The statistical probability factor in triplet mediated photon upconversion: a case study with perylene.

Triplet-triplet annihilation photon upconversion (TTA-UC) is a process where two low-energy photons are converted into one higher-energy photon. A crucial component for an efficient upconversion process is the statistical probability factor (f), defined as the probability of the formation of a high-energy singlet state upon coupling of two low-energy triplet states. Theoretically, f depends on the energy level distribution, molecular orientation, inter-triplet exchange coupling of triplet dyads, and spin-mixing of resulting spin states (singlet, triplet, and quintet). However, experimental values of f for acene-based annihilators have been subject to large variations due to many factors that have resulted in the reporting of different f values for the same molecule. In this work, we discuss these factors by studying perylene as a case study annihilator, for which by far the largest variation in f = 16 to 100% has been reported. We systematically investigated the TTA-UC of PdTPBP:perylene, as a sensitizer-annihilator pair and obtained the experimental f = 17.9 ± 2.1% for perylene in THF solution. This limits the maximum TTA-UC quantum yield to 9.0% (out of 50%) for this annihilator. We found that such a low f value for perylene is largely governed by the energy-gap law where higher non-radiative losses due to the small energy gap between 2 × T1 and T2 affect the probability of singlet formation. Interestingly, we found this observation true for other acene-based annihilators whose emission ranges from the UV to the yellow region, thus providing a blueprint for future design of efficient TTA-UC systems.

Fullerene Complexation in a Hydrogen-Bonded Porphyrin Receptor via Induced-Fit: Cooperative Action of Tautomerization and C-H···π Interactions.

A supramolecular chiral hydrogen-bonded tetrameric aggregate possessing a large cavity and tetraarylporphyrin substituents was assembled using alternating 4H- and 2H-bonds between ureidopyrimidinone and isocytosine units, respectively. The aggregation mode was rationally shifted from social to narcissistic self-sorting by changing urea substituent size only. The H-bonded tetramer forms a strong complex with C60 guest, at the same time undergoing remarkable structural changes. Namely, the cavity adjusts to the guest via keto-to-enol tautomerization of the ureidopyrimidinone unit and as a result, porphyrin substituents move apart from each other in a scissor blade-like opening fashion. The rearrangement is accompanied by C-H···π interaction between the alkyl solubilizing groups and the nearby placed porphyrin π-systems. The latter interaction was found to be crucial for the guest complexation event, providing energetic compensation for otherwise costly tautomerization. We showed that only the systems possessing sufficiently long alkyl chains capable of interacting with a porphyrin ring are able to form a complex with C60. The structural rearrangement of the tetramer was quantitatively characterized by electron paramagnetic resonance pulsed dipolar spectroscopy measurements using photogenerated triplets of porphyrin and C60 as spin probes. Further exploring the C-H···π interaction as a decisive element for the C60 recognition, we investigated the guest-induced self-sorting phenomenon using scrambled tetramer assemblies composed of two types of monomers possessing alkyl chains of different lengths. The presence of the fullerene guest has enabled the selective scavenging of monomers capable of C-H···π interaction to form homo-tetrameric aggregates.

Triplet and singlet exciton diffusion in disordered rubrene films: implications for photon upconversion.

Triplet and singlet exciton diffusion plays a decisive role in triplet-triplet annihilation (TTA) and singlet fission (SF) processes of rubrene (Rub) films at low excitation power, and therefore has an important implication for TTA-mediated photon upconversion (UC). Although triplet diffusion in crystalline Rub was studied before, there is no quantitative data on diffusion in disordered Rub films most widely employed for NIR-to-Vis UC. The lack of these data hinders the progress of TTA-UC applications relying on a Rub annihilator (emitter). Herein, a time-resolved PL bulk-quenching technique was employed to estimate the exciton diffusion coefficient (D) and diffusion length (LD) in the neat Rub films as well as Rub-doped PS films at 80 wt% doping concentration, previously reported to be optimal in terms of UC efficiency. The impact of commonly utilized singlet energy collector (sink) DBP on exciton diffusion was also assessed, highlighting its importance exclusively on the dynamics of singlets in Rub films. Our study revealed that triplet diffusion lengths (LTD) of 25-30 nm estimated for the disordered Rub films are sufficient for encountering triplets from the neighboring sensitizer molecules at a low sensitizer PdPc concentration (0.1 wt%), thereby enabling the desired TTA domination regime to be reached. Essentially, the performance of Rub-based UC systems was found to be limited by the modest maximal LTD (up to ∼55 nm) in disordered films resulting from a short maximum triplet lifetime τT (∼100 µs) inherent to this emitter. Thus, to enhance the NIR-to-Vis TTA-UC performance, new emitters with a longer triplet lifetime in the solid state are required.

Temporal Dynamics of Solid-State Thermally Activated Delayed Fluorescence: Disorder or Ultraslow Solvation?

Time-resolved emission spectra of thermally activated delayed fluorescence (TADF) compounds in solid hosts demonstrate significant temporal shifts. To explain the shifts, two possible mechanisms were suggested, namely, slow solid-state solvation and conformational disorder. Here we employ solid hosts with controllable polarity for analysis of the temporal dynamics of TADF. We show that temporal fluorescence shifts are independent of the dielectric constant of the solid film; however, these shifts evidently depend on the structural parameters of both the host and the TADF dopant. A ≤50% smaller emission peak shift was observed in more rigid polymer host polystyrene than in poly(methyl methacrylate). The obtained results imply that both the host and the dopant should be as rigid as possible to minimize fluorescence instability.

Conformational disorder enabled emission phenomena in heavily doped TADF films.

Thermally activated delayed fluorescence (TADF) compounds doped in solid hosts are prone to undergo solvation effects, similar to those in the solution state. Emission peak shifts and changes in emission decay rates usually follow solid-state solvation (SSS). However, here we show that typical SSS behavior in heavily doped TADF films could be of a completely different origin, mistakenly attributed to SSS. Typically, increasing the doping load was found to redshift the emission peak wavelength and enhance the rISC rate. However, more in-depth analysis revealed that SSS actually is negligible and both phenomena are caused by the specific behavior of delayed emission. Increasing the concentration of the TADF compound was shown to enhance the concentration quenching of long-lived delayed fluorescence from conformer states with the largest singlet energy, eventually leading to a gradual redshift of the delayed emission peak wavelength. Concomitantly, the loss of long-lived delayed fluorescence entailed reverse intersystem crossing rate enhancement, though the rate-governing singlet-triplet energy gap was gradually increasing. The observed phenomena are highly unwanted, burdening molecular structure and OLED performance optimization.

TADF Parameters in the Solid State: An Easy Way to Draw Wrong Conclusions.

The successful development of thermally activated delayed fluorescence (TADF) OLEDs relies on advances in molecular design. To guide the molecular design toward compounds with preferable properties, special care should be taken while estimating the parameters of prompt and delayed fluorescence. Mistakes made in the initial steps of analysis may lead to completely misleading conclusions. Here we show that inaccuracies usually are introduced in the very first steps while estimating the solid-state prompt and delayed fluorescence quantum yields, resulting in an overestimation of prompt fluorescence (PF) parameters and a subsequent underestimation of the delayed emission (DF) yield and rates. As a solution to the problem, a working example of a more sophisticated analysis is provided, stressing the importance of in-depth research of emission properties in both oxygen-saturated and oxygen-free surroundings.

Impact of t-butyl substitution in a rubrene emitter for solid state NIR-to-visible photon upconversion.

Solid state NIR-to-visible photon upconversion (UC) mediated by triplet-triplet annihilation (TTA) is necessitated by numerous practical applications. Yet, efficient TTA-UC remains a highly challenging task. In this work palladium phthalocyanine-sensitized NIR-to-vis solid UC films based on a popular rubrene emitter are thoroughly studied with the primary focus on revealing the impact of t-butyl substitution in rubrene on the TTA-UC performance. The solution-processed UC films were additionally doped with a small amount of emissive singlet sink tetraphenyldibenzoperiflanthene (DBP) for collecting upconverted singlets from rubrene and in this way diminishing detrimental singlet fission. Irrespective of the excitation conditions used, t-butyl-substituted rubrene (TBR) was found to exhibit enhanced TTA-UC performance as compared to that of rubrene at an optimal emitter doping of 80 wt% in polystyrene films. Explicitly, in the TTA dominated regime attained at high excitation densities, 4-fold higher UC quantum yield (ΦUC) achieved in TBR-based films was caused by the reduced fluorescence concentration quenching mainly due to suppressed singlet fission. Under low light conditions, i.e. in the regime governed by spontaneous triplet decay, even though triplet exciton diffusion was obstructed in TBR films by t-butyl moieties, the subsequently reduced TTA rate was counterbalanced by both suppressed singlet fission and non-radiative triplet quenching, still ensuring higher ΦUC of these films as compared to those of unsubstituted rubrene films.

Achieving Submicrosecond Thermally Activated Delayed Fluorescence Lifetime and Highly Efficient Electroluminescence by Fine-Tuning of the Phenoxazine-Pyrimidine Structure.

Thermally activated delayed fluorescence (TADF) materials, combining high fluorescence quantum efficiency and short delayed emission lifetime, are highly desirable for application in organic light-emitting diodes (OLEDs) with negligible external quantum efficiency (EQE) roll-off. Here, we present the pathway for shortening the TADF lifetime of highly emissive 4,6-bis[4-(10-phenoxazinyl)phenyl]pyrimidine derivatives. Tiny manipulation of the molecular structure with methyl groups was applied to tune the singlet-triplet energy-level scheme and the corresponding coupling strengths, enabling the boost of the reverse intersystem crossing (rISC) rate (from 0.7 to 6.5) × 106 s-1 and shorten the TADF lifetime down to only 800 ns in toluene solutions. An almost identical TADF lifetime of roughly 860 ns was attained also in solid films for the compound with the most rapid TADF decay in toluene despite the presence of inevitable conformational disorder. Concomitantly, the boost of fluorescence quantum efficiency to near unity was achieved in solid films due to the weakened nonradiative decay. Exceptional EQE peak values of 26.3-29.1% together with adjustable emission wavelength in the range of 502-536 nm were achieved in TADF OLEDs. Reduction of EQE roll-off was demonstrated by lowering the TADF lifetime.

Minimization of solid-state conformational disorder in donor-acceptor TADF compounds.

Thermally activated delayed fluorescence (TADF) compounds with a flexible donor-acceptor structure suffer from conformational disorder in solid-state, which deteriorates their emission properties as well as OLED performance. Accordingly, TADF materials with predictable solid-state emission properties are highly desirable. In this work, we analyse the relation between the molecular rigidity and solid-state TADF properties. Two TADF compounds, 4,6-bis(2-methyl-4-(10H-phenothiazin-10-yl)phenyl)pyrimidine (PTZ-mPYR) and 1,2,3,4-tetrakis(carbazol-9-yl)-5,6-dicyanobenzene (4CzPN), with similar emission properties in toluene solutions but different rigidity of the molecular structure were systematically studied. The analysis was supplemented by comparison of solid-state TADF properties of PTZ-mPYR with its analogue 4,6-bis(4-(10H-phenoxazin-10-yl)phenyl)pyrimidine (PXZ-PYR), bearing a more sterically constrained planar electron-donor unit. All compounds showed conformational disorder in diluted polymer films; however its extent directly depended on the molecular structure. Large dispersion of singlet-triplet energy gaps resulted in remarkably prolonged TADF lifetime for PTZ-mPYR with a less sterically constrained donor unit. In contrast, weakened conformational disorder in rigid 4CzPN with sterically crowded donor units was shown to ensure rapid TADF decay with only a threefold lower solid-state rISC rate as compared to toluene. Similarly, selection of a more sterically constrained planar electron-donor unit was also shown to be preferable for lowering the conformational disorder. Our findings are important for the future design of compounds with efficient solid-state TADF as well as for the further application in OLEDs with low external quantum efficiency roll-off.

Emission wavelength dependence on the rISC rate in TADF compounds with large conformational disorder.

Large vibronic coupling between the local and charge-transfer triplet states is required for efficient reverse intersystem crossing in TADF compounds. This is ensured by low steric hindrance between donor and acceptor molecular units. However, flexible molecular cores show large conformational disorder and emission wavelength instability in solid films.

Enhancement of triplet-sensitized upconversion in rigid polymers via singlet exciton sink approach.

To increase the practical usefulness of solid-state sensitized upconversion (UC) materials as components of solar energy harvesting systems, it is important to identify and suppress loss mechanisms, and increase the UC quantum yield (Φ UC). Here we focus on a benchmark UC system consisting of the emitter 9,10-diphenylanthracene (DPA) and the sensitizer platinum octaethylporphyrin (PtOEP) in a rigid poly(methyl methacrylate) (PMMA) matrix, and show that one of the major losses originates from Förster resonant energy transfer (FRET) from DPA back to PtOEP. Even though DPA emission lies within the PtOEP transparency window, the quantitative assessment of singlet exciton diffusion for samples with a high DPA content evidences that long-range FRET results in effective exciton trapping by PtOEP. A dramatic factor-of-6 reduction of the DPA emission quantum yield occurs even at PtOEP concentrations as low as 0.05 wt%. To alleviate this problem, we demonstrate a new concept based on the introduction of highly emissive sink sites to trap the singlet excitons produced upon annihilation prior to their quenching by the sensitizer. For DPA/PtOEP blends in PMMA, 1,6-bis-[2,5-di(dodecyloxyphenyl)ethynyl]pyrene is shown to be a useful sink, which results in 1.5-fold increase of the Φ UC. A maximum Φ UC of 2.7% was achieved, which is among the highest reported values for rigid sensitized UC polymers.

Low-Threshold Light Amplification in Bifluorene Single Crystals: Role of the Trap States.

Organic single crystals (SCs) expressing long-range periodicity and dense molecular packing are an attractive amplifying medium for the realization of electrically driven organic lasers. However, the amplified spontaneous emission (ASE) threshold (1-10 kW/cm2) of SCs is still significantly higher compared to those of amorphous neat or doped films. The current study addresses this issue by investigating ASE properties of rigid bridging group-containing bifluorene SCs. Introduction of the rigid bridges in bifluorenes enables considerable reduction of nonradiative decay, which, along with enhanced fluorescence quantum yield (72-82%) and short excited state lifetime (1.5-2.5 ns), results in high radiative decay rates (∼0.5 × 109 s-1) of the SCs, making them highly attractive for lasing applications. The revealed ASE threshold of 400 W/cm2 in acetylene-bridged bifluorene SCs is found to be among the lowest ever reported for organic crystals. Ultrafast transient absorption spectroscopy enabled one to disclose pronounced differences in the excited state dynamics of the studied SCs, pointing out the essential role of radiative traps in achieving a record low ASE threshold. Although the origin of the trap states was not completely unveiled, the obtained results clearly evidence that the crystal doping approach can be successful in achieving extremely low ASE thresholds required for electrically pumped organic laser.

Nanoparticle-doped electrospun fiber random lasers with spatially extended light modes.

Complex assemblies of light-emitting polymer nanofibers with molecular materials exhibiting optical gain can lead to important advance to amorphous photonics and to random laser science and devices. In disordered mats of nanofibers, multiple scattering and waveguiding might interplay to determine localization or spreading of optical modes as well as correlation effects. Here we study electrospun fibers embedding a lasing fluorene-carbazole-fluorene molecule and doped with titania nanoparticles, which exhibit random lasing with sub-nm spectral width and threshold of about 9 mJ cm-2 for the absorbed excitation fluence. We focus on the spatial and spectral behavior of optical modes in the disordered and non-woven networks, finding evidence for the presence of modes with very large spatial extent, up to the 100 µm-scale. These findings suggest emission coupling into integrated nanofiber transmission channels as effective mechanism for enhancing spectral selectivity in random lasers and correlations of light modes in the complex and disordered material.

Structure-property relationship of blue solid state emissive phenanthroimidazole derivatives.

Seven new derivatives of phenanthro[9,10-d]imidazole having differenet substituents at the 1st and the 2nd positions of the phenanthroimidazole moiety were synthesized and characterized. The comparative study of their properties was performed employing thermal, optical, electrochemical and photoelectrical measurements. The properties of the newly synthesized compounds were compared with those of earlier reported derivatives of phenanthroimidazole and several interesting new findings were disclosed. Density functional theory calculations accompanied by optical spectroscopy measurements have shown the possibility of tuning the emission properties (excited-stated decay rate, fluorescence quantum yield, etc.) of phenanthro[9,10-d]imidazole derivatives via attachment of different substituents to the 1st and the 2nd positions. The most polar and bulky substituents linked to the 2nd position were found to have the greatest impact on the emissive properties of compounds causing (i) fluorescence quantum yield enhancement of dilute liquid and solid solutions (up to 97%), (ii) suppression of intramolecular torsion-induced nonradiative excited-state relaxation in rigid polymer films as well as (iii) inhibition of aggregation-promoted emission quenching in the neat films. Most of the studied compunds exhibited ambipolar charge transport character with comparable drift mobilities of holes and electrons. The highest hole and electron mobilities approaching 10-4 cm2 V-1 s-1 were observed for the derivative having a triphenylamino group at the 1st position of the imidazole ring and the phenyl group at the 2nd position. The estimated triplet energies of phenanthro[9,10-d]imidazole compounds were found to be in the range of 2.4-2.6 eV, which is sufficiently high to ensure effective energy transfer to yellow/red emitters.

The Role of Triplet Exciton Diffusion in Light-Upconverting Polymer Glasses.

Light upconversion (UC) via triplet-triplet annihilation (TTA) by using noncoherent photoexcitation at subsolar irradiance power densities is extremely attractive, particularly for enhanced solar energy harvesting. Unfortunately, practical TTA-UC application is hampered by low UC efficiency of upconverting polymer glasses, which is commonly attributed to poor exciton diffusion of the triplet excitons across emitter molecules. The present study addresses this issue by systematically evaluating triplet exciton diffusion coefficients and diffusion lengths (LD) in a UC model system based on platinum-octaethylporphyrin-sensitized poly(methyl methacrylate)/diphenylanthracene (emitter) films as a function of emitter concentration (15-40 wt %). For this evaluation time-resolved photoluminescence bulk-quenching technique followed by Stern-Volmer-type quenching analysis of experimental data was employed. The key finding is that although increasing emitter concentration in the disordered PMMA/DPA/PtOEP films improves triplet exciton diffusion, and thus LD, this does not result in enhanced UC quantum yield. Conversely, improved LD accompanied by the accelerated decay of UC intensity on millisecond time scale degrades TTA-UC performance at high emitter loadings (>25 wt %) and suggests that diffusion-enhanced nonradiative decay of triplet excitons is the major limiting factor.

Concentration effects on spontaneous and amplified emission in benzo[c]fluorenes.

Deep-blue-emitting benzo[c]fluorene-cored compounds featuring twisted peripheral moieties for suppressed concentration quenching of emission were synthesized and investigated as potential materials for light amplification. This detailed study of the effect of concentration on the spontaneous and stimulated emission, excited-state lifetime and susceptibility to form aggregates obtained for different benzofluorenes, has enabled the understanding of the concentration dependence of the amplified spontaneous emission (ASE) threshold and revealed the optimal concentration for the lowest threshold. The weak concentration quenching accompanied by high fluorescence quantum yield (>40%) and radiative decay rate (>5 × 10(8) s(-1)) have enabled the attainment of the lowest ASE threshold in the neat amorphous film of benzofluorene bearing dihexylfluorenyl peripheral moieties. Aggregate formation was found to negligibly affect the emission efficiency of the benzofluorene films; however, it drastically increased ASE threshold via the enhanced scattering of directional stimulated emission, and thereby implied the necessity to utilize homogeneous glassy films as the lasing medium. Although the bulky dihexylfluorenyl groups at the periphery ensured the formation of glassy benzofluorene films with the ASE threshold as low as 900 W cm(-2) (under nanosecond excitation), they adversely affected carrier drift mobility, which implied a tradeoff between ASE and charge transport properties for the lasing materials utilized in the neat form. Such a low ASE threshold attained in air is among the lowest reported for solution-processed neat films. The low threshold and enhanced photostability of benzofluorenes against fluorene compounds in air show great potential for benzofluorene-cored molecular glasses as active media for lasing applications.

Non-symmetric 9,10-diphenylanthracene-based deep-blue emitters with enhanced charge transport properties.

Realization of efficient deep-blue anthracene-based emitters with superior film-forming and charge transport properties is challenging. A series of non-symmetric 9,10-diphenylanthracenes (DPA) with phenyl and pentyl moieties at the 2nd position and alkyl groups at para positions of the 9,10-phenyls were synthesized and investigated. The non-symmetric substitution at the 2nd position enabled to improve film forming properties as compared to those of the unsubstituted DPA and resulted in glass transition temperatures of up to 92 °C. Small-sized and poorly conjugated substituents allowed to preserve emission in the deep blue range (<450 nm). Substitution at the 2nd position enabled to achieve high fluorescence quantum yields (up to 0.7 in solution, and up to 0.9 in the polymer host), although it caused an up to 10-fold increase in the intersystem crossing rate as compared to that of the unsubstituted DPA. Further optimization of the film forming properties achieved by varying the length of the alkyl groups attached at the 9,10-phenyls enabled to attain very high hole drift mobilities (∼5 × 10(-3)-1 × 10(-2) cm(2) V(-1) s(-1)) in the solution-processed amorphous films of the DPA compounds.

Phenylethenyl-substituted triphenylamines: efficient, easily obtainable, and inexpensive hole-transporting materials.

Star-shaped charge-transporting materials with a triphenylamine (TPA) core and various phenylethenyl side arm(s) were obtained in a one-step synthetic procedure from commercially available and relatively inexpensive starting materials. Crystallinity, glass-transition temperature, size of the π-conjugated system, energy levels, and the way molecules pack in the solid state can be significantly influenced by variation of the structure of these side arm(s). An increase in the number of phenylethenyl side arms was found to hinder intramolecular motions of the TPA core, and thereby provide significant enhancement of the fluorescence quantum yield of the TPA derivatives in solution. On the other hand, a larger number of side arms facilitated exciton migration through the dense side-arm network formed in the solid state and, thus, considerably reduces fluorescence efficiency by migration-assisted nonradiative relaxation. This dense network enables charges to move more rapidly through the hole-transport material layer, which results in very good charge drift mobility (µ up to 0.017 cm(2) V (-1) s(-1)).