Temperature-dependent Nonradiative Processes Research Articles

Unveiling the Dual Emission Photo-Deactivation Mechanism of a Neutral Heteroleptic Iridium (III) Complex.

In this article, the photo-deactivation mechanism of dual emission of a neutral iridium (III) complex is explored by using density function theory (DFT) and time-dependent density function theory (TD-DFT) calculations. To explore the phosphorescence quantum yield of the iridium (III) complex, the radiative decay constant of each emission excited state was computed by TD-DFT calculations, including spin-orbit coupling (SOC). In these calculations, factors such as the transition dipole moments, energy gaps, and SOC elements between the emission states and singlet excited states are taken into account in the evaluation of the radiative decay constants. Additionally, the non-radiative decay is revealed by considering the temperature-independent and the temperature-dependent non-radiative processes. The computational results indicate that the order of the two emission excited states can exert a significant effect on the phosphorescence quantum yield, which is beneficial for understanding the properties of photo-deactivation of phosphorescent emitters.

Exciton and trion dynamics in atomically thinMoSe2andWSe2: Effect of localization

We present a detailed investigation of the exciton and trion dynamics in naturally doped MoSe2 and WSe2 single atomic layers as a function of temperature in the range 10-300K under above band-gap laser excitation. By combining time-integrated and time-resolved photoluminescence (PL) spectroscopy we show the importance of exciton and trion localization in both materials at low temperatures. We also reveal the transition to delocalized exciton complexes at higher temperatures where the exciton and trion thermal energy exceeds the typical localization energy. This is accompanied with strong changes in PL including suppression of the trion PL and decrease of the trion PL life-time, as well as significant changes for neutral excitons in the temperature dependence of the PL intensity and appearance of a pronounced slow PL decay component. In MoSe2 and WSe2 studied here, the temperatures where such strong changes occur are observed at around 100 and 200 K, respectively, in agreement with their inhomogeneous PL linewidth of 8 and 20 meV at T~10K. The observed behavior is a result of a complex interplay between influences of the specific energy ordering of bright and dark excitons in MoSe2 and WSe2, sample doping, trion and exciton localization and various temperature-dependent non-radiative processes.

Exciton dynamics in a site-controlled quantum dot coupled to a photonic crystal cavity

Exciton and cavity mode (CM) dynamics in site-controlled pyramidal quantum dots (QDs), integrated with linear photonic crystal membrane cavities, are investigated for a range of temperatures and photo-excitation power levels. The absence of spurious multi-excitonic effects, normally observed in similar structures based on self-assembled QDs, permits the observation of effects intrinsic to two-level systems embedded in a solid state matrix and interacting with optical cavity modes. The coupled exciton and CM dynamics follow the same trend, indicating that the CM is fed only by the exciton transition. The Purcell reduction of the QD and CM decay times is reproduced well by a theoretical model that includes exciton linewidth broadening and temperature dependent non-radiative processes, from which we extract a Purcell factor of 17 ± 5. For excitation powers above QD saturation, we show the influence of quantum wire barrier states at short delay time, and demonstrate the absence of multiexcitonic background emission.

Photophysics of indole-2-carboxylic acid (I2C) and indole-5-carboxylic acid (I5C): Heavy atom effect

In this study the effect of carboxylic group substitution in the 2 and 5 position of indole ring on the photophysics of the parent indole chromophore has been studied. The photophysical parameters crucial in triplet state decay mechanism of aqueous indole-2-carboxylic acid (I2C) and indole-5-carboxylic acid (I5C) have been determined applying our previously proposed methodology based on the heavy atom effect and fluorescence and phosphorescence decay kinetics [Kowalska-Baron et al., 2012]. The determined time-resolved phosphorescence spectra of I2C and I5C are red-shifted as compared to that of the parent indole. This red-shift was especially evident in the case of I2C and may indicate the possibility of hydrogen bonded complex formation incorporating carbonyl CO, the NH group of I2C and, possibly, surrounding water molecules. The possibility of the excited state charge transfer process and the subsequent electronic charge redistribution in such a hydrogen bonded complex may also be postulated. The resulting stabilization of the I2C triplet state is manifested by its relatively long phosphorescence lifetime in aqueous solution (912μs). The relatively short phosphorescence lifetime of I5C (56μs) may be the consequence of more effective ground-state quenching of I5C triplet state. This hypothesis may be strengthened by the significantly larger value of the determined rate constant of I5C triplet state quenching by its ground-state (4.4×108M−1s−1) as compared to that for indole (6.8×107M−1s−1) and I2C (2.3×107M−1s−1). The determined bimolecular rate constant for triplet state quenching by iodide kqT1 is equal to 1×104M−1s−1; 6×103M−1s−1 and 2.7×104M−1s−1 for indole, I2C and I5C, respectively. In order to obtain a better insight into iodide quenching of I2C and I5C triplet states in aqueous solution, the temperature dependence of the bimolecular rate constants for iodide quenching of the triplet states has been expressed in Arrhenius form. The linearity of the obtained Arrhenius plots clearly indicated the existence of one temperature-dependent non-radiative process for the de-excitation of I2C and I5C triplet state in the presence of iodide. This process may be attributed to the solute-quenching by iodide and, most probably, proceeds via reversibly formed exciplex. The activation energies obtained from linear Arrhenius plots (1.89kcal/mol for I5C; 2.55kcal/mol for I2C) are smaller as compared to that for diffusion controlled reactions in aqueous solution (about 4kcal/mol), which may indicate the great importance of the electrostatic interactions between solute and iodide ions in lowering the energy barrier needed for the formation of the triplet-quencher complex. Based on the theoretical predictions (at the DFT(CAM-B3LYP)/6-31+G(d,p) level of theory) and careful analysis of the obtained FTIR spectra it may be concluded that in the solid state I2C and I5C molecules form associates by intermolecular NH⋯OC and OH⋯OC hydrogen bonding interactions, whereas the existence of intramolecular NH⋯OC interactions in the solid state of I2C and I5C is highly unlikely.

Thermal Quenching Mechanisms in II–VI Semiconductor Nanocrystals

The mechanisms of temperature-dependent nonradiative processes, often referred to as thermal quenching, are studied in CdSe, CdSe/ZnSe, and CdTe nanoparticles. These particles exhibit reversible thermal quenching, the extent of which is strongly dependent on the composition of the surface and nature of the surface ligands. Thermal quenching has dynamic (affecting the luminescence lifetimes) and static (affecting the fraction of particles that are bright versus dark) components. The temperature dependence of quantum yields and time-resolved luminescence decays as well as room temperature transient absorption spectroscopy are used to elucidate the thermal quenching mechanisms. Dynamic thermal quenching is due to thermally activated trapping dynamics that occur on the same time scale as the radiative lifetime. This paper focuses on static thermal quenching and several different mechanisms are considered. It is concluded that the dominant mechanism involves thermal promotion of valence band electrons to empty...

Photophysics of 2,6-dimethyl-4H-pyran-4-thione in the vapor phase

Abstract Emission and excitation spectra of 2,6-dimethyl-4H-pyran-4-thione (DPT) vapor have been measured at different buffer gas pressures from 0 to 300 Torr and at different temperatures. Emission property of DPT vapor is found to vary significantly depending on pressure, excitation energy and temperature. The quantum yields of S1 fluorescence (ΦF1) and T1 phosphorescence (ΦP) increase with increasing buffer gas pressure and with decreasing temperature, while that of the S2 fluorescence (ΦF2) is almost unchanged. The values for ΦP and ΦF1 decrease almost exponentially with increasing excitation energy at low pressure, although the decrease of ΦF2 is not so significant compared to ΦF1 and ΦP. These observations have been interpreted in terms of the relaxation model involving the fast S2 → S1 internal conversion followed by the fast S1 → T1 intersystem crossing and the vibrational relaxation in the T1 manifold along with the temperature dependent nonradiative process from T1.

Photophysical properties of tetracene derivatives in solution III. Thermally activated nonradiative processes and triplet state properties

Photophysical properties of several tetracene derivatives were studied in solution and in sucrose octa-acetate (SOA) glass. The photophysical properties of rubrene (5,6,11,12-tetraphenyltetracene) in SOA glass are not significantly different from those in common solvents employed in our earlier studies. Deviating results by Song and Fayer are shown to be due to the neglect of (S 1-S n ) excited state absorption (ESA) and internal conversion (IC). It is demonstrated that the investigation of two temperature-dependent photophysical properties and, in particular, the direct detection of triplet state molecules allows the accurate determination of the experimental activation energy E a (ISC) for thermally activated (S 1-T 2) intersystem crossing (ISC). For rurbrene in SOA glass E a(ISC) = 860 cm -1 was obtained. A consistent treatment of the photophysical processes is presented that allows the description of the temperature-dependent non-radiative processes and of the influences of solvent and internal heavy atom (HAE) effects for the molecules investigated.

Dynamics of S3 luminescence in diamond

Abstract The S 3 centre occurs in natural type I diamond and consists of a strong luminescence band with zerophonon line at 2.496 eV. A detailed analysis of this centre shows that the low temperature luminescence originates in a spin forbidden transition. On increasing temperature a level 125 meV above the former becomes populated and a new luminescence band appears due to a spin allowed transition. No temperature dependent non-radiative processes were found up to room temperature. This centre reveals interesting characteristics that provide tunable stimulated emission in the range of 2.4-2.1 eV.

Effect of temperature on the triplet states of methyl- and phenyl-substituted biphenyl molecules

The temperature dependence of the quantum yields Φ c→t and Φ t→c of the cis → trans and trans → cis isomerization respectively of 1,3-pentadine sensitized by phenyl- and methyl-substituted biphenyls has been studied in fluid media. The triplet-state quantum yields for the substituted biphenyls increased with decreasing temperature but approached maximum values at the lowest temperatures. The sums of the fluorescence and triplet quantum yields Φ F and Φ T for 4-methylbiphenyl, 3-methylbiphenyl, 4,4′-dimethyl-biphenyl and 3,3′-dimethylbiphenyl were less than unity at all temperatures and the nature of the temperature-dependent non-radiative process(es) is discussed. An attempt has been made to compare triplet-state quantum yields determined by the olefin isomerization technique with those obtained using sensitized biacetyl phosphorescence.