Non-radiative De-excitation Channels Research Articles

Enhancing Photothermal Energy Transduction through Inter- and Intramolecular Interactions of Multiple Two-Photon Dyes Appended onto Calix[4]arene.

Organic dyes-based photothermal agents (OPTAs) have received increasing attention as alternative to inorganic materials due to their higher biocompatibility and extensive diversification. Maximizing nonradiative deexcitation channels is crucial to improve the photothermal conversion efficiency (PCE) of OPTAs. This is typically achieved through individual molecular design or collective enhancement using supramolecular strategies. Furthermore, photothermal therapy (PTT) generally relies on linear one-photon absorption of the light source by the OPTA, with less consideration given to nonlinear two-photon absorption (2PA) strategies, despite their potential benefits. Here, a synergistic strategy, which combines intramolecular and intermolecular quenching, is employed to maximize the photothermal efficiency of diphenylamino-substituted distyryl dicyanobenzene (DSB), an outstanding two-photon-absorbing chromophore. One to three DSB units have been introduced on the conic p-tert-butyl-calix[4]arene (CX), serving as a preorganizing platform to allow aggregate formation and promote intramolecular quenching within the multichromophoric systems. Importantly, the multichromophoric molecules had very high two-photon absorption capabilities with cross sections (δ2PA) reaching maximal values of 3290 GM at 810 nm. Experimental data accompanied by large-scale molecular dynamics simulations and time-dependent density functional theory calculations shed light onto the interaction mechanism in those multiple DSB-appended CX compounds to rationalize their optical properties. Then, the formulation with Pluronic F127 amphiphile yields water-dispersible nanoprecipitates (Nps), in which the PCE is further maximized and the photobleaching is reduced due to the combination of intra- and intermolecular quenching. The high two-photon absorption in the near-infrared (NIR) window associated with the high PCE of these nanosized OPTAs could serve as a basis to future in vivo 2P-PTT applications.

Selective thermal emission of directionally solidified Al2O3/Y3−xErxAl5O12 eutectics: Influence of the microstructure, temperature and erbium content

Directionally solidified eutectic rods of Al2O3-Y3−xErxAl5O12 (0 ≤ x ≤ 3) composition have been grown using the laser floating zone method at processing rates of 25mm/h and 750mm/h. For all the samples the microstructure presents the same morphology, consisting of an interpenetrated 3D eutectic network of both Al2O3 and garnet phases, while eutectic interspacing strongly depends on the solidification rate. Optical absorption, luminescence and thermal emission have been investigated as a function of the growth rate and the erbium content. From the luminescence characterization it can be concluded that erbium ions are present in the eutectic only in the garnet phase. Near infrared thermal emission has been measured at temperatures ranging from 1000 to 1500°C. Selective emission consisting of an emission band at 1540nm and a weaker line at 970nm, ascribed to the radiative de-excitation of the thermally excited Er3+ ions from the 4I13/2 and 4I11/2 multiplets to the 4I15/2 ground state respectively, has been detected in all the samples containing erbium. The influence of the erbium content, the microstructure and the temperature on the thermal emission has been investigated. The strongest selective emission has been obtained for samples with x=3 although it does not increase proportionally to the erbium content. The thermal emission becomes more intense when the temperature is increased but the selective emission seems to saturate at temperatures higher than 1300°C. This effect is especially noticeable for x=3 and is attributed to thermally activated non-radiative de-excitation channels. It is suggested that these channels become effective through energy transfer among Er3+ ions and towards killers.

Directionally solidified Al2O3–Yb3Al5O12 eutectics for selective emitters

Al2O3–Yb3Al5O12 eutectic rods were directionally solidified using the laser floating zone method at rates between 25 and 750mm/h. The microstructure consisted of an interpenetrated network of both eutectic phases for all the growth rates. The size of the phases was strongly dependent on the growth rate, the eutectic interspacing decreasing from 4.5μm at the lowest growth rate to 600nm at 750mm/h. The optical transmission of the sample with coarser microstructure was measured and compared with that of an Yb3Al5O12 single crystal grown “ad hoc” using the same method. The apparent “oscillator strength” of the single 2F7/2→2F5/2 Yb3+ absorption band was larger in the eutectic sample than in the single crystal, which was attributed to the increase in the light path caused by multiple refractions at the eutectic interphases. The thermal emission of the eutectic rod was studied between 1000°C and 1500°C. An intense and relatively narrow emission band at about 1μm corresponding to the 2F5/2→2F7/2 Yb3+ electronic transition was observed in the whole temperature range. The intensity of the band increased with the temperature up to about 1300°C. At higher temperatures a saturation of the selective emission was observed which was attributed to the competition between the increase in the thermal population of the excited state and the enhancement of the non-radiative de-excitation channels with the temperature.

Defect roles in the excitation of Eu ions in Eu:GaN

Eu ions in situ doped in GaN with V/III ratios varying from 3200 to 9600 have been investigated using resonant site-selective photoluminescence (PL), power dependent cathodoluminescence (CL), and a unique electron beam power dependent dual excitation experiment combining the techniques of PL and CL. The results of these experiments reveal the role of defects in the electronic excitation of Eu ions and the link between the GaN host and Eu ion dopants. The relative number of beneficial defects present in each sample for a majority Eu site (Eu1) and a specific secondary site (Eu2) are revealed. Also, a room temperature activated non-radiative recombination pathway linked to a specific, sample dependent Eu2 excitation pathway is identified. Unlike conventional GaN LEDs, Eu:GaN device performance does not rely completely on crystalline quality, but on the presence of specific excitation enhancing defects and the absence of non-radiative de-excitation channels.

Structural and optical characterization of Mg-doped GaAs nanowires grown on GaAs and Si substrates

We report an investigation on the morphological, structural, and optical properties of large size wurtzite GaAs nanowires, low doped with Mg, grown on GaAs(111)B and Si(111) substrates. A higher density of vertical nanowires was observed when grown upon GaAs(111)B. Very thin zinc-blende segments are observed along the axis of the nanowires with a slightly higher linear density being found on the nanowires grown on Si(111). Low temperature cathodoluminescence and photoluminescence measurements reveal an emission in the range 1.40–1.52 eV related with the spatial localization of the charge carriers at the interfaces of the two crystalline phases. Mg related emission is evidenced by cathodoluminescence performed on the GaAs epilayer. However, no direct evidence for a Mg related emission is found for the nanowires. The excitation power dependency on both peak energy and intensity of the photoluminescence gives a clear evidence for the type II nature of the radiative transitions. From the temperature dependence on the photoluminescence intensity, non-radiative de-excitation channels with different activation energies were found. The fact that the estimated energies for the escape of the electron are higher in the nanowires grown on Si(111) suggests the presence of wider zinc-blende segments.

Understanding the differences in photochemical properties of substituted aminopyrimidines

The luminescent patterns of several members of the aminopyrimidine family are very different, showing not fluorescence at all, only a fluorescence band, normal or anomalous, or dual fluorescence, depending on the substituents and on the environment (gas phase vs. polar solvents). In this work, we study the lowest excited states of several members of this family that exhibit different fluorescence patterns to try to explain their photochemistry and to understand the effect of the substituents and the environment. We have found that several excited states (local excited (LE), charge transfer (CT) and nN–π* states) have minima on the lowest excited potential energy surface (S1), being their relative energy the determinant factor of the luminescent behavior. If the more stable S1 minima are of nN–π* character, a non-radiative deexcitation channel is the most efficient and the system shows no fluorescence. If the CT and/or LE states are the most stable, the non-radiative deactivation channel is not accessible and the system fluoresces. The relative energies of the CT and LE minima (affected by substituents and by the presence of a polar solvent) and the different magnitude of the oscillator strength for the radiative transition to the ground state determine which emission is more efficient, giving place to normal, anomalous or dual fluorescence. The study has been carried out by CASSCF/CASPT2 computations, including the solvent effect by means of the PCM model.

Photophysical properties of 1- N-methylamino- and 1- N, N-dimethylamino-9,10-anthraquinone dyes: A comparison with 1-amino-9,10-anthraquinone dye

Photophysical properties of 1- N-methylamino-9,10-anthraquinone (1-MAAQ) and 1- N, N-dimethylamino-9,10-anthraquinone (1-DMAAQ) dyes have been investigated in different solvents and solvent mixtures and the results are compared with those of the lower analogue 1-amino-9,10-anthraquinone (1-AAQ) dye [P. Dahiya, M. Kumbhakar, D.K. Maity, T. Mukherjee, A.B.R. Tripathi, N. Chattopadhyay, H. Pal, J. Photochem. Photobiol. A: Chem. 181 (2006) 338]. Qualitatively, 1-MAAQ behaves similar to 1-AAQ dye, displaying unusual deviations in photophysical properties in nonpolar solvents in comparison to the linear solvent polarity dependent changes in the properties for solvents of moderate to higher polarities. Moreover, the fluorescence decays of the dye show strong temperature-dependence in nonpolar solvents though the decays are temperature independent in solvents of moderate to higher polarities. It is inferred from the present results that 1-MAAQ dye adopts two different structures, one nonplanar structure in nonpolar solvents and the other planar intramolecular charge transfer structure in other solvents of moderate to higher polarities. A support for the above has been obtained from ab initio quantum chemical calculations. It is observed that in absolute term the nonradiative deexcitation channel is more dominating in 1-MAAQ than in 1-AAQ dye and this is attributed to the stronger intramolecular hydrogen bond present in the former dye than the latter. This is also supported by the deuterium isotope effect on the fluorescence decays of 1-MAAQ dye. Unlike 1-AAQ and 1-MAAQ dyes, their higher analogue 1-DMAAQ dye is found to be unusually weak in its fluorescence. This is attributed to the possible distortion in the anthraquinone moiety from planarity due to the steric effect of the bulky 1-N(CH 3) 2 group, which is also supported by the results from ab initio quantum chemical calculations.

Effect of protic solvents on twisted intramolecular charge transfer state formation in coumarin-152 and coumarin-481 dyes

Photophysical properties of coumarin-152 and coumarin-481 dyes in different protic solvents indicate that solute–solvent H-bonding interaction stabilizes the twisted intramolecular charge transfer (TICT) states of these dyes. Accordingly, the onset of TICT mediated nonradiative deexcitation channel in these dyes occurs at lower solvent polarity in protic solvents (Δ f ∼ 0.18) than in aprotic solvents (Δ f ∼ 0.2). The interesting and unusual observation that in lower polarity solvents (Δ f < ∼0.17) the fluorescence lifetime of these dyes increases with temperature is interpreted considering an equilibrium between the free and the solute–solvent H-bonding complexes in protic solvents.

Effect of solvent polarity on the photophysical properties of coumarin-1 dye

Effects of solvent polarity on the photophysical properties of coumarin-1 (C1; 7-NEt2-4-CH3-1,2-benzopyrone) dye have been investigated using steady-state and time-resolved fluorescence measurements. In moderate to higher polarity solvents the properties like Stokes’ shifts (Δν̄), fluorescence quantum yields (Φf), fluorescence lifetimes (τf), radiative rate constants (kf), and nonradiative rate constants (knr) follow more or less linear correlation with the solvent polarity function Δf [={(D−1)/(2D+1)}−{(n2−1)/(2n2+1)}]. In nonpolar solvents, namely, hexane, cyclohexane, methylcyclohexane, 3-methylpentane, and decalin, however, all the above-mentioned properties show unusual deviation in comparison to the trend observed in moderate to higher polarity solvents. Thus, the Δν̄ and knr values are unusually lower and the Φf, τf, and kf values are unusually higher in nonpolar solvents. Comparing the results of C1 with those of its lower analogue, namely, coumarin-120 (C120; 7-NH2-4-CH3-1,2-benzopyrone), it has been inferred that like C120, the dye C1 also exists in different structural form in nonpolar solvents than in other solvents of moderate to higher polarities. It is proposed that in nonpolar solvents the dye exists in a nonpolar structure, where the 7-NEt2 group of the dye adopts a pyramidal configuration and consequently out of plane of the 1,2-benzopyrone moiety. In moderate to higher polarity solvents, the dye is supposed to exist in a polar intramolecular charge transfer structure with its 7-NEt2 group in resonance with the 1,2-benzopyrone moiety. The observed differences in some of the properties of C1 and C120 dyes in nonpolar solvents have been rationalized based on the relative rates of the flip-flop motions of the 7-amino groups of the two dyes. It is understood that where a very fast flip-flop motion of the 7-NH2 group in C120 introduces an activation-controlled nonradiative deexcitation channel for the dye excited state in nonpolar solvents, the very slow flip-flop motion of the 7-NEt2 group in C1 cannot introduce any such nonradiative deexcitation channel for the dye.

Photophysical Properties of Coumarin-500 (C500): Unusual Behavior in Nonpolar Solvents

Photophysical properties of 7-NHEt-4-CF 3 -1,2-benzopyrone dye (coumarin-500, C500), have been investigated in different solvents and solvent mixtures using optical absorption and fluorescence measurements. In nonpolar solvents, namely, hexane, cyclohexane, methylcyclohexane, 2-methylpentane, 3-methylpentane, and Decalin, the dye displays unusual properties in comparison to those in other solvents. Thus, in the former solvents, the Stokes' shifts Δν and the fluorescence lifetimes (τ f ) for C500 are found to be unusually low. Though in these solvents the fluorescence quantum yields Φ f are seen to be only marginally higher, the radiative decay rate constants k f are found to be unusually higher than those in the other solvents. From comparing the results of C500 with those reported earlier for its lower analogue, C151 (coumarin-151, 7-NH 2 -4-CF 3 -1,2-benzopyrone; Nad, S.; Pal, H. J. Phys. Chem. A. 2001, 105, 1097), it is inferred that just like C151, in nonpolar solvents the dye C500 also exists in nonpolar structures, whereas in all other solvents the dye exists in a polar intramolecular charge transfer (ICT) structure. Unlike C151, the C500, however, does not show any activation-controlled nonradiative deexcitation channel for the Si state in nonpolar solvents, which arises due to the flip-flop motion of the 7-amino group. Due to the heavier mass, the flip-flop motion of the 7-NHEt group in C500 is supposed to be much slower than that of the 7-NH 2 group in C151. Thus, such a motion cannot introduce any deexcitation channel, which can compete with the intrinsic fluorescence k f and the internal conversion k 1 C rates in the excited C500 dye. Thus, unlike C151, there is no activation-controlled nonradiative deexcitation channel for C500 in nonpolar solvents though both the dyes appear to exist in similar structural forms in these solvents, which are different than those existing in other solvents of lower to higher polarities.

Luminescence of C 60 adsorbed on Ag and In surfaces

The photoluminescence (PL) of 1–2 monolayers (ML) of C 60 adsorbed on cold-deposited Ag and In surfaces is strongly quenched by the metal-C 60 interaction. However, similar to the case of the Raman spectra of C 60 on the same surfaces, the PL intensity increases substantially upon annealing of the surfaces of 300 K. Changes in the PL spectrum indicate that the metal-C 60 interaction, which includes charge transfer from the surface to the adsorbed C 60, perturbs the first C 60 ML more strongly than the second. We attribute these effects to the introduction of nonradiative deexcitation channels involving electronic states near the metal's Fermi level.

Direct observation of the photodecomposition of liquid nitromethane under UV photolysis by sub-picosecond time-resolved CARS experiments

Using a sub-picosecond CARS experiment we observed the direct photolysis of liquid nitromethane after pulse excitation at 299 nm. We measured the dynamical behaviour under excitation of three main ground-state Raman lines: the v(CN) mode at 917 cm −1, the v s (NO 2) and δ s (CH 3) modes near 1400 cm −1 and the V s(CH 3) mode at 2968 cm −1. From the evolution of these modes we deduced that the excited state disappears with a lifetime of 1.1 ± 0.3 ps and we measured a photolysis quantum yield at 299 nm of 24% ± 5%. A non-radiative de-excitation channel from the excited to the ground state has been observed and its possible role in the explosive process is discussed.

Nonradiative deexcitation mechanisms in the visible spectrum of I2 vapour

Vibronically resolved laser photoacoustic (LPA) and laser optogalvanic (LOG) spectra of I 2 vapour have been simultaneously recorded in a specially designed cell at room temperature in the 480–540 nm region. Optogalvanic spectra were recorded at an effective pressure of 15 Torr in a continuously flowing system with a partial pressure of I 2 vapour of about 200 mTorr. The photoacoustic spectra were recorded at effective air pressures of 15 Torr as well as 760 Torr and the range of the spectrum was extended up to 680 nm. It is found that the discrete bands in the photoacoustic spectra correspond to the electronic transition B 3 Π( Ou + )← X 1 Σ g and 1 Π 1u ← X 1 Σ g and A 3 Π 1u ← X 1 Σ g transitions. The contribution of heat signals produced due to nonradiative deexcitation in the generation of optogalvanic signals is discussed in the light of the similarity between the two spectra. Four different nonradiative deexcitation channels have been found to contribute significantly to the generation of photoacoustic signals in the visible absorption region of I 2 and these are discussed in the light of optical Franck—Condon Factors (FCFs). Energy transfer to the O 2 molecule due to the photodissociation of I 2 into 2 P 3 2 + 2 P 1 2 atoms as well as the predissociation of the B 3 Π( Ou + ) state around ν′∼23–27 have been discussed.

Surface-plasmon assisted Auger transitions in grazing collisions of ions with simple metal surfaces

The dependence of surface-plasmon assisted decay of excited atomic p-levels to s-levels in front of simple metal surfaces on the particle velocity parallel to the surface plane is studied within a dielectric linear-response theory. This non-radiative de-excitation channel can be rather likely, provided the atomic transition energy is close to the surface-plasmon energy. Furthermore, a strong orientational effect of this damping process on the mean orbital angular momentum of the atom is found as a consequence of the particle motion parallel to the surface plane. The reverse process, that is, surface-plasmon assisted excitation is also discussed.