DOI: 10.1002/cphc.201300667

Determining the Critical Particle Size to Induce EnhancedEmission in Aggregates of a Highly Twisted TriarylamineAkshay Kokil,*[a] J. Matthew Chudomel,[b] Boqian Yang,[b] Michael D. Barnes,[b]

Paul M. Lahti,*[b] and Jayant Kumar*[a]

Photoluminescent (PL) conjugated organic molecules havefound numerous applications in the area of sensing and organ-ic electronics.[1–5] Solution PL at high fluorophore concentra-tions is typically much weakened or quenched[6, 7] by aggregateformation, which produces multiple non-radiative pathways forexcited-state relaxation to its ground state. Such PL quenchingis often further pronounced in the solid state, with deleteriouseffects on applications in organic electronics and optical sens-ing.[1, 4, 8, 9] Thus, photophysical behavior of organic chromo-phores in aggregate states is an increasingly important topicof activity.[10] Some organic dyes display strongly increased PLupon aggregation, in contrast to the commonly observed PLquenching.[11, 12] This phenomenon has been termed aggrega-tion-induced enhanced emission (AIEE), attributed to the sup-pression of non-radiative pathways for relaxation of the excitedstate through aggregation-assisted charge transfer, intramolec-ular rotations, p–p stacking or a combination of these.[12–14]

AIEE has been reported in donor–acceptor dyes with strongelectron-donating and -withdrawing groups, with increased ag-gregated-state emission attributed to restricted intramolecularrotation, and reduced co-facial intermolecular p overlap.[13, 15, 16]

AIEE occurring just at onset of aggregation has been associat-ed with hypsochromic shift of the emission maximum ata slightly increased intensity, followed by a large rise in emis-sion intensity with a stronger hypsochromic shift.[12, 13, 17] Theconditions under which AIEE onset first occurs can potentiallyprovide fundamental insight in how solid-state particle growthaffects PL, such as the effects of particle size and the surround-ing environment. However, close investigation of AIEE underinitial onset conditions have so far received limited attention.Determining AIEE onset correlations with particle size growthseems particularly germane for understanding changes in PLas fluorophores aggregate and potentially organize into solids.

For the recently reported[18] highly twisted donor–acceptortriarylamine 9-(N,N-dianisylamino)anthracene (9DAAA), wehave not only found a new example of AIEE in binary solvent

mixtures, but correlated the onset of AIEE with growth to a crit-ical aggregate size. The medium in which the aggregates formalso influences the critical size needed to yield AIEE. As the ag-gregates increase beyond this critical size, shielding of the ex-citons from quenching by the dispersion medium leads to fur-ther enhancement in the emission intensity, giving PL spectrathat closely resemble those from neat solid 9DAAA films.

9DAAA (structure in Figure 1) is an ambipolar molecule witha weak electron-accepting anthracene unit attached to an elec-tron-rich dianisylamine unit. Steric interactions between theanthracene peri-CH bonds and the dianisylamine unit requirea highly twisted ground-state conformation that favors intra-molecular charge transfer (ICT) upon photoexcitation. Asa result, 9DAAA exhibits strongly solvent-dependent emissionmaxima and PL decay lifetimes due to excited-state chargeseparation in polar solvents.[18] The PL of 9DAAA is stronglyquenched in polar media, due to preferential ICT and nonradia-tive decay in solution, but neat solid-state 9DAAA exhibits read-ily observable PL, possibly due to a lack of anthracene p stack-ing in its crystal lattice. The restricted and twisted conforma-tional geometry of 9DAAA, its tendency for photoexcited ICT,and its combination of PL quenching in polar solutions with PLemission in the neat solid state, made this molecule a highlypromising AIEE candidate.

AIEE investigations for 9DAAA were performed in solvent–non-solvent mixtures, using tetrahydrofuran (THF) or acetoni-trile (ACN) as solvent, and distilled water as the non-solvent,since these components are mutually miscible. The 9DAAAconcentration and total solvent–non-solvent mixture volumeswere held constant. The solvent–non-solvent mixture wasfreshly prepared for every measurement and a predeterminedaliquot of 9DAAA stock solution added, followed by vigorousmixing with an orbital shaker for 1 min. After equilibrating theobtained solutions/dispersions for 2 min, absorption and PLspectra were recorded. The dispersions remained dependablystable for at least 30 min, during which the spectroscopic in-vestigations were conducted.

The dielectric constants of THF, ACN, and water are 7.6, 38.0,and 81.0, respectively.[19] 9DAAA absorbs in similar regions ofthe UV/vis spectrum in THF or in ACN, but its emission in THFis blue-shifted by 40 nm and about five times more intensecompared to ACN (Figure S1, Supporting Information). This isconsistent with the previously reported 9DAAA solvatochromicbehavior,[18] where increasing solvatochromic red-shift and de-creased emission efficiency in higher-polarity solvents were at-tributed to ICT from the dianisylamine donor units to the an-thracene acceptor unit.

[a] Dr. A. Kokil, Prof. J. KumarCenter for Advanced MaterialsUniversity of Massachusetts, LowellOne University Ave. Lowell, MA 01854 (USA)E-mail : akshay_kokil@uml.edu

jayant_kumar@uml.edu

[b] J. M. Chudomel, Dr. B. Yang, Prof. M. D. Barnes, Prof. P. M. LahtiDepartment of ChemistryUniversity of Massachusetts, Amherst710 North Pleasant Street, Amherst, MA 01003 (USA)E-mail : lahti@chem.umass.edu

Supporting information for this article is available on the WWW underhttp://dx.doi.org/10.1002/cphc.201300667.

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A 5 mm solution of 9DAAA in ACN excited at 440 nm displaysa weak emission maximum at 612 nm. As the ACN content de-creases from 100 % to 40 % in ACN:water mixtures, the 9DAAAPL intensity decreases and the PL peak maximum red-shifts bya few nanometers (Figure 1 a, and Figure S2). This behavior isagain consistent with previously reported behavior for 9DAAA,where increased polarity (more water) in the environmentgives more PL quenching.

At 30 % v/v ACN:water, the PL spectrum broadens abruptlywith a hypsochromic shift of ~30 nm (~110 meV) and contin-ued weak intensity (Table 1, details in Figure S2 of the Support-ing Information). At 20 % v/v ACN:water, the 9DAAA PL maxi-mum shows a further hypsochromic shift of 12 nm, band nar-rowing, and a dramatic increase in PL intensity (Figure 1 a).This is signature behavior for AIEE in a solvent–non-solventmixture.[12] The hypsochromic shift can be attributed toa change in the emitting fluorophores from individual mole-cules surrounded by polar solvent, to emitters that are incor-porated into a less polar environment within aggregates of

9DAAA molecules. The continu-ing low PL intensity at 30 % v/vACN:water (AIEE onset) can beexplained by consideration ofthe size of the aggregates underthose conditions, as describedbelow.

For 90 % water in the mixtures(10 % v/v ACN:water), the aggre-gation-induced PL intensity en-hancement is 25-fold versus theminimum PL intensity at 30–40 % v/v ACN:water. This corre-sponds to a net 7-fold increaserelative to the PL intensity inpure ACN (Figure 1 b). Thechanges in emission color andintensity with increased watercontent are manifest not only inthe Figure 1 a spectra, but evento the naked eye, as shown in

Figure 1 c. Notably, for 90 % or greater water content in theACN:water mixtures, the 9DAAA PL spectrum essentially over-lays the 9DAAA drop-cast solid film PL spectrum[18] (see Fig-ure S3). This supports the hypothesis that emission in >70 %water content mixtures arises primarily from 9DAAA moleculesin a solid-like environment, that is, in aggregates.

The absorption spectra of 9DAAA in the ACN:water mixturesalso support aggregation at high water content. The UV/Vislong-wavelength absorption maximum for 50 mm 9DAAA dis-plays a small hypsochromic shift from 443 to 437 nm as watercontent increases from 0 % to a fraction of 60 % v/v (Fig-ure 2 a). However, at 70 % v/v water, the absorption bandbroadens, and at 80–90 % v/v water a broad absorption peakcentered at ~460 nm is observed, a significant bathochromicshift versus the pure ACN value. Also, the excitation spectrumfor 9DAAA in 90 % v/v water displays a 24 nm bathochromicshift of its long-wavelength peak relative to the pure ACNresult (Figure 2 b), corresponding with the absorption spectralshifts. Both spectra at high water content also correspond wellwith the neat solid-film absorption spectrum for 9DAAA (Fig-ure S4),[17] again consistent with a dominant solid-like aggre-gated condition for 9DAAA in high-water-content mixtures.

In dilute solutions of fluorophores, dynamic intramolecularrotations help quench the excited state through nonradiativepathways. ICT-induced nonradiative quenching also is favoredby higher polarity media in ambipolar systems like 9DAAA.[20]

But, in an aggregated state, intramolecular rotations should berestricted, reducing nonradiative pathways and enhancing re-laxation of the photoexcited state through radiative chan-nels.[12] 9DAAA inherently favors a bisected ground-state ge-ometry for which rotation of the dianisylamine group relativeto the anthracene unit may occur in solution, but is unlikely ina highly aggregated state. The twisted ground-state conforma-tion of 9DAAA[17] may also hinder co-facial intermolecular p

overlap in an aggregate, as is seen in the known[18] 9DAAAcrystal structure. The lack of co-facial overlap favors radiative

Figure 1. a) Photoluminescence spectra of 9DAAA (5 mm) in different ACN:water mixtures. b) Photoluminescenceintensity of 9DAAA relative to pure ACN samples as a function of the water content. c) Vials containing 9DAAA indifferent ACN:water mixtures (100 % ACN left to 10:90 ACN:water (v/v) right).

Table 1. Fluorescence and aggregation properties of 9DAAA aggregatesin ACN:water mixtures.

CH3CN[%]

PL maxi-mum[nm]

RMS ag-gregatediameter[nm]

Weightedmean life-time[a]

<t> [ns]

A1[a]

[%]t1

[a]

[ns]A2

[a]

[ns]t2

[a]

[ns]

100 612 n/a 4.2 4.250 615 84 2.3 2.340 616 96 4.0 63.0 1.7 37.0 8.130 584 131 3.0 54.5 1.2 45.5 5.220 572 171 4.2 47.6 1.4 52.4 6.710 570 254 6.3 41.0 2.2 59.0 9.2

[a] Wavelengths used for the time-resolved photoluminescence measure-ments were 609, 611, 611, 580, 580, and 574 nm, respectively, goingdown the columns in this table. See Figure 4.

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channel exciton decay in a solid, rather than intermolecularquenching. So, the AIEE observed in 9DAAA may be due a com-bination of reduced intramolecular molecular motions and re-duced co-facial p overlap in the aggregated state.

To investigate 9DAAA aggregate size evolution below andabove the AIEE onset concentration (30:70 v/v ACN:water), dy-namic light scattering (DLS) studies were carried out using thesame ACN–water samples that were used for the PL studies.The observed DLS mean particle size is plotted versus waterfraction in Figure 3. Aggregation of 9DAAA was detected for�30 % v/v water in the ACN/water mixture, above which theaverage particle size increased with increasing water content.Although DLS thereby indicated some aggregation of 9DAAAmolecules for water content less than 70 % v/v in the ACN:wa-ter mixtures, the mean particle size grew to 131 nm beforeAIEE onset was observed at 30:70 v/v ACN:water.

The hypsochromic PL spectral shift at �70 % water contentindicates sufficient aggregation to shield the emitting 9DAAAfluorophores somewhat from polar solvent molecules, but the

low PL intensity indicates that relaxation of the excited state isstill strongly influenced by the surrounding polar medium. Thiscan be explained by ready hopping of any photo-generatedexcitons to the surface of smaller aggregates (<131 nm),where they are exposed to the polar ACN–water mixture envi-ronment, and so still relax primarily by nonradiative mecha-nisms. As the aggregate size increases further with increasingwater fraction, exciton migration to the surface is more diffi-cult, and there is a greater fraction of relaxation inside the ag-gregate by radiative mechanisms with enhanced PL intensity,giving spectra qualitatively similar to the emission in neat solidfilm 9DAAA.

To probe the photophysical changes associated with AIEE,time-resolved photoluminescence (TRPL) spectroscopy wasperformed using 5 mm 9DAAA in ACN:water mixtures, monitor-ing lifetimes at the emission maximum. Representative curvesare shown in Figure 4. The PL lifetime decay curves for 100 %ACN and 50:50 ACN:water fit well to mono-exponential model-ing, but for �40 % water content samples a double-exponen-tial model was required, I(t) = A1exp(�t/t1) + A2exp(�t/t2),where t1 and t2 represent different decay channels. Overallweighted mean lifetimes <t> of PL decay were calculated ac-

Figure 2. a) Normalized UV/Vis absorption spectra for 9DAAA in different ACN:water mixtures. b) Normalized excitation spectra recorded at emission maxi-mum for 9DAAA in 100 % ACN (a) and ACN:water mixture (10:90 ACN:water, c).

Figure 3. 9DAAA particle size as a function of water content in ACN:watermixtures. Error bars represent the standard deviation of the mean particlesize obtained by averaging three dynamic light-scattering measurements.

Figure 4. TRPL decay curves for 9DAAA in ACN:water mixtures. Percentagesare v/v ACN, and wavelengths represent position of the peak maximumwhose lifetime was monitored.

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cording to <t> = (A1t1 + A2t2)/(A1+A2), and are given inTable 1.

The fast lifetime TRPL process t1 is attributable to fluores-cence decay of solvated 9DAAA molecules; this lifetime de-creases as solvent polarity increases. Both the magnitude andtrends with polarity for t1 are consistent with the previous9DAAA studies[17] in varying polarity organic solvents. We pre-sume the slow decay process t2 to be associated with emissionfrom excitons formed with aggregates that are shielded fromsolvent-induced quenching; their lifetimes increase as aggre-gate size increases. As water content increases above 40 %, theslow decay channel process appears and becomes more prom-inent relative to the fast channel process (Table 1); this behav-ior is qualitatively similar to that of AIEE previously reportedfor other systems.[13, 21] Also, as the water fraction increasesfrom pure ACN, the overall weighted mean lifetime at first de-creases—consistent with excited state quenching in morepolar media[1, 18, 22]—but then increases in concert with increasein the slow decay channel lifetime above the 70 % water frac-tion, where AIEE onset is observed. The overall longer lifetimesin high water content correlate with the increased PL intensityunder those conditions.[21]

In a purely solvated system, a sharp increase in PL intensitywould correlate with faster radiative decay rates. However, theTable 1 results are complicated because they reflect both sol-vated and aggregated 9DAAA emission, in varying relativeamounts. The observed sharp PL intensity increase in the ag-gregation regime fits a major contribution from PL scalingwith particle size. The number of emitting excitons in an ag-gregate particle with radius r scales roughly as r3 and inverselywith exciton radiative lifetime, so increasing particle size rapid-ly increases PL intensity if the mean lifetime does not dropcommensurately. The increase in static PL intensity seen for de-creasing the ACN content from 20 to 10 % (Figure 1 a) matchesreasonably well to that expected when increasing particle sizefrom 171 nm to 254 nm coupled with a mean relaxation timeincrease from 4.2 ns to 6.3 ns. The increase in mean excitonlifetime shown in Table 1 is dominated by the t2 decay lifetimeincreases within the aggregates, due to reduced solvent-in-duced quenching. So, a combination of substantially increasedparticle size and mildly increased mean lifetime for all emittersyields a strong net PL intensity rise in the sample.

To evaluate the effects with a solvent–non-solvent mixturewith different polarity changes, the AIEE of 9DAAA was also in-vestigated by static spectroscopy in THF:water mixtures. ThePL and UV/Vis absorption spectra of 9DAAA in THF–water mix-tures display similar trends and variation with water content asthe ACN:water mixtures (see the Supporting Information), witha transition to AIEE behavior at 70 % water in THF. For 90 %water content (10:90 THF:water), the aggregation-induced PLintensity enhancement is 15-fold versus the minimum PL inten-sity in 70 % v/v water. This is a 2-fold increase relative to thePL intensity in pure THF. This relatively lower increase in PL in-tensity from AIEE can be attributed to the increased quantumyield of 9DAAA in less polar THF as compared to the morepolar ACN.

In the THF:water mixtures, the AIEE behavior was observedwhen the DLS-determined critical mean particle size reached104 nm. Since the THF:water mixtures are relatively less polarcompared to the same v/v-composition ACN:water mixtures,the change in polarity from solvated to aggregated environ-ments is less dramatic, and the changes in PL intensity are ac-cordingly smaller. The smaller critical 9DAAA aggregate size togive AIEE in the less polar THF:water mixtures suggests thatthe minimum aggregate size required to shield photogenerat-ed excitons from solvent-induced quenching is related to thepolarity (dielectric constant) of the surrounding medium.

In summary, the twisted triarylamine 9DAAA shows strongPL emission enhancement and spectral blue-shifting as aggre-gates form when a polar non-solvent fraction drives the fluoro-phore out of solution. The resulting AIEE photophysical behav-ior, including average PL decay lifetime, correlates with thesize of the formed particles. A critical aggregate size, which de-pends on the solvent mixture conditions utilized, is requiredfor AIEE to manifest effectively in these cases. This provides in-sight about the distance scale over which exciton hopping tothe surface of an aggregate solid phase can lead to quenchingof the exciton, a useful parameter for studies of excitons in or-ganic light-emitting diodes and solar cells, with varying lengthscales of phase separation between photo-excitable and exci-ton-quenching regions. Further study is underway to deter-mine the longer-term stability and photophysical behavior ofindividual aggregate particles, and to determine whether theAIEE-inducing aggregates are crystalline and how that mightvary under different conditions.

Experimental Section

9DAAA was synthesized as previously[18] reported. It has a molarabsorptivity of about 1400 cm�1

m�1 at 407 nm and a crystallo-

graphic density of 1.296 g cc�1. Acetonitrile and THF were pur-chased from Sigma–Aldrich and used as received. PL spectra wererecorded using a PerkinElmer LS-55 spectrometer. The UV/Vis ab-sorption spectra were recorded using an Agilent 8453 photodiodearray UV/Vis spectrometer. Dynamic light scattering (DLS) experi-ments were performed on a custom-built instrument usinga 50 mW He-Ne Laser, an photodiode detector (BI-APD), a digitaltime correlator (BI-9000), and a custom Brookhaven data acquisi-tioning interface. The autocorrelation functions were de-convolut-ed using a mulitple pass non-negatively constrained least-squaresalgorithm.[22]

Time-resolved photoluminescence (TRPL) measurements were car-ried out using a previously reported[18] time-correlated single-photon counting (TCSPC) approach. Solution samples were excitedwith PicoQuant pulsed diode laser at a wavelength of 407 nm witha repetition rate of 20 MHz and a typical excitation pulse width of50 ps. The sample PL was spectrally dispersed with a monochroma-tor, detected with an avalanche photodiode (Id Quantique 100-APD) at the emission maximum with a bandwidth of 5 nm, and an-alyzed with a TCSPC system (PicoQuant PicoHarp 300). The laserand detection systems provided a 70 ps time resolution.For PL/DLS/UV measurements, ACN, THF and water were filteredthrough a 0.1 mm PTFE syringe filter. A stock solution of 9DAAAwas prepared either in ACN or in THF. A constant volume of thesolvent–non-solvent mixture and 9DAAA concentration was uti-lized for all the measurements. For each set of measurements, the

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ACN:water or THF:water mixtures were freshly prepared. A prede-termined aliquot of the stock solution was added to the preparedsolvent:non-solvent mixtures and the sample mixed vigorously for1 min on an orbital shaker. The sample was then allowed to standand equilibrate for 2 min prior to recording PL/UV spectra. DLSwas performed on the spectroscopy samples immediately after re-cording the PL spectra.

Acknowledgements

Photophysical studies (A.K. , J.K. , B.Y. , M.D.B.) were supported aspart of Polymer-Based Materials for Harvesting Solar Energy, anEnergy Frontier Research Center funded by the U.S. Departmentof Energy, Office of Basic Energy Sciences under Award NumberDE-SC0001087. J.M.C. , P.M.L. and M.D.B. acknowledge supportfrom the US Department of Energy (Program Manager : LarryRahn, DE-FG02-05ER15695) for developing the 9DAAA synthesis.The authors declare no competing financial interest.

Keywords: aggregate fluorescence · aggregation-inducedenhanced emission · photoluminescence · solvatochromism ·triarylamine fluorescence

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Received: July 20, 2013

Published online on && &&, 2013

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COMMUNICATIONS

A. Kokil,* J. M. Chudomel, B. Yang,M. D. Barnes, P. M. Lahti,* J. Kumar*

&& –&&

Determining the Critical Particle Sizeto Induce Enhanced Emission inAggregates of a Highly TwistedTriarylamine

Towards highly luminescent aggre-gates: A highly twisted triphenylaminedisplays aggregation-induced enhancedemission. A solvent-specific critical mo-lecular aggregate size, once reached,gives rapid onset of enhanced emissionin polar solvent mixtures that favorquenching of solvated individual mole-cule excited states.

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