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We investigate the ability of dynamic fluorescence probes to accurately track populations of multi-excitonic states in molecular dyads based on conjugated acenes capable of intramolecular singlet fission (iSF). Stochastic simulations of reported photophysical models from time-resolved spectroscopic studies of iSF dyads based on large acenes (e.g., tetracene and pentacene) are used to extrapolate population and fluorescence yield dynamics. The approach entails the use of repetitive rectangular-shaped excitation waveforms as a stimulus, with durations comparable to triplet lifetimes. We observe unique dynamics signatures that can be directly related to relaxation of multi-exciton states involved over the entire effective time of singlet fission in the presence and absence of an excitation light stimulus. In particular, time-dependent fluorescence yields display an abrupt decay followed by slower rise dynamics appearing as a prominent "dip" feature in responses. The initial fast decrease in the fluorescence yield arises from the formation of triplet pairs and separated triplets that do not produce emission resembling a complete ground state bleach effect. However, relaxation of one separated triplet allows the system to absorb, and in some cases, this increases the fluorescence yield, causing rise dynamics in the emissive response. Our approach also permits extrapolation of all multi-exciton state population dynamics up to steady state conditions in addition to the ability to explore consequences of alternative relaxation channels. The results demonstrate that it is possible to resolve unique signatures of singlet fission events from dynamic fluorescence studies, which can augment detection capabilities and extend sensitivity limits and accessible time scales. [ABSTRACT FROM AUTHOR]

Effective diversion of surge currents is vital to prevent unwanted damage to sensitive electronics. Among the most successful and efficient strategies relies on a dielectric stimulated arc breakdown mechanism with high permittivity ceramic granules in a spark‐gap geometry. Although generally regarded as a self‐healing process, substantial energy deposition may occur that, over time, diminishes the ability to withstand repeated electrical assaults. We investigate the susceptibility of lead–magnesium–niobate–lead titanate (PMN–PT) granule microstructure and composition changes following many exposures to high voltage impulses resulting in arc breakdown. Scanning electron microscopy and energy‐dispersive spectroscopy mapping reveal a broad range of thermal and mechanical defects entailing thermal reduction of constituent PMN–PT metal ions and recasting due to rapid eruption of volatile species. Additionally, evidence of local melting and microcracking are apparent that can have deleterious impact on the proper function of the granules, namely, the ability to concentrate electric fields across air gaps to establish and sustain discharge pathways. We propose that the localized nature of damage and stochasticity associated with the dielectric stimulated breakdown mechanism may allow granules to maintain functionality provided no permanent conduction paths are established. [ABSTRACT FROM AUTHOR]

Resolving the population dynamics of multiple triplet excitons on time scales comparable to their lifetimes is a key challenge for multiexciton harvesting strategies, such as singlet fission. We show that this information can be obtained from fluorescence quenching dynamics and stochastic kinetic modeling simulations of single nanoparticles comprising self-assembled aggregated chains of poly(3-hexylthiophene) (P3HT). These multichromophoric structures exhibit the elusive J-aggregate type excitonic coupling leading to delocalized intrachain excitons that undergo facile triplet formation mediated by interchain charge transfer states. We propose that P3HT J-aggregates can serve as a useful testbed for elucidating the presence of multiple triplets and understanding factors governing their interactions over a broad range of time scales. Stochastic kinetic modeling is then used to simulate discrete population dynamics and estimate higher order rate constants associated with triplet-triplet and singlet-triplet annihilation. Together with the quasi-CW nature of the experiment, the model reveals the expected amounts of triplets at equilibrium per molecule. Our approach is also amenable to a variety of other systems, e.g., singlet fission active molecular arrays, and can potentially inform design and optimization strategies to improve triplet harvesting yields. [ABSTRACT FROM AUTHOR]

Resolved vibronic structure in electronic spectra provides a detailed view into how molecular structure changes after absorption or emission of a photon. We report temperature- and pressure-dependent luminescence spectra of trans-[Re[O.sub.2][(pyridine).sub.4]I. Low-temperature spectra reveal long vibronic progressions in the totally symmetric O = Re = O (907 [cm.sup.-1] and Re-pyridine (211 [cm.sup.-1] stretching modes, indicating large structural displacements along these normal coordinates. The luminescence band maximum is at ca. 15 500 [cm.sup.-1]. Room-temperature spectra are somewhat less-resolved; however, intervals closely matching the O = Re = O frequency (~870 [cm.sup.-1] persist at higher temperatures. The variable pressure spectra exhibit distinct changes in the vibronic patterns, and luminescence energies decrease by 16 [+ or -] 2 [cm.sup.-1]/kbar (1 bar = 100 kPa). Low-temperature spectra are modeled using two-dimensional potential energy surfaces to represent the initial and final electronic states, from which the quantitative normal coordinate offsets can be determined. We then adapt this model to the room-temperature, pressure-dependent data where it is possible to determine how the offsets and other important spectroscopic parameters vary with the pressure-induced changes of the molecular structure. Key words: trans-[Re[O.sub.2] [(pyridine).sub.4] I, low-temperature luminescence spectroscopy, high-pressure luminescence spectroscopy, vibronic structure, emitting state distortions.

Luminescence spectra of trans-dioxo bis-(tetramethylethylenediamine)-rhenium(V) show pressure-dependent vibronic structure, frequency and the intensity distribution within the progression, involving the high-frequency Raman-active O=Re=O stretching mode. The influence of pressure on the shape and relative positions of the ground and emitting state potentials energy surfaces are calculated along the normal coordinate of the vibrational mode.

Photoinduced oxidation (doping) of conjugated polymers by complexation with oxygen can have a significant impact on electronic properties and performance in device environments. Nanofiber model forms of poly(3‐hexylthiophene) (P3HT) are investigated using single molecule spectroscopy that possess similar morphological qualities as their bulk thin film counterparts yet, heterogeneity is confined to the spatial dimensions of these particles. Specifically, P3HT nanofibers assembled in anisole solutions contain both aggregated and nonaggregated (amorphous) chains with distinct electronic properties. Excitation intensity dependent photoluminescence (PL) emission imaging is then used to expose differences in oxygen affinity and reactivity upon photoexcitation. Nanofiber regions with low PL yields tend to show faster PL intensity saturation that also degrade much faster following periods of high excitation intensity soaking. Conversely, other regions show gains in PL intensity and virtually no saturation. These PL "gainer" and "loser" behaviors are assigned as originating from amorphous and aggregated P3HT chains, respectively. The apparent propensity of aggregated chains to undergo latent oxygen doping indicates a greater affinity probably due to a larger extent of electronic delocalization in these structures. The results shed new light on degradation factors studied frequently at the bulk material level, which often lacks sufficient sensitivity to specific structural forms. [ABSTRACT FROM AUTHOR]

Optical emission spectra of poly[2-methoxy-5-[3′,7′-dimethyloctyloxy)-1,4-phenylenevinylene] (MDMO-PPV) in dilute solutions exhibit a vibronic progression interval (∼1225 cm-1) that does not correspond to any ground state vibrational mode frequency. This phenomenon is assigned as the missing mode effect (MIME) in which five key displaced polymer backbone vibrational modes in the range of 800-1600 cm-1 contribute to the MIME interval. Emission spectra are calculated by analytically solving the time-dependent Schrödinger equation using estimates of mode-specific vibrational displacements determined independently from preresonance Raman intensities. Emission spectra of MDMO-PPV thin films and nanoparticles are measured and lineshapes show an increase of the MIME frequency to ∼1340 cm-1 in addition to changes in vibronic intensity distributions and energies. Composite blend thin films consisting of MDMO-PPV and a fullerene derivative (1:1 w/w) exhibit a substantially larger MIME interval (∼1450 cm-1) that arises from an increase in polymer chain planarity. This structural change is most apparent from large decreases of the excited state displacement of an out-of-plane C[Single_Bond]H bending mode (961 cm-1) that becomes forbidden in the planar structure. [ABSTRACT FROM AUTHOR]

Fluorescence emission spectra were recorded for isolated polymer chains of the polyfluorene copolymer, F8BT [poly(9,9-di-n-octylfluorene-alt-benzothiadiazole)], at 298 and 20 K for two molecular weights (chain lengths). Condon analysis reveals a decrease in emitting state displacements between spectra measured at 298 and 20K, suggesting that temperature-induced structural displacements are responsible for the change in the bimodal emission.

Vibronic progressions in both metal-ligand and ligand-centered symmetric stretching modes are described. Low-temperature luminescence spectra are used to determine these progressions.

The luminescence properties of thiocyanate and selenocyanate platinum (II) and Palladium (II), that shows strong variations with temperature and pressure, are described. The temperature and pressure-dependant spectroscopic results show large variations from ambient conditions.

We investigate a class of non-emissive conjugated polymers with very short excited state lifetimes believed to undergo singlet fission and relaxation to mid-gap forbidden excited states. Poly(3-decylthieneylenvinylene) (P3DTV) and its heavy atom analog, poly(3-decylseleneylenvinylene) (P3DSV), are strongly aggregating conjugated polymers that experience large excited state displacements along multiple vibrational modes. We demonstrate this Franck–Condon vibrational activity effectively disperses excitation energy into multiple non-radiative channels that can be explained using a simple, two-state potential energy surface model. Resonance Raman spectroscopy is sensitive to early Franck–Condon vibrational activity and we observe rich harmonic progressions involving multiple high frequency CC backbone symmetric stretching motions (∼1000–1600 cm−1) in both systems reflecting mode-specific excited state geometrical displacements. Transient absorption spectra confirm that efficient non-radiative processes dominate excited state relaxation dynamics which are confined to π-stacked aggregated chains. Surprisingly, we found little influence of the heteroatom consistent with efficient vibrational energy dissipation. Our results highlight the importance of aggregation and multi-dimensional Franck–Condon vibrational dynamics on the ability to harvest excitons, which are not usually considered in materials design and optimization schemes. [ABSTRACT FROM AUTHOR]

The four effects that contribute to the large and unexpected increase in luminescence intensity of the [Pd(SCB)4]2- complex with increasing pressure are described. It is likely that all four effects described contribute to the surprising pressure-induced luminescence properties of the title complex.

Triplet formation and interactions with emissive singlet excitons are investigated in poly(3-hexylselenophene) (P3HS) using single molecule spectroscopy. P3HS is a heavy atom analog of the more commonly studied poly(3-hexylthiophene) (P3HT), a benchmark polymer for solar cells. P3HS tends to aggregate strongly which necessitates dilution to ultra-low levels within a solid inert host in order to resolve photophysical responses of single chains. Fluorescence excitation intensity modulation is performed on isolated P3HS chains using a sequence of rectangular pulses of varying intensities to probe the presence of spin-forbidden triplet excitons. Triplet population dynamics originating from singlet–triplet and triplet–triplet interactions appear as quenching of the initial fluorescence intensity to steady-state levels on characteristic time scales of ∼1–10 μs. Over 80% of all molecules studied display significant fluorescence intensity modulation (quenching depths ＞50%) indicative of efficient intersystem crossing and large triplet occupancies. Because triplets are highly localized and singlet–triplet and triplet–triplet annihilation rate constants are comparable to those of intersystem crossing, multiple triplets are present at any given time on single P3HS chains. Triplet lifetimes were estimated to be ∼4 μs (upper limit) determined from recovery to the ground electronic singlet state in the absence of light and, surprisingly, triplets vanish at the onset of P3HS aggregation. This result was unexpected since P3HS triplet formation takes place on time scales ＜30 ps making this process competitive with most accessible non-radiative deactivation pathways. [ABSTRACT FROM AUTHOR]

Single molecule photoluminescence (PL) spectroscopy of conjugated polymers has shed new light on the complex structure-function relationships of these materials. Although extensive work has been carried out using polarization and excitation intensity modulated experiments to elucidate conformation-dependent photophysics, surprisingly little attention has been given to information contained in the PL spectral line shapes. We investigate single molecule PL spectra of the prototypical conjugated polymer poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) which exists in at least two emissive conformers and can only be observed at dilute levels. Using a model based on the well-known "Missing Mode Effect" (MIME), we show that vibronic progression intervals for MEH-PPV conformers can be explained by relative contributions from particular skeletal vibrational modes. Here, observed progression intervals do not match any ground state Raman active vibrational frequency and instead represent a coalescence of multiple modes in the frequency domain. For example, the higher energy emitting "blue" MEH-PPV form exhibits PL maxima at ∼18,200 cm-1 with characteristic MIME progression intervals of ∼1200-1350 cm-1, whereas the lower energy emitting "red" form peaks at ∼17,100 cm-1 with intervals in the range of ∼1350-1450 cm-1. The main differences in blue and red MEH-PPV chromophores lie in the intra-chain order, or, planarity of monomers within a chromophore segment. We demonstrate that the Raman-active out-of-plane C-H wag of the MEH-PPV vinylene group (∼966 cm-1) has the greatest influence in determining the observed vibronic progression MIME interval. Namely, larger displacements (intensities)-indicating lower intra-chain order-lower the effective MIME interval. This simple model provides useful insights into the conformational characteristics of the heterogeneous chromophore landscape without requiring costly and time-consuming low temperature or single molecule Raman capabilities. [ABSTRACT FROM AUTHOR]

Blends of alternating ‘push–pull’ donor/acceptor (d/a) co-polymers with soluble fullerenes as active materials have shown promise for increasing power conversion efficiencies in organic photovoltaic (OPV) devices. We investigate morphology-dependent optical and electronic properties of poly[2,6-(4,4-bis-(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b′]dithiophene)-alt-4,7(2,1,3-benzothiadiazole)] (PCPDTBT) blended with [6,6]-phenyl C61 butyric acid methyl ester (PCBM) using electronic absorption and resonance Raman spectroscopies. Selective resonance excitation spanning the entire PCPDTBT absorption envelope (∼400–900 nm) was used to probe via Raman spectroscopy the degree of composition- and conformation-dependent charge transfer character along the polymer backbone. Raman intensities of characteristic PCPDTBT backbone donor/acceptor moieties vary with excitation wavelength. We perform density functional theory (DFT) calculations to assign Raman-active vibrational modes and correlate polymer backbone conformations to the degree of intra-chain donor/acceptor charge transfer character. We find the best agreement between experimental and simulated spectra for planarized PCPDTBT backbone consistent with strong charge transfer character along the backbone, which also gives rise to a new red-shifted absorption band appearing in PCBM blends. Resonance Raman and photocurrent imaging experiments were next used to spatially map morphology-dependent vibrational signatures of PCPDTBT donor/acceptor moieties within functioning solar cell devices. Solvent additives were applied using 1,8 octanedithiol (ODT) to modify PCPDTBT:PCBM morphologies and compared to as-cast blends. Raman and photocurrent images indicate a well-mixed morphology that we propose induces planarization of the PCPDTBT backbone. [ABSTRACT FROM AUTHOR]

The effect of the strong electron acceptor, 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4-TCNQ), on poly(3-hexylthiophene) (P3HT) aggregates is studied. F4-TCNQ is commonly used as a dopant for P3HT, however, relatively little is currently known about its effect on polymer conformation and packing in the presence of fullerenes. Resonance Raman and optical spectra of pristine P3HT or blends with [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) doped with F4-TCNQ up to ∼10% w/w show a loss of pristine-type P3HT aggregates with increasing dopant concentration. Complexed P3HT chains possess greater backbone planarity due to hole injection which is corroborated from density functional theory (DFT) calculations of oligothiophene surrogates and F4-TCNQ. Morphologies of doped P3HT/PCBM systems are characterized using scanning transmission electron microscopy (STEM) with electron energy loss spectroscopy (EELS) detection and images reveal mixed clusters of P3HT/F4-TCNQ fibril-like domains that increase in size with dopant loading. The apparent preference of F4-TCNQ for P3HT aggregates is attributed to efficient charge separation stemming owing to the more polarizable nature of chains comprising the aggregate π-stack. [ABSTRACT FROM AUTHOR]

Resonance Raman spectroscopic imaging is introduced as a physical probe to identify and spatially map morphology-dependent variations of intra- and interchain interactions and order in poly-3- hexyithiophene (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) photovoltaic blend thin films. Absorption spectra and C=C symmetric stretching Raman modes of P3HT/PCBM blend films show contributions from two distinct species that are assigned as aggregated and unaggregated P3HT chains with characteristic Raman frequencies of ∼1450 (Ic=cagg) and ∼1470 cm-1 (Ic=cun), respectively. Upon thermal annealing of blend films, the relative concentrations of Ic=cagg and Ic=cun species (R = Ic=cagg/Ic=cun) changes on average from 0.79 ± 0.20 (as-cast) to 2.45 ± 0.77 (annealed). It is proposed that R values report on the relative densities of states (DOS) of aggregated and unaggregated species, and resonance Raman imaging is then used to spatially map morphology-dependent variations of R values and uncover subclassifications of these species. From both Rand frequency dispersion resonance Raman images of Ic=cagg and Ic=cun species, four distinct types of P3HT chains are identified and mapped in annealed P3HT/PCBM blend thin films: (i) highly aggregated/crystalline, (ii) partially aggregated, (iii) interfacial, and (iv) unaggregated/PCBM rich. The change in aggregation upon annealing is attributed to an increase in planarity of the P3HT chains that is determined from the ratios of C=C/C-C symmetric stretching mode intensities. [ABSTRACT FROM AUTHOR]

Despite intense, long-term interest in organic semiconductors from both an applied and fundamental perspective, key aspects of the electronic properties of these materials remain poorly defined. A particularly challenging problem is the molecular nature of positive charge carriers, that is, holes or oxidized species in organics. Here, the unique ability of single-molecule spectroelectrochemistry (SMS-EC) to unravel complex electrochemical process in heterogeneous media is used to study the oxidation of nanoparticles of the conjugated polymer poly(9,9-dioctylfluorene-co-benzothiadiazole). A reversible hole-injection charging process has been observed that occurs primarily by initial injection of shallow (untrapped) holes, but soon after the injection, a small fraction of the holes becomes deeply trapped. Good agreement between experimental data and simulations strongly supports the presence of deep traps in the studied nanoparticles and highlights the ability of SMS-EC to study energetics and dynamics of deep traps in organic materials at the nanoscale. [ABSTRACT FROM AUTHOR]

Low-temperature luminescence spectra from three salts of the trans-[OsO2(NCS)4]2- complex exhibit highly resolved vibronic structure in both metal-ligand high-frequency O=Os=O (885 cm-1) and lower-frequency Os-N(CS) (255 cm-1) symmetric stretching modes as well as in a ligand-centered CS stretching mode (858 cm-1). Band maxima range from 10000 to 12000 cm-1, and spectra contain irregular frequency intervals that correspond to transitions from more than one origin and phonon sidebands. Experimental band shapes are distinctly different for all three compounds and are calculated assuming harmonic potential energy surfaces for both the ground and emitting states. Normal-coordinate offsets along all displaced vibrational modes are determined and compared for the three compounds. The analyses reveal emitting-state displacement of high-frequency ligand-centered (CS) and metal-ligand (O=Os=O) symmetric stretching modes, leading to observed high-frequency intervals (855–880 cm-1) that do not match any frequencies determined from ground-state Raman spectra. The values for the high-frequency normal-coordinate offsets, ΔQO=Os=O and AQCS, were found to be on the order of 0.06 Å. Offsets along the 255 cm-1 Os-N mode varied noticeably between the three compounds and were largest for the compound with the largest value of ΔQCS. [ABSTRACT FROM AUTHOR]

Resolved vibronic structure in electronic spectra provides a detailed view into how molecular structure changes after absorption or emission of a photon. We report temperature- and pressure-dependent luminescence spectra of trans-[ReO2(pyridine)4]I. Low-temperature spectra reveal long vibronic progressions in the totally symmetric O=Re=O (907 cm–1) and Re-pyridine (211 cm–1) stretching modes, indicating large structural displacements along these normal coordinates. The luminescence band maximum is at ca. 15 500 cm–1. Room-temperature spectra are somewhat less-resolved; however, intervals closely matching the O=Re=O frequency (~870 cm–1) persist at higher temperatures. The variable pressure spectra exhibit distinct changes in the vibronic patterns, and luminescence energies decrease by 16 ± 2 cm–1/kbar (1 bar = 100 kPa). Low-temperature spectra are modeled using two-dimensional potential energy surfaces to represent the initial and final electronic states, from which the quantitative normal coordinate offsets can be determined. We then adapt this model to the room-temperature, pressure-dependent data where it is possible to determine how the offsets and other important spectroscopic parameters vary with the pressure-induced changes of the molecular structure. [ABSTRACT FROM AUTHOR]

The luminescence properties of thiocyanate and selenocyanate platinum(II) and palladium(II) complexes show strong variations with temperature and pressure. The d-d luminescence band maxima for [Pt(SCN)[sub 4]](PPh[sub 4])[sub 2] (1), [Pt(SCN)[sub 4]](n-Bu[sub 4]N)[sub 2] (2), and [Pt(SeCN)[sub 4]](n-Bu[sub 4]N)[sub 2] (4) complexes are centered at ca. 14500 cm[sup -1] whereas those of the [Pd(SCN)[sub 4]](n-Bu[sub 4]N)[sub 2] (3) and [Pd(SeCN)[sub 4]](n-Bu[sub 4]N)[sub 2] (5) complexes are approximately 2000 cm[sup -1] lower in energy. Low-temperature luminescence spectra from single-crystal samples have broad bands with highly resolved vibronic structure indicating large displacements of the emitting-state potential energy minimum along several metal-ligand normal coordinates. The largest displacements involve the totally symmetric (a[sub 1g]) stretching modes with frequencies of 295 cm[sup -1] (1), 303 cm[sup -1] (2), 274 cm[sup -1] (3), 195 cm[sup -1] (4), and 185 cm[sup -1] (5). The lower frequencies of these dominant progression-forming modes for the selenocyanate complexes lead to luminescence bands that are narrower by ca. 500 cm[sup -1] (fwhm) than those observed from the thiocyanate complexes. Under external pressures, the room-temperature luminescence intensities and lifetimes show considerable enhancement at pressures up to 40 kbar. This effect is largest for the palladium(II) complexes with lifetimes increasing from approximately 350 ns at ambient pressure up to 62 µs at 30 kbar, an increase by more than 2 orders of magnitude. The platinum(Il) complexes exhibit a significant, but noticeably lesser increase of luminescence lifetimes and intensities with increasing pressure. The temperature- and pressure-dependent luminescence decay behavior is rationalized using the emittingstate molecular geometry determined from the resolved low-temperature luminescence spectra combined with the strong-coupling limit of radiationless decay theory. [ABSTRACT FROM AUTHOR]

The pressure dependence of stretching vibrations involving metal-oxo multiple bonds is reported and discussed. Metal–ligand vibrational frequencies involving double and triple bonds increase with pressure according to the nature of the metal center and ancillary ligands. Pressure-dependent luminescence spectra, at room temperature, of a trans-dioxo rhenium(V) complex exhibit resolved vibronic structure with a progression in a high frequency metal-oxo stretching mode that shows a change of the intensity distribution with pressure. Raman and luminescence spectra are used to rationalize the unusual decrease of the metal-oxo stretching frequency observed for two complexes at pressures over 30 kbar. [Copyright &y& Elsevier]

Examines the effect of pressure on coupled electronic ground and excited states determined from luminescence spectra of trans-dioxorhenium(V) complexes. Exhibition of vibronic structure; Evidence of spectroscopic features from coupled electronic states; Presence of largest pressure effects in coupled electronic states close in energy.

Resonance Raman spectra of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester blend thin films display progressions of overtone and combination bands (up to two harmonics) involving the dominant symmetric C=C backbone stretching mode of P3HT that encode excited state vibrational displacements and dynamics information. Contributions from both crystalline (aggregated) and amorphous (unaggregated) P3HT domains are resolved and intensities are analyzed using the time-dependent theory of spectroscopy. Raman spectra, excitation profiles, and absorption spectra are simulated with the same parameters using a single electronic state description for each P3HT form. Time-dependent wavepacket overlaps expose vibrational coherence on sub-100 fs timescales, which is usually difficult to extract from conventional ultrafast pump-probe spectra and transients of polymer/fullerene blends. The results demonstrate the potential of simpler CW resonance Raman approaches to uncover excited state geometry changes and early vibrational dynamics from distinct morphological forms in polymer/fullerene blends. [ABSTRACT FROM AUTHOR]

Examines the luminescence intensity of [Pd(SCN)[sub 4](n-Bu[sub 4]N)[sub 2] single crystals at room temperature shows an unprecedented pressure-induced increase by several orders of magnitude. Variation of the luminescence intensities; Use of raman spectroscopy; Impact of temperature on luminescence intensity.