氟苯并芴核心聚合物中光谱发光和激光增益阈值特性检测方案(激光产品)

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ResearchGateSee discussions， stats， and author profiles for this publication at： https：//www.researchgate.net/publication/282657087 CrossMark1.0×10°s-l) and smaller concentration quenching， fluorene-cored oligomers exhibiteddown to one order of magnitude lower ASE thresholds at higher concentrations as compared tothose of benzofluorene counterparts. The lowest threshold (300 W/cm²) obtained for the fluorene-cored oligomers at the concentration of 50 wt % in polymer matrix is among the lowest reportedfor solution-processed amorphous films in ambient conditions， what makes the oligomers promis-ing for lasing application. Great potential in emission amplification was confirmed by high maxi-mum net gain (77cm-) revealed for these compounds. Although the photostability of theoligomers was affected by photo-oxidation， it was found to be comparable to that of various or-ganic lasing materials including some commercial laser dyes evaluated under similar excitationconditions. O 2015 AIP Publishing LLC. [http：//dx.doi.org/10.1063/1.4927555] Organic lasers offer a number of advantages over thecoherent light sources based on conventional semiconductors.The main benefits include simple device fabrication via solu-tion processing of organic compounds (e.g.， ink-jet printing)，wide emission-wavelength tunability due to the broad gainspectra of organic lasing materials，negligible temperaturedependence of stimulated emission threshold， output power，and emission wavelength as a result of strongly localizedexcited states(excitons)) involved inlight emission.34Therefore， organic lasers are considered to be attractive forapplications in sensing， optical data communications， mem-ory， displays， and spectroscopy in general. One of the mainissues preventing realization of organic electrically drivenlaser is related to poor carrier mobility，which in the case ofamorphous films may reach 0.01-1 cm²/V/s and 43cm²/V/sin highly aligned films.8 The low mobility induces polaronabsorption， which prevents from reaching sufficiently highcurrent densities， and thus， threshold exciton concentrationsrequired for stimulated transitions. Additional and no less im-portant factor limiting concentration is exciton-exciton anni-hilation. .10 Since carrier/exciton hopping efficiency dependsstrongly on molecular packing morphology in the solid films，both issues (low carrier mobility and exciton-exciton annihila-tion) are primarily governed by intermolecular interactions.Strong intermolecular interactions are conditioned by highchromophore density， which also favors enhanced carrier mo-bility and is crucial for realization of efficient laser.On theother hand， high chromophore concentrations facilitateexciton-exciton annihilation and unfortunately， in most cases， degrade emission efficiency as a result of the formation ofpoorly emissive aggregate or excimer species. The efforts todecrease intermolecular coupling， e.g.， by diluting chromo-phores in a host material or exploiting bulky dendritic or spirocompounds，may preserve the emission efficiency and ensurelow amplified spontaneous emission (ASE) threshold； how-ever， the reduced coupling will inevitably deteriorate chargetransport. And indeed， recent studies suggested a tradeoffbetween ASE and charge transport properties for the lasingmaterials utilized in the condensed form.Obviously， delib-erate control of intermolecular interaction is necessary toobtain both high carrier drift mobility and high emission effi-ciency (for low ASE threshold) at high concentrations. Here， we report on the ASE properties of blue-emittingfluorene-based oligomers as a function of their concentrationin polymer matrix and demonstrate their superiority in lightamplification at high concentrations. Featuring excellent car-rier drift mobilities (close to 10-’cm²/V/s) in the amorphousfilms， the oligomers were designed to contain either fluoreneor benzo[c]fluorene core and singly bonded fluorenyl periph-eral groups decorated with different-length alkyl moieties forinvestigating molecular packing effects on the concentrationquenching of emission. The chemical structures of the stud-ied oligomers (BF-et， BF-hex， F-et， F-hex) are shown in theinsets of Fig.1. For the synthesis and carrier drift mobilitiesof the compounds， see supplementary material and thedetails provided therein. Spontaneous emission of the compounds was excited atthe absorption band maximum by a xenon lamp coupled tothe monochromator (FWHM<10 meV) and measured usinga back-thinned CCD spectrometer PMA-11 (Hamamatsu). Wavelength (nm) FIG. 1. Spontaneous (dashed line) and amplified (solid line) emission spectraof the benzofluorene-cored (a) and fluorene-cored (b) oligomers in PS matrixat 10 wt % concentration. Insets： chemical structure of the oligomers. ASE measurements were carried out by exciting the filmswith a 40-um-wide laser stripe focused on the film surfacenear its freshly cleaved edge by using a cylindrical lens (theso-called thin excitation stripe geometry)'14，15and detectingthe emission propagating along the stripe. Wavelength-tunable optical parametric amplifier (EKSPLA) pumped bynanosecond Nd：YAG laser (pulse duration 5 ns， repetitionrate 10Hz) was used to excite the films at the absorptionband maximum. Fluorescence quantum yields (Dp) wereestimated using the integrating sphere. Fluorescence transi-ents were measured at fluorescence band maximum using atime-correlated single photon counting system PicoHarp 300(PicoQuant)， which utilized a semiconductor diode laser(repetition rate 1 MHz， pulse duration 70 ps， emission wave-length 370 nm) as an excitation source. Figure 1 shows spontaneous emission and ASE spectraof the oligomers in a rigid polymer (polystyrene PS) matrixat a concentration of 10 wt %. The oligomers were excited atthe absorption band maxima， i.e.， 367 nm for BF-et， BF-hexand 354 nm for F-et， F-hex. The employed concentration issufficiently low for these compounds to neglect intermolecu-lar interactions. Clearly， atsuchlow1 concentrations.different-length alkyl moieties (ethyl or hexyl) play no rolein the emission， since they are not conjugated. The spontane-ous emission spectra are broad and exhibit characteristicvibronic structure. The spectra of BF-et and BF-hex oligom-ers are shifted by 25 nm to longer wavelengths as comparedto those of F-et and F-hex as a result of enhanced conjuga-tion of benzofluorene core. Much narrower ASE bandsobtained at high pumps emerge at the 0-1 vibronic transition of spontaneous emission， which is typical for a quasi-four-level vibronic systems.3 Since high chromophore concentration is essential forattaining high carrier mobility， and ultimately， for efficientlasing， low emission quenching and high radiative relaxationrates (k，) must be ensured at such conditions. To this end， con-centration quenching of spontaneous emission was evaluatedby dispersing fluorene- and benzofluorene-cored oligomers inan inert PS matrix and estimatingDp dynamics vs moleculeconcentration in the range of 0.5-100 wt %(Fig.2). Dp valuesin the Fig. 2 are accompanied by excited state lifetime (t)data estimated from fluorescence transients measured at dif-ferent oligomer concentrations in PS. At low concentrations，fluorene-cored oligomers expressed somewhat higher Pr(90%) and shorter t (0.7ns) as compared to these values(@p=78% and t=1.0ns) for benzofluorene-cored com-pounds. This implies significantly larger k， (1.3×10s) influorene-cored oligomers F-et and F-hex in respect to that(7.8×10°s) in BF-et and BF-hex making the formeroligomers more promising for light amplification. It is worthnoting that increasing compound concentration up to 10 wt %(for benzofluorenes) or 20 wt % (for fluorenes) had negligibleinfluence on the Dp and t， whereas further increase of the con-centration up to 100 wt% resulted in roughly 2-fold drop ofthese values. Only 2-fold reduction of Dp and t in the neatfilms is caused by enhanced nonradiative relaxation inducedby exciton migration and infers rather weak emission quench-_ing7，16，17and thereby weak intermolecular coupling. This wasalso confirmed by the small red shift of emission spectra aswell as single exponential decay of the transients preservedup to the highest oligomer concentrations in PS. FIG. 2. Fluorescence quantum yields (solid points) and average fluorescencelifetimes (empty points) of the benzofluorene-cored (a) and fluorene-cored(b) oligomers as a function of their concentration in PS matrix. A similar quenching efficiency observed for oligomersBF-et(F-et) and BF-hex(F-hex) with increasing concentrationindicates minor influence of the length of alkyl moietiesattached to the peripheral fluorenyl groups. Meanwhile， theonset of quenching at higher concentrations (>20 wt%) forfluorene-cored oligomers is due to the longer alkyl moietiesattached to the core. The longer alkyls result in a less denseintermolecular packing and， therefore， are more efficient insuppressing concentration effects. Generally， the weak con-centration quenching observed for the studied oligomers canbe also attributed to their twisted molecular geometries result-ing from significant steric hindrance of the neighboring bulkyfragments as confirmed by the density functional theory cal-culations. The calculations revealed large dihedral angles (upto 59°) between the core and peripheral fragments. The impact of intermolecular coupling on the amplifiedemission of oligomers was investigated by evaluating ASEthreshold (IASE) as a function of their concentration in PS ma-trix. IASE is one of the key parameters describing the feasibil-ity of the material to be employed in lasing systems as anactive medium.18-21 IrASE was determined from the pumppower dependences of the edge emission spectra like thoseshown for the oligomer F-et in Fig. 3(a). The sudden emissionband narrowing with the increasing pump， which was accom-panied by an abrupt change in the emission intensity from lin-ear to superlinear (inset of Fig.3(a))， served as a signature ofIASE. All the studied oligomers exhibited continuous decreaseof IASE with increasing concentration up to a certain point(optimal concentration) and further increase of IASE after thispoint (Fig. 3(b)). The initial decrease of IASE is related to thereduction of absorptive optical losses due to the reducedintermolecular distances. In fact， compensation for the opticallosses by expressing IASE in the units of absorbed (not inci-dent) power density should result in the independence of IAS1SEas a function of compound concentration up to the pointwhere intermolecular interactions are still negligible (see Fig.S2 in the supplementary material3). However， the laterincrease of IASE occurring above the optimal concentration of6-10 wt % for BF-et and BF-hex，and 10-50 wt % for F-hexand F-et is determined by the concentration quenching effect E Wavelength (nm) Concentration in PS (wt%) FIG.3. (a) Excitation power dependence of the edge emission spectra of F-et in PS matrix at 50 wt % concentration. Inset： emission peak intensityvsexcitation power density (Iexc)； arrow indicates onset of ASE. (b) ASEthreshold as a function of oligomer concentration in PS matrix. Lines areguides for the eye. as discussed above. Evidently， the smaller concentrationquenching obtained for fluorene-cored oligomers (see Fig.2)favored higher optimal concentrations for ASE. Furthermore，owing to the larger k，， fluorene-cored oligomers exhibiteddown to one order of magnitude lower ASE thresholds ascompared to those (~3kW/cm) of benzofluorene counter-parts. The lowest irASEth of 300W/cm² (1.5 pJ/cm)) wasattained for the fluorene-cored oligomer F-et at the concentra-tion of 50 wt % in PS. Remarkably， this is one of the lowestASE thresholds reported for solution-processed amorphousfilms prepared and measured in ambient conditions undernanosecond excitation.20，22-26The optimal concentration(50 wt %) is also very high as compared to that found forother lasing materials. It emphasizes strongly suppressed con-centration quenching for F-et oligomer. The optical gain in the light amplifying oligomers wasestimated by using the variable stripe length technique.The PS films containing optimal oligomer concentrationwere pumped at different excitation densities with the pumplength gradually increasing from 0 to 0.2 cm. The emissionintensity， I(2)， recorded as a function of the stripe length， l，was modeled using the Eq.(1) to estimate the net opticalgain coefficient， g(A)， where AIs(a，l) defines spontaneous emission proportional tothe pumping intensity. The modeling of I(2) was carried outat the wavelength corresponding to maximum of ASE inten-sity. The fitted data for BF-hex and F-hex oligomers are dis-played in Fig. 4. The inconsistence of experimental andmodeling results for large stripe lengths (1>0.1 cm) wascaused by the gain saturation. The variation of pump inten-sity from 2×IASE to 8×IASE resulted in the boost of g from14 to 45cm- and from 34 to 77 cm- for BF-hex andF-hex， respectively. The obtained maximum experiment lim-ited g of 77cmis among the highest reported for organiccompounds.26-30 High gain is advantageous since it enables FIG. 4. ASE peak intensity as a function of excitation stripe length for BF-hex (a) and F-hex (b) in PS matrix at 10 wt % concentration. Pump densities(2×IASE，4×IASE，8×IASE) and estimated optical gain values， indicated. manufacturing of compact lasers， which are more tolerant tofabrication defects. To assess the photostability of the oligomers at the con-ditions approaching those of a working laser device， ASE in-tensity was monitored as a function of the number of pumppulses at the pump density equal to 2×I (Fig. 5). Thephotostability lifetime (t) of each oligomer was thendefined as the number of pump pulses causing a decrease ofASE intensity to half its initial value. tase values were esti-2mated for the oligomers in air and nitrogen atmosphere toreveal the influence of detrimental photo-oxidation， i.e.， gen-eration of carbonyl defects.9iBF-et and BF-hex expressedsimilar t4SE of 1.3×10 and 4×10+ pump pulses in air andinert atmosphere， respectively， whereas， for F-et and F-hex，these lifetimes were 2×10 and 6×10 pulses. The greaterphotostability revealed for fluorene-cored oligomerswasmainly due to the reduced ASE threshold， whereas 3-foldworse photostability obtained in air as that in inert atmos-phere was evidently conditioned by photo-oxidation. Theestimated rAsE well exceeding 1 ×10 pump pulses was found/2 Wto be comparable or even better as the lifetimes of othersmall-molecule or polymer compounds as well as some com-mercial laser dyes evaluated in a similar nanosecond/picosec-ond pulse regime.16.31 To summarize， spontaneous and amplified emissionproperties of fluorene and benzofluorene oligomers wereassessed as a function of their concentration in polystyrenematrix to reveal their potential for application in organiclasers. Unlike the majority of high-emission-efficiency chro-mophores， the oligomers were found to express small concen-tration quenching of emission， and thus， weak intermolecularinteractions as a result of sterically hindered fluorenyl periph-eral groups endowed with different-length alkyl moieties.Furthermore， superior charge transport properties (with car-rier drift mobilities close to 10-2cm²/V/s) revealed in thewet-casted amorphous films of blue-emitting oligomersimplied potential for reducing threshold current density inoligomer-based lasing devices. Exhibiting higher emission Number of pump pulses FIG. 5. Normalized ASE intensity dependence on the number of pumppulses in air (empty points) and nitrogen atmosphere (solid points) for F-hexand BF-hex dispersed in PS matrix at the concentration corresponding to thelowest IASE. The excitation power densities were 2×IASE. quantum yields(90%)and larger radiative decay rates(>1.0×10°s-l)aaccompaniedbbyyweak concentrationquenching of emission fluorene-cored oligomers demon-strated down to one order of magnitude lower ASE thresholdsat higher oligomer concentrations as compared to those ofbenzofluorene-cored compounds. This made the fluorene-cored oligomers more attractive as the light amplifying me-dium. High value of maximum net gain (77cm-) followedby the lowest ASE threshold (300W/cm) attained for thefluorene-cored oligomers at remarkably high concentration(50 wt %) in polymer matrix indicated great potential of thesecompound for lasing application. Importantly， the photo-stability of the studied oligomers was demonstrated to be asgood as that of the other organic lasing compounds. The research was funded by Grant No. VP1-3.1-SMM-10-V-02-023 from the European Social Fund Agency. ( S . R. Forrest， N ature 4 28，9 1 1 ( 2 004). ) ( 2 J. Clark and G. Lanzani， Nat. 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Reuse of AIP content is subject to the terms at： http：//scitation.aip.org/termsconditions. Downloaded to IP： Deliberate control of intermolecular interactions in fluorene- and benzofluorene-cored oligomerswas attempted via introduction of different-length alkyl moieties to attain high emission amplificationand low amplified spontaneous emission (ASE) threshold at high oligomer concentrations.Containing fluorenyl peripheral groups decorated with different-length alkyl moieties, the oligomerswere found to express weak concentration quenching of emission, yet excellent carrier driftmobilities (close to 102 cm2/V/s) in the amorphous films. Owing to the larger radiative decayrates (>1.0109 s1) and smaller concentration quenching, fluorene-cored oligomers exhibiteddown to one order of magnitude lower ASE thresholds at higher concentrations as compared tothose of benzofluorene counterparts. The lowest threshold (300W/cm2) obtained for the fluorenecoredoligomers at the concentration of 50 wt % in polymer matrix is among the lowest reportedfor solution-processed amorphous films in ambient conditions, what makes the oligomers promisingfor lasing application. Great potential in emission amplification was confirmed by high maximumnet gain (77 cm1) revealed for these compounds. Although the photostability of theoligomers was affected by photo-oxidation, it was found to be comparable to that of various organiclasing materials including some commercial laser dyes evaluated under similar excitationconditions.