was systematically investigated using steady state and time-resolved UV-vis absorption and fluorescence measurements combined with cyclic voltammetry. Analysis of the frontier orbital localization of the global minimum geometry and the vertical transitions was carried out from DFT calculations. The first singlet excited state was found to be π-π* in all derivatives regardless of the polarity of the solvent. Charge separation/recombination dominates the singlet excited state deactivation for carbazole-containing FODs. Intersystem crossing (ISC) operates exclusively in the 3,6-disubstituted variants as evidenced by phosphorescence experiments. In the case of CPAFO36, ISC competes disadvantageously with CT deactivation.

Insights into the effect of ketylimine, aldimine, and vinylene group attachment and regiosubstitution on the fluorescence deactivation of fluorene

The spectroscopic and electrochemical properties of a 9-substituted fluorene ketylimine (3) were investigated and compared with those of its vinylene analogue (4) to determine the origins of the quenched fluorescence of these compounds. The predominate mode of singlet excited state deactivation of the heteroatomic fluorene was found to be internal conversion involving bond rotation. Meanwhile, its carbon counterpart was found to undergo deactivation preferentially by intersystem crossing to form its triplet, which was confirmed by laser flash photolysis. Both 3 and 4 quenched the fluorescence of fluorene with diffusion-controlled rate constants, implying that the singlet excited states of 3 and 4 are also quenched by intramolecular photoinduced electron transfer (PET). This deactivation mode was found to be exergonically favorable (–90 kJ/mol for 3 and –81 kJ/mol for 4) according to the Rehm–Weller equation. The position of the heteroatomic bond on the fluorene moiety was further found to influence the singlet excited state deactivation pathway. The 2-substituted regioisomer decayed predominately by intramolecular PET and its fluorescence can be restored by acid protonation. Conversely, the PET mechanism is a minor deactivation mode for the 9-substituted fluorene derivative and its fluorescence can be enhanced by suppressing bond rotational modes, possible at low temperature and potentially in thin films.

Comparison of the Thermodynamic Landscapes of Unfolding and Formation of the Energy Dissipative State in the Isolated Light Harvesting Complex II

In biochemistry and cell biology, understanding the molecular mechanisms by which physiological processes are regulated is regarded as an ultimate goal. In higher plants, one of the most widely investigated regulatory processes occurs in the light harvesting complexes (LHCII) of the chloroplast thylakoid membranes. Under limiting photon flux densities, LHCII harvests sunlight with high efficiency. When the intensity of incident radiation reaches levels close to the saturation of the photosynthesis, the efficiency of light harvesting is decreased by a process referred to as nonphotochemical quenching (NPQ), which enhances the singlet-excited state deactivation via nonradiative dissipative processes. Conformational rearrangements in LHCII are known to be crucial in promoting and controlling NPQ in vitro and in vivo. In this article, we address the thermodynamic nature of the conformational rearrangements promoting and controlling NPQ in isolated LHCII. A combined, linear reaction scheme in which the folded, quenched state represents a stable intermediate on the unfolding pathway was employed to describe the temperature dependence of the spectroscopic signatures associated with the chlorophyll fluorescence quenching and the loss of secondary structure motifs in LHCII. The thermodynamic model requires considering the temperature dependence of Gibbs free energy difference between the quenched and the unquenched states, as well as the unfolded and quenched states, of LHCII. Even though the same reaction scheme is adequate to describe the quenching and the unfolding processes in LHCII monomers and trimers, their thermodynamic characteristics were found to be markedly different. The results of the thermodynamic analysis shed light on the physiological importance of the trimeric state of LHCII in stabilizing the efficient light harvesting mode as well as preventing the quenched conformation of the protein from unfolding. Moreover, the transition to the quenched conformation in trimers reveals a larger degree of cooperativity than in monomers, explained by a small characteristic entropy (ΔHq = 85 ± 3 kJ mol−1 compared to 125 ± 5 kJ mol−1 in monomers), which enables the fine-tuning of nonphotochemical quenching in vivo.

Energy and Electron Transfer in β-Alkynyl-Linked Porphyrin−[60]Fullerene Dyads

Three porphyrin-fullerene dyads, in which a diyne bridge links C(60) with a beta-position on a tetraarylporphyrin, have been synthesized. The free-base dyad was prepared, as well as the corresponding Zn(II) and Ni(II) materials. These represent the first examples of a new class of conjugatively linked electron donor-acceptor systems in which pi-conjugation extends from the porphyrin ring system directly to the fullerene surface. The processes that occur following photoexcitation of these dyads were examined using fluorescence and transient absorption techniques on the femtosecond, picosecond, and nanosecond time scales. In sharp contrast to the photodynamics associated with singlet excited-state decay of reference tetraphenylporphyrins (ZnTPP, NiTPP, and H(2)TPP), the diyne-linked dyads undergo ultrafast (<10 ps) singlet excited-state deactivation in toluene, tetrahydrofuran (THF), and benzonitrile (PhCN). Transient absorption techniques with the ZnP-C(60) dyad clearly show that in toluene intramolecular energy transfer (EnT) to ultimately generate C(60) triplet excited states is the dominant singlet decay mechanism, while intramolecular electron transfer (ET) dominates in THF and PhCN to give the ZnP(*+)/C(60)(*-) charge-separated radical ion pair (CSRP). Electrochemical studies indicate that there is no significant charge transfer in the ground states of these systems. The lifetime of ZnP(*+)/C(60)(*-) in PhCN was approximately 40 ps, determined by two different types of transient absorption measurement in two different laboratories. Thus, in this system, the ratio of the rates for charge separation (k(CS)) to rates for charge recombination (k(CR)), k(CS)/k(CR), is quite small, approximately 7. The fact that charge separation (CS) rates increase with increasing solvent polarity is consistent with this process occurring in the normal region of the Marcus curve, while the slower charge recombination (CR) rates in less polar solvents indicate that the CR process occurs in the Marcus inverted region. While photoinduced ET occurs on a similar time scale in a related dyad 15 in which a diethynyl bridge connects C(60) to the para position of a meso phenyl moiety of a tetrarylporphyrin, CR occurs much more slowly; i.e., k(CS)/k(CR) approximately equal to 7400. Thus, the position at which the conjugative linker is attached to the porphyrin moiety has a dramatic influence on k(CR) but not on k(CS). On the basis of electron density calculations, we tentatively conclude that unfavorable orbital symmetries inhibit charge recombination in 15 vis a vis the beta-linked dyads.

Orientation-Dependent Electron Transfer Processes in Fullerene−Aniline Dyads

Newly synthesized ortho- and para-substituted fullerene−aniline dyads exhibit orientation-dependent electron transfer under photoexcitation. Minimum energy conformation based on molecular mechanics calculations yield folded conformations for ortho-substituted dyads and extended ones for the para-substituted dyads. 1H NMR studies also provided evidence for the folding of the anilinic group in the case of ortho-substituted dyads. Through-space charge transfer processes in these dyads were investigated using steady state fluorescence and lifetime measurements. The decrease in the spatial distance between the donor and acceptor moieties of the ortho-substituted dyads facilitated an efficient electron transfer. The marked increase in the singlet excited state deactivation rate constants and quantum yields of charge separation observed in the case of the ortho-substituted dyads, in polar solvents, further support a topographically controlled electron transfer process.

Radiationless Deactivation of Singlet Excited States of Complex Molecules in the Gas Phase

Abstract The ways of degradation of electronic excitation energy of singlet states of complex molecules in the gas phase with various character of dependence of the quantun yield and lifetime of fluorescence on the excess vibrational energy of excited molecules, ΔE∗vib, are discussed. From the analysis of literature data on phthalimide and anthracene derivatives and perylene, and also from the results of investigations conducted in the present study for 9,10-dimethylanthracene and 9,10-diphenylanthracene it follows that in the majority of cases radiationless degradation of electronic excitation energv is realized both by means of intersystem crossing and internal conversion. For all studied molecules, with the exception of anthracene, an increase in intersystem crossing with increasing ΔE∗vib of excited singlet state is observed, while the internal conversion probability may either increase or decrease. In particular, an