Naphthalene Triplet Research Articles

Trap or Triplet? Excited-State Interactions in 2D Perovskite Colloids with Chromophoric Cations.

Movement of energy within light-harvesting assemblies is typically carried out with separately synthesized donor and acceptor species, which are then brought together to induce an interaction. Recently, two-dimensional (2D) lead halide perovskites have gained interest for their ability to accommodate and assemble chromophoric molecules within their lattice, creating hybrid organic-inorganic compositions. Using a combination of steady-state and time-resolved absorption and emission spectroscopy, we have now succeeded in establishing the competition between energy transfer and charge trapping in 2D halide perovskite colloids containing naphthalene-derived cations (i.e., NEA2PbX4, where NEA = naphthylethylamine). The presence of room-temperature triplet emission from the naphthalene moiety depends on the ratio of bromide to iodide in the lead halide sublattice (i.e., x in NEA2Pb(Br1-xIx)4), with only bromide-rich compositions showing sensitized emission. Photoluminescence lifetime measurements of the sensitized naphthalene reveal the formation of the naphthalene triplet excimer at room temperature. From transient absorption measurements, we find the rate constant of triplet energy transfer (kEnT) to be on the order of ∼109 s-1. At low temperatures (77 K) a new broad emission feature arising from trap states is observed in all samples ranging from pure bromide to pure iodide composition. These results reveal the interplay between sensitized triplet energy transfer and charge trapping in 2D lead halide perovskites, highlighting the need to carefully parse contributions from competing de-excitation pathways for optoelectronic applications.

Bioinspired Luminescent Europium-Based Probe Capable of Discrimination between Ag+ and Cu.

Due to their similar coordination properties, discrimination of Cu+ and Ag+ by water-soluble luminescent probes is challenging. We have synthesized LCC4Eu, an 18 amino acid cyclic peptide bearing a europium complex, which is able to bind one Cu+ or Ag+ ion by the side chains of two methionines, a histidine and a 3-(1-naphthyl)-l-alanine. In this system, the naphthyl moiety establishes a cation-π interaction with these cations. It also acts as an antenna for the sensitization of Eu3+ luminescence. Interestingly, when excited at 280 nm, LCC4Eu behaves as a turn-on probe for Ag+ (+150% Eu emission) and as a turn-off probe for Cu+ (-50% Eu3+ emission). Shifting the excitation wavelength to 305 nm makes the probe responsive to Ag+ (+380% Eu3+ emission) but not to Cu+ or other physiological cations. Thus, LCC4Eu is uniquely capable of discriminating Ag+ from Cu+. A detailed spectroscopic characterization based on steady-state and time-resolved measurements clearly demonstrates that Eu3+ sensitization relies on electronic energy transfer from the naphthalene triplet state to the Eu3+ excited states and that the cation-π interaction lowers the energy of this triplet state by 700 and 2400 cm-1 for Ag+ and Cu+, respectively. Spectroscopic data point to a modulation of the efficiency of the electronic energy transfer caused by the differential red shift of the naphthalene triplet, deciphering the differential luminescence response of LCC4Eu toward Ag+ and Cu+.

Interlayer Triplet Energy Transfer in Dion-Jacobson Two-Dimensional Lead Halide Perovskites Containing Naphthalene Diammonium Cations.

Recently, hybrid perovskites have gained attention as sensitizers for molecular triplet generation. Layered, two-dimensional (2D) perovskites are especially well-suited for this purpose because the triplet donor (inorganic framework) and triplet acceptor (organic layer) are self-assembled into adjacent sheets, so that with the appropriate energetics, triplets can be driven across the interface. Here we examine interlayer energy transfer in a series of mixed-halide Dion-Jacobson 2D perovskites containing divalent naphthalene cations. We find that the sensitized phosphorescence in these compounds is dominated by naphthalene triplet excimer emission, but when the inorganic exciton is tuned near resonance with the naphthalene triplet, naphthalene monomer phosphorescence competes with triplet excimer formation. The interlayer energy-transfer process is further revealed by ultrafast transient absorption spectroscopy through kinetic variations in triplet excimer formation times. Ultimately, gaining control over interlayer interactions in 2D perovskites through cation design will help uncover new functions and applications for these materials.

Sensitized Molecular Triplet and Triplet Excimer Emission in Two-Dimensional Hybrid Perovskites.

Two-dimensional (2D) organic-inorganic hybrid perovskites are promising materials for next-generation optoelectronic devices owning to their structural and functional versatility and enhanced ambient stability. Recent studies have started to focus on engineering the molecular properties of the organic cations to induce inorganic-to-organic energy/charge transfer for new functionalities, yet many puzzles regarding the inorganic-organic interaction mechanisms remain to be resolved. Here we fabricate 2D lead halide perovskites containing naphthalene methylamine (NMA) cations to study naphthalene triplet sensitization by inorganic excitons. We find that triplet sensitization proceeds via a two-step mechanism initiated by subpicosecond hole transfer from the inorganic layer to naphthalene. We also provide spectroscopic evidence for triplet excimer formation, i.e., the association between triplet and ground state molecules. The intensity ratio between the excimer and triplet emissions can be tuned via the percentage of the NMA cations in the organic layer, offering a route to tunable white-light emitters using 2D hybrid perovskites.

Triplet Sensitization by Lead Halide Perovskite Thin Films for Efficient Solid-State Photon Upconversion at Subsolar Fluxes

Summary We investigate the rubrene triplet sensitization by perovskite thin films based on methylammonium formamidinium lead triiodide (MAFA) of varying thicknesses. The power-law dependence of both the MAFA photoluminescence (PL) intensity and upconverted emission is tracked as a function of the incident power density. Bimolecular triplet-triplet annihilation (TTA) exhibits a unique power-law dependence with a slope change from quadratic-to-linear at the threshold Ith. The underlying MAFA PL power-law dependence dictates the power law of the upconverted PL: (1) below Ith, the slope of the upconverted PL is twice the value of the MAFA PL; (2) above Ith, it follows the same power law as the underlying recombination of mobile electrons and holes in the MAFA films. We find that the Ith shifts to subsolar incident laser powers when increasing the MAFA thickness above 30 nm. For the thickest MAFA film of 380 nm we find an upconversion threshold of Ith = 7.1 mW/cm2.

Naphthalene Triplet Excited State as a Probe for the Assessment of Drug Distribution in Binary Protein Systems

Three drugs containing the naphthalene (NP) chromophore, namely, naproxen (NPX), propranolol (PPN), and cinacalcet (CIN), but with different affinities toward serum albumins (SAs) and α-1-acid glycoproteins (AAGs) have been employed for the assessment of drug distribution in binary SA/AAG systems. These three drugs represent an appropriate choice for checking whether a methodology based on transient absorption spectroscopy of a given reporter can be employed for discrimination between different distribution patterns in multicompartmental biological media. Thus, upon laser flash photolysis (LFP) of NPX, PPN, and CIN in the presence or absence of proteins, the NP triplet excited state ((3)NP*) at ∼420 nm was always detected, although the kinetics of the decay traces was structure- and medium-dependent. In aerated PBS, only a very short triplet lifetime (τ(T)) was found (1-2 μs). By contrast, in the presence of SAs, two longer triplet lifetimes (5-76 μs) were observed, ascribed to (3)NP* within site I and site II. Upon binding to AAGs, only a long τ(T) (15-47 μs) was found. When the two proteins were present simultaneously in the same media, fitting of the decay traces was clearly consistent with a distribution of the drug between the different biological compartments and the bulk solution, which correlates well with the known protein affinities of every drug. Experiments were performed in both human (HSA/HAAG) and bovine protein media (BSA/BAAG). The results showed that SAs are the major carriers for NPX; by contrast, PPN binds preferentially to AAGs. An intermediate situation was found for CIN, which presents comparable affinity for both proteins. The results obtained for the two enantiomers of each drug were very similar, although a small stereodifferentiation was observed between the triplet lifetimes in the protein binding sites.

ChemInform Abstract: Synthesis and Adiabatic Photochemistry of a 1,4-Difluorobenzene - Naphthalene Biplanemer.

A naphtalene-1,4-difluorobenzene biplanemer 1a has been prepared through the electrolytic oxidative didecarboxylation. The direct irradiation of 1a gave 1,4-difluorobenzene and excited naphthalene in both a singlet state (Φf = 0.098) and a triplet state (Φp = 0.029) at 77 K in EPA. In the triplet-sensitized reaction, the biplanemer 1a underwent intramolecular [2π+2π] cycloaddition to give a cage compound instead of the formation of the triplet naphthalene.

Nuclear‐Spin Polarization in Electron‐Transfer Reactions of Amines

AbstractChemically induced dynamic nuclear polarization (CIDNP) observed during electron transfer (ET) reactions of tertiary amines such as DABCO (1) or Et3N (2) with a wide range of electron acceptors support the involvement of amine radical‐cations (e.g., 1.+ or 2.+) as key intermediates. Radical ions such as 2.+ may be deprotonated, generating neutral aminoalkyl radicals (e.g., 2.). When generated by reaction with an electron acceptor of energetically low triplet state such as naphthalene (1Naph*), the resulting pair 2.+/Naph.− reacts mostly by reverse electron transfer (RET) from triplet pairs populating the naphthalene triplet state.

Triplet exciplexes as energy transfer photosensitisers

Experimental evidence is provided for the occurrence of triplet-triplet energy transfer from benzoylthiophene-indole exciplexes to naphthalenes with a remarkable stereodifferentiation; chiral recognition is also observed in the decay of the generated naphthalene triplets.

Intramolecular Energy Transfer Involving Heisenberg Spin-Coupled Dinuclear Iron−Oxo Complexes

The synthesis, structure, and physical properties of a series of oxo-bridged dinuclear Fe(III) complexes containing pendant naphthalene groups are described. The compounds [Fe(2)O(O(2)CCH(2)-C(10)H(7))(tren)(2)](BPh(4))(NO(3))(2) (8), [Fe(2)O(O(2)CCH(2)-C(10)H(7))(TPA)(2)](ClO(4))(3) (9), Fe(2)O(O(2)CCH(2)-C(10)H(7))(2)(Tp)(2) (10), and Fe(2)O((O(2)CCH(2)CH(2))(2)-C(10)H(6))(Tp)(2) (11) (where tren is tris(2-aminoethyl)amine, TPA is tris(2-pyridyl)amine, and Tp is hydrotrispyrazolylborate) have been characterized in terms of their structural, spectroscopic, magnetic, and photophysical properties. All four complexes exhibit moderately strong intramolecular antiferromagnetic exchange between the high-spin ferric ions (ca. -130 cm(-)(1) for H = -2JS(1).S(2)). Room-temperature steady-state emission spectra for compounds 8-11 in deoxygenated CH(3)CN solution reveal spectral profiles similar to methyl-2-naphthyl acetate and [Zn(2)(OH)(O(2)CCH(2)-C(10)H(7))(2)(TACN-Me(3))(2)](ClO(4)) (13, where TACN-Me(3) is N,N,N-1,4,7-trimethyltriazacyclononane) but are significantly weaker in intensity relative to these latter two compounds. Time-resolved emission data for the iron complexes following excitation at 280 nm can be fit to simple exponential decay models with tau(obs)(S)()1 = 36 +/- 2, 32 +/- 4, 30 +/- 5, and 39 +/- 3 ns for compounds 8-11, respectively. The decays are assigned to the S(1) --> S(0) fluorescence of naphthalene; all of the lifetimes are less than that of the zinc model complex (tau(obs)(S)()1 = 45 +/- 2 ns), indicating quenching of the S(1) state by the iron-oxo core. Nanosecond time-resolved absorption data on [Zn(2)(OH)(O(2)CCH(2)-C(10)H(7))(2)(TACN-Me(3))(2)](ClO(4)) reveal a feature at lambda(max) = 420 nm that can be assigned as the T(1) --> T(n) absorption of the naphthalene triplet; the rise time of 50 +/- 10 ns corresponds to an intersystem crossing rate of 2 x 10(7) s(-1). A similar feature (though much weaker in intensity) is also observed for compound 8. The order-of-magnitude reduction in the T(1) lifetime of the pendant naphthalene for all of the iron-oxo complexes (tau(obs)(T)1 = 5 +/- 2 micros vs 90 +/- 10 micros for [Zn(2)(OH)(O(2)CCH(2)-C(10)H(7))(2)(TACN-Me(3))(2)](ClO(4))) indicates quenching of the naphthalene triplet with an efficiency of >90%. Neither the naphthalene radical cation nor the reduced Fe(II)Fe(III) species were observed by transient absorption spectroscopy, implying that energy transfer is the most likely origin for the quenching of both the S(1) and T(1) states. Spectral overlap considerations strongly support a Förster (i.e., dipolar) mechanism for energy transfer from the S(1) state, whereas the lack of phosphorescence from either the free naphthyl ester or the Zn model complex suggests Dexter transfer to the diiron(III) core as the principal mechanism of triplet quenching. The notion of whether spin exchange within the diiron(III) core is in part responsible for the unusual ability of the iron-oxo core to engage in energy transfer from both the singlet and triplet manifolds of naphthalene is discussed.

FT-EPR study on the sign of exchange interaction in triplet naphthalene-galvinoxyl encounter pair

Chemically induced dynamic electron polarization (CIDEP) created in the quenching of triplet naphthalene by galvinoxyl were investigated by time-resolved Fourier-transform electron paramagnetic resonance (FT-EPR) measurements with monitoring a free induction decay signal of a pulsed microwave irradiation. Transient FT-EPR spectra of galvinoxyl with CIDEP were observed in various nonpolar solvents with different viscosity. A transient FT-EPR signal phase shows remarkable dependence on the viscosity: FT-EPR signal phases were absorption and emission in the solvents with low and high viscosity, respectively. Time evolutions of the FT-EPR signal of galvinoxyl were well simulated by a model of the radical-triplet pair mechanism (RTPM) for CIDEP. A sign of theJ value in the triplet naphthalene-galvinoxyl system in various solvents were discussed on the basis of the sign rule in the RTPM and the transient FT-EPR signal phase. One of possible explanation for the solvent viscosity dependence of the transient FT-EPR signal phase was pressented on the basis of hypothetical model of theJ value.

Time resolved ESR study on energy difference of quartet and doublet states in radical-triplet encounter pairs

The energy difference J of quartet and doublet states in a radical-triplet pair (RTP) has been studied in pairs of triplet naphthalene (Np) derivative/galvinoxyl(Galv) and triplet biphenyl (Biph) derivative/Galv systems by time resolved ESR spectroscopy. Most of the systems studied show an unusual positive sign for J that corresponds to the relative location of the quartet state being below the doublet state. In order to explain this unusual sign of J in RTPs, the effects of intermolecular charge transfer (CT) states were examined. The observation was explained by introducing configuration interaction between zero-order RTP (RTP0) and CT (CT0) states in the encounter pair, where the RTP0 state is located above the CT0 state. Among the RTP systems examined, triplet 1-chloro-Np/Galv, triplet 1-bromo-Np/Galv and triplet 4-cyano-Biph/Galv systems show a normal negative value of J. On the basis of the estimated energy gap between the RTP0 and CT0 states, it was concluded that configuration interaction is not significant in these three RTP systems. The value of J in these RTP systems is dominated by exchange interaction, giving a negative sign. The solvent effect on the sign of J in a triplet Np/Galv system is also reported, as evidence that the sign of J is controlled by the CT0 state.

Investigations of triplet excimer of naphthalenophane by emission and transient absorption measurements in solution

By using syn-[3.4](1,5)naphthalenophane (44NPP) whose naphthalene rings are in a parallel form, photophysical and photochemical properties of naphthalene triplet excimer are reported. 34NPP emits only excimer fluorescence at 295 and 77 K, and phosphorescence was absent even at 77 K. Based on the transient absorption spectra of 34NPP, it is shown that the excimeric triplet state of 34NPP is formed without the locally excited triplet at 295 and 77 K. The triplet excimer of 34NPP is produced via intersystem crossing from the singlet excimer state with a rate of 1.4×10 7 s −1 at 295 K. The triplet excimer in a parallel configuration is shown to be non-phosphorescent.

Electronic to vibrational energy conversion and charge transfer contributions during quenching by molecular oxygen of electronically excited triplet statesDedicated to Professor Frank Wilkinson on the occasion of his retirement.

The bimolecular rate constants kqT for quenching of some substituted naphthalene triplet states by molecular oxygen (O2(3Σg−)) in acetonitrile and cyclohexane and the efficiencies, fΔT, with which singlet oxygen (O2*(1Δg)) is thereby produced are reported for naphthalene derivatives with a wider range of oxidation potentials than those previously measured. The magnitude of kqT and fΔT are inversely correlated, and both parameters exhibit pronounced sensitivity to the oxidation potential (EMOX) of the naphthalene derivative and to the solvent polarity. The modified charge transfer mediated mechanism of quenching based on singlet and triplet channels for oxygen quenching is invoked to discuss these results. In cyclohexane the maximum value for fΔT of one is observed for compounds with high oxidation potentials indicating no contribution from the triplet channel whilst in acetonitrile the limit for fΔT of 0.25 expected when singlet and triplet channels give equal contributions, when spin statistics is taken into account, is observed for 2,6-dimethoxynaphthalene, which is the derivative with the lowest oxidation potential. These results are combined with those previously reported by ourselves in cyclohexane in order to examine the dependence of the quenching rate constants due to energy transfer on the energy gap (ET–E1Σ). This allows the quenching rate constants due to energy transfer in cyclohexane to be separated into contributions with and without charge transfer assistance. The latter contribution shows a smooth dependence on (ET–E1Σ) and the same dependence on the electronic energy, which has to be converted into vibrational energy, probably applies when the solvent is acetonitrile since making this assumption results in similar dependences of the charge transfer contributions in both singlet and triplet channels in acetonitrile. The free energy of activation for charge transfer assisted quenching is shown to have a linear dependence on the free energy change for full charge transfer but the indications are that quenching is via singlet and triplet charge transfer complexes with only partial charge transfer character being 12.5% and 17% in acetonitrile and cyclohexane respectively.

1,4‐Naphthoquinonediazide‐2‐carboxylic acid: a diazo compound with a long‐lived triplet excited state

Laser flash photolysis (308 nm) of 1,4-naphthoquinonediazide-2-carboxylic acid 1 in 2-methyltetrahydrofuran (MTHF) leads to the detection of a long-lived (5.3 μs) triplet excited state, whose triplet energy, obtained by measuring the phosphorescence spectrum of 1 in an Ar- matrix at 10 K, is ca 55 kcal mol−1 (1 kcal­= 4.184 KJ). The first triplet excited state of 1 is quenched efficiently by 1,3-dienes and molecular oxygen. By calculating the spin density distribution it can be shown that 31 is not a typical naphthalene triplet, as it bears significant spin density at the diazo moiety. Copyright © 2001 John Wiley & Sons, Ltd.

Substituent effects on the excited states of phenyl-capped phenylene vinylene tetramers

The singlet and triplet excited states of phenyl-capped tetramers of phenylene vinylene with different alkyl, alkoxy or cyano substituents, were investigated in benzene solution. The lowest singlet states were studied by laser flash-photolysis with time-resolved microwave conductivity and fluorescence. The triplets were generated by pulse radiolysis, via energy transfer from the solvent excited singlet followed by inter-system crossing or by energy transfer from the naphthalene triplet. Their absorption spectra and rates of quenching by oxygen were determined.

1,4‐Naphthoquinonediazide‐2‐carboxylic acid: a diazo compound with a long‐lived triplet excited state

AbstractLaser flash photolysis (308 nm) of 1,4‐naphthoquinonediazide‐2‐carboxylic acid 1 in 2‐methyltetrahydrofuran (MTHF) leads to the detection of a long‐lived (5.3 μs) triplet excited state, whose triplet energy, obtained by measuring the phosphorescence spectrum of 1 in an Ar‐ matrix at 10 K, is ca 55 kcal mol−1 (1 kcal­= 4.184 KJ). The first triplet excited state of 1 is quenched efficiently by 1,3‐dienes and molecular oxygen. By calculating the spin density distribution it can be shown that 31 is not a typical naphthalene triplet, as it bears significant spin density at the diazo moiety. Copyright © 2001 John Wiley & Sons, Ltd.

Pulse Radiolysis−Optical Absorption Studies on the Triplet States of p-Phenylenevinylene Oligomers in Solution

The triplet excited states (3Bu) of phenyl-capped, n-octyloxy substituted p-phenylenevinylene oligomers have been generated by pulse radiolysis in benzene at room temperature. They were characterized by their UV/vis absorption spectra, kinetics of formation and decay, and quenching by oxygen. The triplet states were formed either by energy transfer from the solvent excited singlet followed by inter-system crossing (φISC ≈ 0.06) or by energy transfer from the naphthalene triplet. They exhibited a strong absorption (e ≈ 105 M-1 cm-1) at photon energies that depended strongly on the chain length. Comparison with poly(2-methoxy-5-(2‘-ethylhexoxy)-p-phenylenevinylene) (MEH−PPV) suggests that the conjugation in the polymer extends over approximately 15 monomer units, i.e., similar to the conjugation length in the solid state at low temperature. In oxygen-free solution, the triplets had lifetimes of the order of tens of microseconds. They were quenched by oxygen with bimolecular rate constants of ca. 109 M-1 s-1...

Singlet−Singlet, Triplet−Triplet, and “Optically-Controlled” Energy Transfer in Polychromophores. Preliminary Models for a Molecular Scale Shift Register

The photophysics of two trichromophoric molecules have been studied by a combination of absorption, fluorescence, phosphorescence, and laser flash photolysis. TRI-1 consists of phenanthrene and naphthalene terminal chromophores joined to a central biphenyl group by methyl ester bridges. Intramolecular singlet−singlet energy transfer (SSET) between the biphenyl and terminal chromophores occurs efficiently with kSSET > 6 × 1010 s-1. Longer range SSET from the naphthalene moiety to the phenanthrene group takes place with a lower rate, k ∼ 2.5 × 108 s-1. In TRI-2, the biphenyl moiety is replaced by a benzophenone group. SSET here occurs from the phenanthrene chromophore to the central benzophenone chromophore although with a significantly lower rate than the biphenyl → phenanthrene rate in TRI-1. This is likely due to the change in configuration (ππ* to nπ*) of the excited singlet states involved in the energy transfer. Triplet−triplet (TTET) energy transfer between biphenyl and the terminal chromophores is not observed as a result of a low biphenyl triplet population. That both the phenanthrene and naphthalene triplets are observed following laser photolysis and have different lifetimes and different excitation wavelength concentration ratios indicates that there is no significant TTET between these two groups on the time scale at which the triplets are decaying. This is attributed to the large interchromophore distance (∼12.5 Å) as indicated by modeling studies. In TRI-2, TTET from benzophenone to the terminal chromophores is indicated by both phosphorescence and laser flash photolysis results. Two-laser flash photolysis of the phenanthrene triplet in TRI-1 results in the production of the naphthalene triplet by the following suggested route: (i) production of the upper excited triplet state of the phenanthrene group, (ii) energy transfer to the central biphenyl moiety, and (iii) further energy transfer to the naphthalene chromophore. The utility of this two-laser behavior as the basis for the operation of an optically coupled molecular scale shift register is discussed.

Photophysics and photochemistry of triplet exciplexes between triplet naphthalene derivatives and benzophenone

Abstract