Olivier Mongin et al- Organic nanodots for multiphotonics: synthesis and photophysical studies

8/3/2019 Olivier Mongin et al- Organic nanodots for multiphotonics: synthesis and photophysical studies

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Organic nanodots for multiphotonics: synthesis and photophysicalstudies w

Olivier Mongin, a Anna Pla-Quintana, b Francesca Terenziani, ac Delphine Drouin, a

Ce line Le Droumaguet, a Anne-Marie Caminade, b Jean-Pierre Majoral* b andMireille Blanchard-Desce* a

Received (in Montpellier, France) 15th February 2007, Accepted 31st May 2007 First published as an Advance Article on the web 8th June 2007 DOI: 10.1039/b702452p

Organic nanodots based on the gathering of an exponentially increasing number of two-photonuorophores on a dendritic platform of controlled size and symmetry represent a promising non-toxic alternative to quantum dots for (bio)imaging purposes. This modular route offers a numberof advantages in terms of versatility but also raise a number of questions to be addressed. Inparticular, possible interactions between uorophores, due to connement effects, have to betaken into account. With this aim in mind we have investigated and compared the photophysicaland two-photon absorption (TPA) properties of two series of organic nanodots of different

geometries: spherical-like organic nanodots derived from a dendritic scaffold built from acyclotriphosphazene core and dumbbell-like organic nanodots derived from a dendritic scaffoldbuilt from an elongated rod-like chromophore. The study provides evidence that the differenttopology and nature of the dendritic architecture lead to signicant changes in photoluminescencecharacteristics as well as to subtle variations of the TPA efficiency. As a result, the dumbbell-likenanodots although less promising in terms of two-photon induced uorescence (due to partialquenching of uorescence efficiency) also demonstrate that improvement of the TPA efficiency canbe achieved by playing on the nature and topology of the dendritic scaffold of the nanodots.

Introduction

Molecular two-photon absorption (TPA) has attracted a lot of interest over the last decade owing to its applications invarious elds including spectroscopy, 1,2 three-dimensionaloptical data storage, 35 microfabrication, 68 laser up-conver-sion, 9,10 high-resolution three-dimensional imaging of biolo-gical systems, 1113 and photodynamic therapy. 14 Among these,two-photon-excited uorescence (TPEF) has gained wide-spread popularity in the biology community and given riseto the technique of two-photon laser scanning uorescencemicroscopy. 1113 Using a two-photon excitation process ( i.e. anonlinear process involving the simultaneous absorption of two photons) instead of a conventional one-photon excitationoffers a number of advantages. These include the ability for ahighly conned excitation (and intrinsic three-dimensionalresolution) and increased penetration depth by replacingtypical one-photon excitation in the UVvisible blue region

by two-photon excitation in the visible redNIR region, owingin particular to the reduction of scattering losses.

Most of these applications strongly benet from the use of chomophores displaying very high TPA cross-sections inorder to signicantly decrease the excitation intensity orprovide improved excitation efficiency and selectivity. In-deed, in recent years a considerable effort has been devotedto the design and investigation of chromophores with largeTPA cross-sections, exploring in particular dipolar 10,1520

and quadrupolar, 17,2138 structures. Lately, attention hasturned towards multipolar 3943 and conjugated branchedstructures 4451 including dendrimers. 5255

Depending on the targeted applications, two-photon chro-mophores also have to satisfy additional requirements. Forinstance, the use of such chromophores for biological multi-photonic imaging calls for the design of uorophores combin-ing a high uorescence quantum yield ( F ) and TPA cross-section ( s 2) in the spectral range of interest ( i.e. 7001200nm) 56 several orders of magnitude larger than endogenouschromophores (whose TPA cross-sections range between 10 5

to about 1 GM). 13 This would open the route for selectiveexcitation of two-photon chromophores leading to signi-cantly improved signal to noise ratio (reduction of back-ground uorescence) and reduction of photodamage. 57 Low(photo)toxicity and high photostability are important addi-tional criteria.

Recently semiconductor nanocrystals (quantum dots:QDs) have been shown to provide a particularly effectiveapproach to uorescent nano-objects with extremely large

a Synthe se et Electrosynthe se Organiques (CNRS, UMR 6510),Institut de Chimie, Universite de Rennes 1, Campus Scientique deBeaulieu, Ba t. 10A, F-35042 Rennes Cedex, France. E-mail:[email protected]; Fax: + 33 2 23 23 62 77

b Laboratoire de Chimie de Coordination, CNRS, 205 route deNarbonne, F-31077 Toulouse Cedex 4, France. E-mail: [email protected]; Fax: +33 5 61 55 30 03

c Dipartimento di Chimica GIAF, Universita ` di Parma, Parco Areadelle Scienze 17/A, 43100 Parma, Italy

w This paper was published as part of the special issue on Dendrimersand Dendritic Polymers: Design, Properties and Applications.

1354 | New J. Chem., 2007, 31 , 13541367 This journal is c the Royal Society of Chemistry and the Centre National de la Recherche Scientique 2007

PAPER www.rsc.org/njc | New Journal of Chemistry


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section as a function of the topology. Additive contributions of the two-photon uorophores to the overall TPA efficiency of the nanodot was observed in the case of the SOND series. 61

Since the environment and the packing of the decoratinguorophores may differ in the two series, modication of theirTPA efficiency cannot be excluded. Finally whereas in the caseof the SOND series, energy transfer can occur among similaruorophores on the periphery (excitation energy migration),

energy transfer can in principle (depending on the nature andspectroscopic characteristics of the core chromophore) occurboth along the surface (homotransfer) and between the core andthe surface of the nanodot (heterotransfer) in the DOND case.

Results and discussion

Synthesis

The synthesis of the spherical-like rst generation dendrimerG1 was carried out by grafting 12 equivalents of the TPA

chromophore F to the P(S)Cl 2 end groups of the rst genera-tion phosphorus dendrimer 6769 built from a cyclotriphospha-zene core 70 (Scheme 1). The same reaction carried out with thesecond, third and fourth generations of the same series of dendrimers afforded the spherical dendrimers G2 , G3 and G4 ,respectively (Fig. 3), as previously reported. 61

Two series of dumbbell-like dendrimeric compounds weresynthesized, both issued from the same biphenolic chromo-

phore 271 (Scheme 2) as the starting compound. The rst series(C0 , C1 ) was synthesized to serve as model for the photo-physical properties of the core uorophores of the other series(i.e. DOND series). The synthesis of the generation 0 of themonochromophoric series C0 necessitates rst the isolation of the pentasubstituted cyclotriphosphazene 1, which is thenreacted with chromophore 2. The substitution of the sixth Clof the cyclotriphosphazene with such bulky substituent is slowand needs 7 days to go to completion (Scheme 2). 31 P NMR isparticularly suitable to monitor this reaction; indeed the

Fig. 2 Schematic representation of heterochromophoric dumbbell-like organic nanodots (DOND).

Scheme 1 Synthesis of spherical dendrimer G1 .

Fig. 3 Structures of spherical-type dendrimers G2G4 .



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spectrum of compound 1 displays an ABB 0 system, whereasthe sixth substitution induces an AA 0A00 system for C0 ,affording a narrow multiplet.

The generation 0 of the multichromophoric series of dumb-bell-like dendrimers ( i.e. DOND series) is synthesized using adifferent route. Indeed, in this case N 3P3Cl 6 (large excess) is

reacted with the biphenol 2 to afford compound 3.71

In thesecond step, 10 equivalents of the two-photon uorophore F61

are grafted to afford after a prolonged heating (37 days) thegeneration 0 of the dumbbell-like multichromophoric dendri-mer G 00 (Scheme 3). In this case also, 31 P NMR is aninvaluable tool, since the completion of the reactions is shownby the appearance of a narrow multiplet, observed after aseries of intermediate compounds displaying ABB 0 typespectra.

First generation compounds of both monochromophoricand multichromophoric dumbbell-like dendrimers are synthe-sized by grafting phenols to the P(S)Cl 2 end groups of dendrimer 4.71 The monochromophoric model compound

C1 is obtained by reaction with HOC 6H 4 OMe, whereasthe multichromophoric compound G 01 is obtained by reactionwith uorophore F (Scheme 4, Fig. 2). In both cases, thenarrow multiplet (due to the unsymmetrical substitution onthe N 3P3 groups) observed at 62.4 ppm for the P(S)Cl 2 endgroups by 31 P NMR is replaced by another narrow multipletat 64.5 ppm. An intermediate signal observed at 69.4 ppm andcorresponding to the monosubstitution (P(S)ClOAr) totallydisappears when the reaction is over. The same method isapplied for the synthesis of the second generation of thedumbbell-like multichromophoric dendrimer G 02. This com-

pound possesses two types of chromophoric units: one at thecore and 40 as end groups (Scheme 5, Fig. 2).

Photophysical properties

One-photon absorption. All dendrimers show a strong ab-

sorption band in the near UVvisible-blue region (Table 1).The normalized absorption spectra of the multichromophoricdendrimers of the SOND series overlap almost perfectly withthe absorption spectrum of the isolated decorating uoro-phore F with only a slight broadening at the red tail (Fig. 4).The molar extinction coefficients increase almost linearly withthe number of decorating uorophores leading to giant ab-sorption coefficients for the G4 SOND dendrimer (Table 1). 72

These trends suggest that no noticeable interactions occur inthe ground state.

The absorption spectra of dendrimers of the dumbbell-likeseries (DOND dendrimers) are broader and slightly blue-shifted with respect to that of the SOND dendrimers (Table

1, Fig. 4). The absorption spectra of the core chromophores(C0 for G 00 and C1 for G 01 and G 02) mostly overlap, havecomparable extinction coefficient and are only slightly blue-shifted with respect to that of peripheral uorophores F (Table1, Fig. 5). However, comparison of the absorption spectra of dendrimers of the DOND series with that corresponding tothat expected for an additive contributions of core and per-ipheral chromophores F (Fig. 6) shows that the assembly of two-photon uorophores F in the DOND series leads tobroader spectra and lower effective absorption coefficientper uorophore. This indicates a larger inhomogeneous

Scheme 2 Synthesis of monochromophoric model C0 .

Scheme 3 Synthesis of dumbbell-like multichromophoric compound G 00 (only half of the molecule is represented for G 00).

This journal is c the Royal Society of Chemistry and the Centre National de la Recherche Scientique 2007 New J. Chem ., 2007, 31 , 13541367 | 1357


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broadening suggesting that the decorating uorophores of theDOND series experience a larger range of environments (anddisorder) in relation with the different geometry and lowersymmetry of their arrangement within the nanodots.

Fluorescence. The emission spectra of dendrimers of theSOND series clearly show a change of the emission band shapeand position with increasing generation number (Fig. 4). Achange of the vibronic relative intensities is observed alongwith a red-shift of the emission maxima (Table 1). Whereas the00 transition is clearly the more intense vibronic band in the

emission spectrum of the parent isolated uorophore F , the01 transition becomes more intense in the dendrimers andthis effect becomes stronger on going from G1 to G2 com-pounds (Fig. 4). The change in vibronic structure reveals anincrease in reorganization energy in the dendrimers as com-pared to the isolated uorophore, indicating that the emittinguorophore has stronger interactions with its environmentwhen gathered on the surface of the dendrimer. Indeed theproximity of adjacent chromophores modies the local envir-

onment of the emitting uorophore as compared to an isolateduorophore surrounded only by solvent molecules. The loss of ne vibronic structure, red-shift and spectral broadening of theemission band with increasing generation number suggeststhat the polarity of the environment of the emitting uoro-phores is increasing. It should be stressed that the emission of quadrupolar chromophore F is very sensitive to polarity 73

allowing its use as a probe of its local environment. Fig. 7shows emission spectra calculated by the aid of the modelreported in ref. 73, for different environmental reorganizationenergies ( eor ). Parameters used are the same as reported in ref.

73, since the chromophore is the same. In the adopted model,the evolution of position and shape (together with inhomoge-neous broadening effects) of the emission band of somequadrupolar chromophores (belonging to class I as denedin ref. 73), is ascribed to symmetry breaking in the relaxedexcited state. Polar environments stabilize broken-symmetrystates, corresponding to dipolar states. The dipolar natureof the broken-symmetry relaxed excited state is thus respon-sible for the solvatochromic behavior of uorescence bands. In

Scheme 4 Synthesis of monochromophoric dendrimer C1 and dumbbell-like dendrimer G 01 (only half of the molecule is represented for G 01).

Scheme 5 Synthesis of dumbbell-like dendrimer G 02 (only half of the molecule is represented for G 02).



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Fig. 7, the variation of eor nicely describes the evolution of theemission spectrum in different dendrimer generations andsymmetries (this correspondence is summarized in the legend).Increasing dendrimer generation corresponds to increased eor

values: chromophores act as dielectrics, more polar than thedissolving solvent (toluene). The effect of solvent polarity andthat of dendrimer generation/shape are very similar, on the

band position and shape. For example, we can estimate thatthe polarity of the nanodots varies from lower than that of CHCl 3 for low-generation nanodots, up to almost that of acetone for high-generation nanodots.

A distinct feature of the emission characteristics of dendri-mers of the SOND series is their low anisotropy values. Parentuorophore F exhibits anisotropy of 0.18 in toluene, whereas

Table 1 Photophysical data for uorophore F and dendrimers G1G4 (SOND), C0C1 (model dumbbell-like chromophores) and G 00G 02(DOND) in toluene

No. of uorophores MW/g mol 1 l abs /nm e/M 1 cm 1 l em /nm F

a t /ns b rc s 2 (max.)d /GM

F 1 F 841 386 84 900 420 0.83 0.67 0.18 765G1 12 F 11 485 385 1 004 000 423 0.75 0.71 0.03 8880G2 24 F 24 102 386 2 035 000 426 0.71 0.69 0.02 17 700G3 48 F 49 335 386 3 785 000 441 0.62 0.71 0.01 29 800G4 96 F 99 804 386 7 101 000 445 0.48 0.66 0.01 55 900C0 1 C0 2014 377 73 100 410 0.81 0.82 0.22 124C1 1 C1 5377 378 71 500 412 0.68 0.85 0.28 146G 00 1 C0 + 10 F 9184 382 805 000 443 0.44 e 0.88 f g 7100G 01 1 C1 + 20 F 19 698 382 1 537 000 444 0.11 e 0.71 f g 14300G 02 1 C1 + 40 F 40 726 384 3 000 000 445 0.26 e 0.65 f g 32800a Fluorescence quantum yield determined relative to uorescein in 0.1 M NaOH. b Experimental uorescence lifetime measured by time-correlatedsingle photon counting. c Steady-state uorescence anisotropy ( l exc = 382 nm).

d 1 GM = 10 50 cm 4 s photon

1. e Determined for l exc =l abs (max.).

f Determined for l em = l em (max.).g Anisotropy is wavelength-dependent (see Fig. 9).

Fig. 4 Normalized absorption and emission spectra ( l exc = l abs (max.)) in toluene of SOND G1G4 , and DOND G 00G 02. The absorption andemission spectra of uorophore F in toluene are also included for evaluation of the effect of the gathering of uorophores F on the surface of dendritic nanodots.



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G1G4 display much lower anisotropies, with values rangingbetween 0.03 and 0.01 (Table 1). Since rotational diffusion of the SOND dendrimers is expected to be much slower than thatof F due to their size (the radius of dendrimer G4 is estimated

to about 4 nm), the depolarization can be ascribed to fastenergy transfer between adjacent decorating uorophores 7479

(excitation energy migration or homotransfer). 80 We observethat the uorescence lifetimes remain roughly unchanged(Table 1) which is consistent with the hypothesis of homo-chromophoric energy transfer. Indeed homotransfer is possi-ble via a Fo rster mechanism 81 given the non-negligible overlapbetween absorption and emission spectra of decorating uoro-phores. We also observe a continuous decrease of the uores-cence quantum yield with increasing generation number (Ta-ble 1). This could be due to the occurrence of traps leading touorescence quenching or formation of excimers (consistentwith an increasing red-tail) with lower uorescence quantum

yields.82

The number of traps is expected to statisticallyincrease with increased generation number which could ex-plain the variation of uorescence quantum yield.

The analysis of the variation of the emission of dendrimersof the DOND series is more intricate because their emissionstems from both core and peripheral uorophores (Fig. 4). Thelow-energy absorption bands of model core ( C0 , C1 ) andperipheral ( F) compounds show a major overlap (Fig. 5) andsimilar intensities (Table 1) making selective one-photon ex-

citation of only one type of chromophore unattainable. Inaddition, we observe that the emission spectra of modelcompounds in toluene show a signicant overlap, the emissionof uorophore F being slightly red-shifted (Fig. 5). Emissionspectra of DOND dendrimers G 00G 02 show signicant differ-ences as compared to their SOND analogues bearing similarnumber of decorating uorophores ( i.e. G1G3 ). The emissionband of DOND dendrimers G 00G 02 are clearly red-shiftedand broadened (in particular in the red side) with respect totheir SOND analogues G1G3 (Fig. 8). A possible explanationis that the emitting uorophores in the DOND dendrimersexperience a different environment than in the correspondingSOND dendrimers. This could be attributed to the proximitynot only of adjacent identical decorating uorophores but alsoof the core chromophore. The decorating uorophores locatedclose to the core (shown in hashed in Fig. 2) clearly experiencedifferent environment. Simulations based on the model re-ported in ref. 73 also indicate that DONDs are more polarobjects than corresponding SONDs (see Fig. 7), suggesting acloser packing of chromophores in DONDs. This nds apossible conrmation in the lower uorescence quantum yieldof DONDs with respect to corresponding ( i.e. containingapproximately the same number of chromophores) SONDs.

In addition, interchromophoric interactions in the excitedstate (leading to formation of excimer, or hetero-excimer if the core chromophore is involved) can be facilitated in thecase of the DOND series due to the larger inhomogeneousbroadening and difference (disorder) in packing of thedecorating uorophores on the surface of the DOND den-drimers. Such phenomenon could explain the even morepronounced red tail in the emission band of dendrimers of the DOND series (Fig. 8). Interestingly, the more pro-nounced red tail is observed for G 01 which also gives riseto the largest inhomogeneous broadening of the absorptionspectra (Fig. 4).

As opposed to the case of homochromophoric dendrimersof the SOND series whose anisotropy values remain constantin the whole emission range (400600 nm), a continuous

Fig. 5 Normalized absorption and emission spectra ( l exc = l abs(max.)) in toluene of model chromophore F and model dendrimersC0C1 .

Fig. 6 Comparison of experimental and theoretical (evaluated fromadditive contributions of isolated constituting chromophores F andC1 ) absorption spectra of dendrimer G 01.

Fig. 7 Emission spectra calculated according to model presented inref. 73 using parameters relevant to the model chromophore ( F ) fordifferent solvent reorganization energies (see legend, eV units).



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decrease of the anisotropy value with increasing emissionwavelength (from B 0.1 at 400 nm to B 0.02 at 600 nm) isobserved in the case of heterochromophoric dendrimers of theDOND series (Fig. 9). This can be attributed to the contribu-tion of two types of uorophores (namely the core chromo-phore and the peripheral uorophores) with differentindividual anisotropies, the combination of which results in awavelength-dependent global anisotropy. Whatever the wave-length, the anisotropy values are lower than that of theirconstituting chromophoric units (see r values for F and C0or C1 in Table 1). 83 These low values for such large nano-objects indicate that fast energy transfer also takes place in theDONDs. For wavelengths higher than 550 nm, where emissionstems only from the peripheral chromophores (see Fig. 4 and5), the anisotropy values are similar to that of homochromo-phoric SONDs indicative of fast energy transfer involving theperipheral chromophores. The larger values obtained at lowerwavelength can be related to the increasing contribution of thecore chromophore, whose emission is blue-shifted with respectto that of peripheral uorophores (Fig. 6). 84

Another important feature is the much lower uorescencequantum yield of the dendrimers of the DOND series ascompared to those of the SOND series (Table 1). 85 This couldbe related to interactions between chromophores giving rise tocompeting processes such as fast energy transfer between

adjacent chromophores and formation of (red-shifted) exci-mers or hetero-excimer with lower uorescence quantum yield.This is consistent with the rise of a red tail in the emission bandwhich is even broader than in the case of the SOND dendri-mers (Fig. 8). Strikingly dendrimer G 01 which is the lessuorescent of the DOND series shows the more pronouncedred tail. Also, the larger disorder (as testied by the inhomo-geneous broadening) within decorating uorophoresdue tothe different geometry of the dendrimers of the DOND seriesas compared to the SOND series-can be responsible fordifferent overlap and interactions between the chromophoreson the periphery.

Two-photon absorption

The TPA of the dendrimers was studied by investigating theTPEF of SOND and DOND dendrimers in toluene. TPEFmeasurements allow for direct measurement of the TPEFaction cross-section s 2F , the relevant gure of merit forimaging applications.

The TPA spectra of dendrimers C0 and C1 were determinedas reference for the TPA response of core chromophores in theDOND series. These model chromophores clearly show muchlower TPA cross-section than decorating uorophore F overthe whole spectral range (Fig. 10) and their maximum TPAcross-sections are more than ve times smaller (Table 1). As a

Fig. 8 Comparison of the emission spectra of corresponding spherical-type and dumbbell-like nanodots in toluene.

Fig. 9 Emission anisotropy of monochromophoric dendrimersC0C1 and dumbbell-like nanodots G 00G 02 in toluene ( l exc =382 nm).

Fig. 10 Two-photon absorption spectra of model uorophore F andmonochromophoric dendrimers C0C1 determined by femtosecondTPEF measurements in toluene.



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result, the TPA response of dendrimers of the DOND series isexpected to be dominated by the contribution of decoratinguorophores given their larger molecular TPA response andlarge excess (with a F /C ratio of, respectively 10/1, 20/1 and40/1 for G 00, G 01 and G 02). In addition the TPA response of reference compounds C0 and C1 over 750 nm is clearlynegligible with respect to that of uorophore F . Hencealthough selective one-photon excitation of decorating uoro-phores of the DOND series was not permitted (similar absorp-tion coefficient and almost complete overlap of the low energy-absorption bands), selective two-photon excitation of thedecorating uorophores of heterochromophoric DOND den-drimers becomes possible. This is a major advantage of TPexcitation.

The TPA spectra of dendrimers of the SOND and DONDseries are shown in Fig. 11. The TPA cross-section in eachseries of dendrimers increases almost linearly with the numberof decorating uorophores leading to giant TPA cross-sec-tions. This is apparent from the quasi overlap of the TPAspectra normalized by the number of decorating chromo-phores in the 700740 nm region (Fig. 12). 86 We observehowever a noticeable difference in the TPA spectra of den-drimers of the SOND 87 and DOND series at wavelengthshigher than 740 nm. In that spectral range, dendrimers of theDOND series show denitely higher TPA cross-sections thandendrimers of the SOND series. For example at 760 nm,dendrimers of the DOND series show a TPA cross-sectionnormalized by the number of chromophores which is 60%larger than dendrimers of the SOND series. This indicates thatthe decorating uorophores (or at least some of them) of theDOND dendrimers show a denitely larger TPA cross-sectionin the 750850 nm region. This difference between DOND andSOND nanostructures can be directly related to their differenttopology and thus to the presence of the core chromophore.Hence although the core chomophores ( C0 or C1 ) show muchsmaller TPA activity than decorating uorophores (Fig. 10),their presence affects the TPA response of peripheral uoro-phores. This can possibly be attributed to a change of arrangement (relative orientation, packing) of decorating

uorophores due to the different topology of the dendriticarchitecture or to direct interactions between the core anddecorating uorophores located close to it. Hence, the presentstudy demonstrates that the different topology of the dendri-mers of the SOND and DOND series modies both the PLand TPA properties of the organic nanodots, providing evi-dence that arrangement and/or environment of the two-photon uorophores in the organic nanodots can signicantlyaffect their response.

Conclusion

The present study demonstrates that the topology and natureof the dendritic platform considerably inuences the PL andTPA properties of the organic nanodots. For comparablenumbers of decorating two-photon uorophores, the PLefficiency of the DOND dendrimers is much poorer due mostprobably to interactions between the core chromophore andproximal excited uorophores (combined with fast energytransfer which allows excitation energy migration towardsthose traps). In contrast, the TPA efficiency is increased forthe DOND dendrimers as compared to the SOND dendrimersin a large portion of the investigated spectral range (740900nm). Such phenomenon may have different causes. It might berelated to the change in topology caused by the differentgeometry of the core, thus resulting in a different packingand self-orientation of the decorating two-photon uoro-phores on the surface of the nanodots. Such effect can indeedlead to changes of the TPA response of decorating uoro-phores as compared to isolated ones. 66 Alternatively, thechange in TPA response could be due to a direct interactionof the core with its proximal decorating uorophores. Repla-cing the chromophoric core by a non-conjugated core havingthe same shape would allow to discriminate these causes. Inany case, this raises an important question since the globalTPA efficiency is found to be different for the two series. Thisopens an interesting route towards improved TPA efficiency of

Fig. 11 Two-photon absorption spectra of SOND ( G1G4 ) andDOND ( G 00G 02) dendrimers determined by femtosecond TPEFmeasurements in toluene.

Fig. 12 Comparison of the TPA efficiency (normalized by thenumber of TP uorophores per nanodot) of SOND and DOND.Inset: zoom of the 720860 nm region (in the same units).



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the nanodots by controlling the relative orientation of thedecorating uorophores and or their environment.

We have also demonstrated in this study that all-organicnanodots can exhibit two-photon absorption and two-photonbrightness comparable to the highest values reported forquantum dots 58 and represent thus a promising complemen-tary approach towards uorescent labels for TPEF imagingapplications. The highly modular design of nanodots allowsadjunction of supplementary layers, decoration of the peri-phery with hydrosolubilizing groups, and insertion of func-tional groups that could be utilized for bioconjugation

purposes. In that way, we have very recently reported 71biocompatible water-soluble monochromophoric nanodotswith shielding dendrimeric layers and peripheral ammoniumgroups, that maintain their quantum efficiency in water and inphysiological environment, and the efficiency of which ascontrast agents has been demonstrated for in vivo TPEFimaging on living animal.

Experimental

General

All manipulations were carried out with standard high vacuumand dry-argon techniques. The solvents were freshly dried anddistilled (THF and ether over sodium/benzophenone, pentaneand CH 2Cl 2 over phosphorus pentoxide). Reactions weremonitored by thin layer chromatography on Merck silica gel60 F254 precoated aluminum sheets. Column chromatogra-phy: SDS silica gel Si 60 A CC (70200 mm, 230400 mesh) orFlorisil s (60100 mesh). Classical 1H, 13 C, 31 P NMR spectrawere recorded with Bruker AC 200, AC 250, or Avance 300,500 spectrometers. References for NMR chemical shifts are85% H 3PO 4 for 31 P NMR, SiMe 4 for 1H and 13 C NMR. Theattribution of 13 C NMR signals has been done using J mod , twodimensional HMBC, and HMQC, Broad Band or CW 31 Pdecoupling experiments when necessary. The multiplicity of 13 C NMR signals is indicated only when the signal is not asinglet. The numbering used for the assignment of NMRsignals is shown in Fig. 13.

Syntheses

Compound 1. Sodium hydride (0.18 g, 7.04 mmol) and dryTHF (20 mL) were introduced into a 50 mL Schlenk. 4-Hydroxyanisole (0.74 g, 5.91 mmol) was added to this suspen-sion. Once the solution was clear, it was cooled to 0 1 C in awaterice bath and hexachlorocyclotriphosphazene (0.40 g,1.14 mmol) was added at once. The reaction was stirred whileallowing the temperature to rise slowly to room temperature.The reaction was considered to be nished after 40 h

(31 P NMR monitoring), then the salts were centrifuged, andthe solvent was removed. The pentasubstituted core wasseparated from the tetra- and hexasubstituted analogues bycolumn chromatography on silica gel using mixtures of hex-anes and ethyl acetate of increasing polarity (9.1 to 8.2). Thepure product (0.50 g, 57% yield) was obtained as a colorlessoil.

1H NMR (CDCl 3, 300.1 MHz), d (ppm) = 3.78 (s, 15H,OCH 3), 6.72 (d, J = 9.0 Hz, 2H, C 3A0 H), 6.73 (d, J = 9.0 Hz,4H, C 3B0 H), 6.79 (d, J = 9.0 Hz, 4H, C 3B0 H), 6.87 (dd, J =9.0 Hz/ J = 1.8 Hz, 2H, C 2A0 H), 6.92 (d, J = 9.0 Hz, 4H,

C 2B0 H), 7.06 (d, J = 9.0 Hz, 4H, C 2B0 H). 31 P{1H} NMR(CDCl 3, 121.5 MHz), d (ppm) = 8.07 (d, J = 81.5 Hz, P

B0 ),

8.08 (d, J = 78.0 Hz, P B0 ), 23.10 (dd, J = 78.0 Hz/ J = 81.5 Hz,PA0 ).

13 C NMR (CDCl 3 , 75.5 MHz), d (ppm) = 157.04 (d, J =2.26 Hz, C 4A0 ), 156.87 (C 4B0 ), 156.74 (C 4B0 ), 144.01 (pseudo t,J = 3.81 Hz, C 1B0 ), 143.85 (pseudo t, J = 3.81 Hz, C

1B0 ), 143.75

(pseudo d, J = 9.81 Hz, C 1A0 ), 122.15 (pseudo t, J = 2.26 Hz,C 2B0 ), 122.09 (m, C

2A0 ), 121.84 (pseudo t, J = 2.26 Hz, C

2B0 ),

114.39 (C 30), 55.53 (OCH 3), 55.51 (OCH 3).

Model compound C0. The pentasubstituted cyclotriphospha-zene 1 (0.11 g, 0.14 mmol) and dry THF (2.5 mL) wereintroduced into a 50-mL Schlenk. The biphenolic chromo-

phore 271

(0.036 g, 0.07 mmol) and caesium carbonate (0.05 g,0.14 mmol) were added to this solution, and the resultingmixture was stirred at room temperature for two days, thenve more days at 50 1 C ( 31 P NMR monitoring). The salts werecentrifuged off and the solvent was removed by vacuumevaporation. The pure product (0.10 g, 55% yield) wasobtained after a column chromatography on silica gel usingmixtures of hexaneethyl acetate of increasing polarity aseluent (9 : 1 - 8 : 2 - 7 : 3 - 5 : 5).

1H NMR (CDCl 3, 300.1 MHz), d (ppm) = 0.630.75 (m,10H, H q , H o ), 1.14 (m, 4H, H p ), 2.07 (br m, 4H, H n ), 3.76 (s,12H, CH 3O), 3.80 (s, 18H, CH 3O), 6.696.75 (m, 20H, C 30 H),6.846.93 (m, 20H, C 20 H), 6.98 (d, J = 8.6 Hz, 4H, H b ), 7.16

(m, 4H, H e, H f ), 7.37 (d, J = 8.6 Hz, 4H, H c), 7.50 (s, 2H, H k ),7.53 (d, J = 8.6 Hz, 2H, H h ), 7.71 (d, J = 7.8 Hz, 2H, H i).31 P{1H} NMR (CDCl 3, 121.5 MHz), d (ppm) = 9.86 (m, P 0).13 C{1H} NMR (CDCl 3 , 62.9 MHz), d (ppm) = 13.89 (C q ),23.14 (C p ), 26.03 (C o ), 40.44 (C n ), 54.97 (C m ), 55.53 (OCH 3),114.34 (C 30), 120.01 (C i), 120.67 (C k ), 121.34 (m, C b ), 121.92(m, C 20), 125.66 (C h ), 127.02 (C e), 127.33 (C c), 129.16 (C f ),134.22 (C g), 136.24 (C d ), 140.71 (C j), 144.30 (m, C

10), 150.07

(m, C a), 151.60 (C l), 156.56 (C 40).

Model compound C1. The rst generation dendrimer 471

with the chromophore immobilized in the core (0.038 g, 10.68mmol), 4-hydroxyanisole (0.030 g, 240.04 mmol), caesium

Fig. 13 Numbering used for NMR assignments (only half of the molecule is represented).



11/14

carbonate (0.150 g, 457.00 mmol), and an acetoneTHF (1 : 1)mixture (8 mL) were introduced in a 50-mL Schlenk. Theresulting heterogeneous solution was stirred at 50 1 C for 20 h.The salts were ltered and the solvent removed when thereaction was nished ( 31 P NMR monitoring). The crudeproduct was dissolved in minimum amount of chloroformand added dropwise to 300 mL of pentane under vigorousstirring, and the precipitated product recovered by ltration(for three times) to afford the pure chromophoric core modelC1 (0.051 g, 89% yield).

1H NMR (CDCl 3 , 300.1 MHz), d (ppm) = 0.530.69 (m,10H, H q , H o ), 1.09 (m, 4H, H p ), 2.02 (br m, 4H, H n), 3.25 (d,3J H P = 10.2 Hz, 30H, CH 3N), 3.70 (s, 20H, CH 3O), 3.72 (s,10H, CH 3O), 3.73 (s, 30H, CH 3O), 6.78 (d, J = 9.0 Hz, 40H,C 31 H), 6.997.16 (m, 68H H c, H e, H f , C 20 H, C 21 H), 7.367.44(m, 8H, H h , H k , H b ), 7.567-66 (m, 32H, H i, C

30 H, CH Q N).

31 P{1H} NMR (CDCl 3 , 121.5 MHz), d (ppm) = 8.47 (m, P 0),64.30 (s, P 1), 64.40 (s, P 1).

13 C{1H} NMR (CDCl 3 , 62.9 MHz),d (ppm) = 13.91 (C q ), 23.08 (C p ), 26.00 (C o ), 33.11 (d, J C P =12.1 Hz, CH 3N), 40.69 (C n ), 55.00 (C m ), 55.52 (OCH 3), 114.49(C 31), 120.06 (C i), 120.67 (C k ), 121.12 (m, C b ), 121.41 (m, C 20),122.28 (d, J C P1 = 4.4 Hz, C

21), 125.54 (C h ), 126.73 (C e),

127.50 (C c), 129.41 (C 30), 129.41 (C f ), 132.24 (m, C 40), 134.72(C g), 136.02 (C d ), 138.28 (d, J C P1 = 13.8 Hz, C 0HQ N),140.72 (C j), 144.14 (d, J C P1 = 6.92 Hz, C 11), 149.57 (m, C a ),151.24 (m, C 10), 151.56 (C l), 156.98 (C

41).

Dendrimer G 00. The zeroth generation dendrimer 371 (0.005g, 4.40 mmol), the terminal chromophore F61 (0.039 g, 46.34mmol), caesium carbonate (0.029, 89.03 mmol), and a 1 : 1tetrahydrofuranacetone mixture (6 mL) were introduced in a50 mL Schlenk, and stirred at 45 1 C for 20 days, then for 17days at 60 1 C. The salts were ltered off and the solvent wasremoved when the reaction was nished ( 31 P NMR monitor-ing). The product was puried by washing the orange solidwith a 1 : 1 acetonemethanol mixture to afford the orangebichromophoric product (0.0182 g, 45% yield).

1H NMR (CDCl 3 , 500.3 MHz), d (ppm) = 0.600.64 (m,44H, H S, H o ), 0.650.70 (m, 66H, H U , H q ), 0.94 (br s, 60H,H Z 0), 1.10 (m, 44H, H T , H p ), 1.22 (m, 30H, H G ), 1.36 (m,120H, H X , H Y , H Z ), 1.62 (m, 40H, H W ), 1.99 (m, 44H, H R ,H n), 3.31 (br s, 40H, H V ), 3.423.57 (m, 20H, H F ), 3.633.79(m, 20H, H E ), 3.974.14 (m, 20H, H D ), 6.61 (d, J = 7.6 Hz,20H, H Q ), 6.65 (m, 20H, H H ), 6.70 (d, J = 7.4 Hz, 20H, H C ),6.91 (m, 20H, H B), 7.00 (m, 4H, H b ), 7.17 (m, 4H, H e, H f ), 7.44(m, 44H, H

I, H

P, H

c), 7.467.52 (m, 44H, H

J, H

L, H

N, H

O, H

h,

H k ), 7.63 (m, 22H, H K , H M , H i).31 P{1H} NMR (CDCl 3, 121.5

MHz), d (ppm) = 9.68 (m, P 0). 13 C{1H} NMR (CDCl 3, 125.8MHz), d (ppm) = 12.28 (C G ), 13.88 (C U , C q ), 14.08 (C Z 0 ),22.71 (C Z ), 23.12 (C T , C p ), 25.89 (C S, C o ), 26.84 (C X ), 27.23(C W ), 31.74 (C Y ), 40.25 (C n ), 40.34 (C R ), 45.63 (C F ), 49.59(C E ), 50.98 (C V ), 55.06 (C R 0 , Cm ), 65.82 (C D ), 88.22 (C N 00 ),88.61 (C I 00 0 ), 90.78 (C I 00 ), 91.22 (C N 00 0 ), 108.76 (C P 0 ), 110.07(C I 0 ), 111.23 (C Q ), 111.46 (C H ), 114.97 (C C ), 119.73 (C K , C M ),120.09 (C i), 120.71 (C k ), 121.24 (m, C b ), 121.97 (m, C B), 122.51(C O 00 ), 122.80 (C K 0 ), 125.54 (C L , CO , C h ), 126.89 (C e), 127.38(C c), 129.17 (C f ), 130.37 (C J , CN ), 132.89 (C P ), 133.03 (C I),134.31 (C g), 136.12 (C d ), 136.12 (C d ), 140.00 (C N 0), 140.22

(C L 0), 140.72 (C j), 144.56 (m, C B 0 ), 147.37 (C H 0 ), 147.94 (C Q 0 ),150.12 (m, C a ), 150.93 (C K 00 , CO 0), 151.71 (C l), 155.65 (C C 0 ).

Dendrimer G 01. The rst generation dendrimer 471 (0.010 g,2.83 mmol), the terminal chromophore F61 (0.049 g, 58.2mmol), caesium carbonate (0.038, 113.94 mmol) and tetrahy-drofuran (7 mL) were introduced in a 50 mL Schlenk, andstirred at room temperature overnight then for 6 days at 45 1 C.The salts were ltered off and the solvent removed when thereaction was nished ( 31 P NMR monitoring). The product waspuried by column chromatography (Florisil s , CHCl 3 toTHFCH 3OH as eluent mixtures) and further diethyl etherwashings to afford the orange bichromophoric product(0.0191 g, 34% yield).

1H NMR (CDCl 3, 500.3 MHz), d (ppm) = 0.570.63 (m,84H, H S, H o ), 0.630.70 (m, 126H, H U , H q ), 0.860.96 (m,120H, H Z 0), 1.041.12 (m, 84H, H T , H p ), 1.121.23 (m, 60H,H G ), 1.301.39 (m, 240H, H X , H Y , H Z ), 1.521.67 (m, 80H,H W ), 1.912.06 (m, 84H, H R , H n ), 3.183.30 (m, 30H,P1 NCH 3), 3.273.36 (m, 80H, H V), 3.373.49 (m, 40H,H F ), 3.603.74 (m, 40H, H E ), 3.924.06 (m, 40H, H D ),6.536.69 (m, 80H, H H , H Q ), 6.696.83 (m, 40H, H C ),7.027.16 (m, 68H, H B , C20 H, H b , H e, H f ), 7.377.44 (m,84H, H I , H P , H c), 7.447.51 (m, 84H, H J , H L , H N , H O , H h ,H k ), 7.487.53 (m, 30H, C 30 H, CH Q N), 7.587.69 (m, 42H,H K , H M , H i).

31 P{1H} NMR (CDCl 3 , 202.5 MHz), d (ppm) =8.58 (m, P 0), 64.58 (m, P 1).

13 C{1H} NMR (CDCl 3 , 125.8MHz), d (ppm) = 12.22 (C G ), 13.86 (C U , C q ), 14.05 (C Z 0),22.68 (C Z ), 23.09 (C T , C p ), 25.88 (C S, C o ), 26.80 (C X ), 27.17(C W ), 31.70 (C Y ), 33.05 (m, P 1 NCH 3), 40.30 (C R , C n ), 45.60(C F ), 49.50 (C E ), 50.99 (C V), 55.05 (C R 0, C m ), 65.69 (C D ), 88.54(C I 00 0 , C N 00 ), 91.03 (C I 00 , C N 00 0 ), 108.81 (C P 0 ), 109.91 (C I 0), 111.41(C H , C Q ), 115.04 (C C ), 119.72 (C K , C M ), 121.36 (m, C 20, C b , C i,C

k), 122.37 (m, C

B), 122.75 (C

K0 , C

O00 ), 125.53 (C

L, C

O, C

h),

127.17 (m, C c, C e), 128.38 (C 30), 129.01 (C f ), 130.37 (C J , CN ),132.17 (m, C 40), 132.88 (C P , C I), 133.81 (C g), 136.14 (C d ),140.17 (m, C L 0 , CN 0, C0HQ N, C j), 144.27 (m, C B 0), 147.33(C H 0), 147.85 (C Q 0 ), 150.93 (m, C K 00 , CO 0, C

10, C a , C l), 156.03

(C C 0 ).

Dendrimer G 02. The second generation dendrimer 571 (0.078g, 0.91 mmol), the terminal chromophore F61 (0.031 g, 36.48mmol), caesium carbonate (0.026 g, 78.09 mmol) and a 1 : 1acetoneTHF mixture (6 mL) were introduced in a 50-mLSchlenk, and stirred at 50 1 C for 21 h. The salts were lteredand the solvent removed when the reaction was nished ( 31 PNMR monitoring). The orange solid obtained was repeatedlywashed with diethyl ether until no F chromophore was ob-served by TLC. Then the solid was taken in dichloromethane,ltered, and the solvent was removed to afford the orangebichromophoric product (0.034 g, 91% yield).

1H NMR (CDCl 3, 500.3 MHz), d (ppm) = 0.500.74 (m,410H, H S, H U , H o , H q ), 0.850.96 (m, 240H, H Z 0), 0.991.19(m, 284H, H G , H T , H p ), 1.241.29 (m, 480H, H X , H Y , H Z ),1.521.67 (m, 160H, H W ), 1.852.09 (m, 164H, H R , H n ),3.123.49 (m, 330H, H F , H V , P1-N-CH 3 , P2-N-CH 3),3.533.72 (m, 80H, H E ), 3.874.07 (m, 80H, H D ), 6.526.67(m, 160H, H H, , H Q ), 6.686.81 (m, 80H, H C ), 7.007.25 (m,148H, H B , C

20 H, C

21 H, H b , H e, H f ), 7.347.44 (m, 164H, H I ,



12/14

H P , H c), 7.447.54 (m, 194H, H J , H L , H N , H O , C 0HQ N,C 1HQ N, H h , H k ), 7.547.72 (m, 142H, H K , H M , C 30 H, C 31 H,H i).

31 P{1H} NMR (CDCl 3 , 121.5 MHz), d (ppm) = 8.43 (m,P0), 62.34 (br s, P 1), 64.62 (br s, P 2).13 C{1H} NMR (CDCl 3,125.8 MHz), d (ppm) = 12.24 (C G ), 13.87 (C U , C q), 14.07(C Z 0 ), 22.70 (C Z ), 23.09 (C T , C p ), 25.88 (C S, Co ), 26.82 (C X ),27.22 (C W ), 31.72 (C Y ), 33.02 (m, P 1 NCH 3, P 2 NCH 3),40.30 (C R , C n ), 45.59 (C F ), 49.51 (C E ), 50.96 (C V ), 55.04 (C R 0 ,Cm ), 65.72 (C D ), 88.22 (C N 00 ), 88.54 (C I 00 0 ), 90.87 (C I 00 ), 91.23(C N 00 0 ), 108.74 (C P 0), 109.87 (C I 0 ), 111.22 (C Q ), 111.40 (C H ),115.03 (C C ), 119.72 (C K , CM ), 121.86 (m, C 20, C 21 , C b , C i, C k ),122.42 (m, C B), 122.78 (C K 0 , C O 00 ), 125.51 (C L , C O , C h ), 127.11(C c, C e), 128.38 (C 30 , C 31), 129.05 (C f ), 130.36 (C J , CN ), 131.59(C 41), 132.34 (C 40), 132.88 (C P ), 132.98 (C I), 133.67 (C g), 136.18(C d ), 139.98 (C N 0 ), 140.17 (m, C L 0, CH Q N, C j), 144.25 (m,CB 0), 147.33 (C H 0), 147.93 (C Q 0 ), 150.92 (m, C K 00 , CO 0 , C

10, C a ,

C l), 151.25 (m, C11), 156.01 (C C 0).

Photophysical studies

Optical absorption and emission spectroscopy. All photophy-

sical measurements have been performed with freshly-pre-pared solutions in air-equilibrated toluene at roomtemperature (298 K). UV/Vis absorption spectra were re-corded on a Jasco V-570 spectrophotometer. Steady-stateand time-resolved uorescence measurements were performedon dilute solutions ( ca. 10 6 M chromophore concentration,optical density o 0.1) contained in standard 1 cm quartzcuvettes using an Edinburgh Instruments (FLS920) spectro-meter in photon-counting mode. Emission spectra were ob-tained, for each compound, under excitation at the wavelengthof the absorption maximum. Fluorescence quantum yieldswere measured according to literature procedures using uor-escein in 0.1 M NaOH as a standard (quantum yield F =

0.90).88,89

The lifetime values were obtained from the recon-volution t analysis (Edinburgh F900 analysis software) of decay proles obtained using the FLS920 instrument underexcitation with a nitrogen-lled nanosecond ash-lamp. Thequality of the ts was evidenced by the reduced w2 value(w2 o 1.1).

Fluorescence anisotropy. Steady-state uorescence anisotro-pies were measured using the Edinburgh Instruments FLS920instrument, by inserting GlanThompson polarizers in theexcitation and emission paths. Emission anisotropies wereobtained at right angles using vertically polarized excitationlight. Wavelength-dependent polarization correction factors

(G-factors) were measured on the same sample underhorizontally polarized excitation following standard proce-dures.

Two-photon absorption. Two-photon absorption cross sec-tions ( s 2) were obtained from the two-photon excited uores-cence (TPEF) cross sections ( s 2F ) and the uorescenceemission quantum yield ( F ). TPEF cross sections in toluene(10 4 M chromophore concentration) were determined using aTi-sapphire laser delivering 150 fs excitation pulses, accordingto the experimental protocol established by Xu and Webb. 90

This experimental protocol allows avoiding contributionsfrom excited-state absorption that are known to result in

largely overestimated TPA cross-sections. Fluorescein in 0.01M NaOH, whose TPEF cross-sections are well-known, 90

served as the reference, taking into account the necessarycorrections for the refractive index of the solvents. 91 Thequadratic dependence of the uorescence intensity on theexcitation intensity was veried for each data point, indicatingthat the measurements were carried out in intensity regimes inwhich saturation or photodegradation do not occur. Moredetails about the experimental setup have been previouslypublished. 91

Acknowledgements

F. T. acknowledges Universite de Rennes 1 for an invitedassistant professor position. We acknowledge nancial sup-port from CNRS, Re gion Bretagne (Renouvellement desCompe tences Program) and Rennes Me tropole. Thanks aredue to the Fundacio n Ramo n Areces for a grant to A.Pla-Quintana.

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56 Corresponding to the combination of reduced absorption andscattering.

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62 Phosphorus dendrimers have been shown to have low toxicity, see:M. Maszewska, J. Leclaire, M. Cieslak, B. Nawrot, A. Okruszek,A.-M. Caminade and J.-P. Majoral, Oligonucleotides , 2003, 13,193205.

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72 A slight apparent decrease of the effective absorption coefficientper chromophore is observed for dendrimers G3 and G4 (inferiorto that of reference uorophore F ) This is most probably anartifact related to the presence of an increasing amount of solventmolecules trapped in the dendrimer interior. A similar effect isconsequently observed for the maximum TPA cross-section of G3and G4 .

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80 Time-resolved uorescence anisotropy decay measurements studyon dendrimers G1G4 indicates that this energy transfer is indeedvery fast: M. Guo and T. Goodson, III, personal communication.

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appearance of an emission tail at lower energy has also beenobserved in dendrimers decorated with naphthyl units: F. Pina,P. Passaniti, M. Maestri, V. Balzani, F. Vo gtle, M. Gorka, S.-K.Lee, J. Van Heyst and H. Fakhrnabavi, ChemPhysChem , 2004, 5,473480.



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83 Model chromophore F exhibits an anisotropy of 0.18, and refer-ence monochromophoric compounds C0 and C1 display values of 0.22 and 0.28, respectively. Since these two dendrimer-embeddedchromophores differ only in generation number, the increase inanisotropy value can be related to the slowing-down of therotational diffusion for increasing molecule size.

84 DOND G 00 shows the smaller anisotropy value at low wavelength(0.07), which might indicate that energy transfer from core chro-mophore to peripheral ones also takes place in that case (in relationwith the smaller distance between core and peripheral chromo-phores). Wavelength-dependent time-resolved anisotropy measure-ments are required to derive nal conclusion.

85 The uorescence quantum yield is an effective value since both thecore and peripheral chromophores contribute to emission. Its valueis thus expected to depend on the excitation wavelength, whichcontrols the relative contribution (in addition to the relative

number of chromophores) of excitation of each type of chromo-phores. The values given in Table 1 correspond to excitation at theabsorption maximum.

86 Except for dendrimer G 02 that shows slightly higher TPA cross-sections.

87 Even if their PL characteristics show that emitting uorophores of SOND dendrimers experience a more polar environment, theaverage TPA response of the decorating uorophores is similarto that of an isolated molecule (Fig. 12), which was also the casefor one-photon absorption (Fig. 4).

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Olivier Mongin et al- Organic nanodots for multiphotonics: synthesis and photophysical studies

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Transcript of Olivier Mongin et al- Organic nanodots for multiphotonics: synthesis and photophysical studies