J. Phys. Chem. C 2009, 113, 12883–12887
12883
Coherent Mechanism of Exciton Transport in Disordered J-Aggregates Alexander N. Lebedenko, Roman S. Grynyov, Gleb Ya. Guralchuk, Alexander V. Sorokin,* Svetlana L. Yefimova, and Yuri V. Malyukin Institute for Scintillation Materials, NAS of Ukraine, 60 Lenin AVenue, 61001 KharkoV, Ukraine ReceiVed: April 10, 2009; ReVised Manuscript ReceiVed: May 19, 2009
Using a luminescent exciton trap, a mechanism of the exciton migration in disordered J-aggregates of amphiphilic analogue of pseudoisocyanine (amphi-PIC) dye has been investigated in the temperature range of 80-300 K. Due to a strong topological disorder in amphi-PIC J-aggregates observed in a binary dimethylformamide-water (DMF/W) solution with a low water content, two types of excitonic states have been revealed, delocalized exciton state that forms the main part of the J-aggregates absorption band (J-band) and a state of strongly localized excitons that forms the long-wavelength edge of the J-band. These excitonic states are characterized by the different mechanism of the exciton transport: a coherent mechanism for delocalized excitons and an incoherent one for localized excitons. As localized excitons provide a small contribution to the J-band and appear only at high degree of topological disorder, the coherent mechanism of the exciton transport in amphi-PIC J-aggregates has been concluded. Such a result is nontrivial due to a small delocalization length of excitons in amphi-PIC J-aggregates (11 monomers at 80 K) provided by the moderate energetic disorder and strong exciton-phonon coupling. Introduction J-aggregates are specific self-assembled nanoscale luminescent clusters of noncovalently coupled organic dye molecules (usually, cyanines, merocyanines, or porphyrins) organized in the form of linear or closed molecular chains, which, in their turn, form complex cylindrical patterns.1-3 The high order degree in molecular chains leads to a delocalization of electronic excitations and an appearance of Frenkel excitons, which are characterized by a narrow intense absorption band (J-band) redshifted with respect to the monomer band.1-3 Due to a static disorder (mainly, energetic one) in the molecular chain, excitons do not delocalize along the whole chain, but on a segment of the chain, length of which is called delocalization length (Ndel).4,5 Ndel defines optical properties of J-aggregates, such as the width of J-band, luminescence decay time, etc.1-5 A number of experimental and theoretical works are devoted to the exploration of the exciton migration in J-aggregates.6-21 J-aggregates are prospective objects for the creation of artificial energy delivery nanosystems similar to light-harvesting complexes (LHC) in plants.17,21-23 There is a significant contradiction in the efficiency of exciton energy migration reported by different authors.7,11-15,17-20 According to the authors of refs 13-15, and 20, an exciton migrates in J-aggregates over up to several hundred molecules, while in refs 7, 11, 12, 17, and 18, an exciton motion over 104 molecules at room temperatures is stated. One of the main questions to be answered in this connection is the mechanism of the exciton transport in J-aggregates (coherent or incoherent). Mainly, the exciton transport in J-aggregates is considered to be coherent, i.e., the exciton propagates along the molecular chain as a wave packet.5 A high efficiency of the exciton migration reported in refs 12, 17, 18 is explained within the model of a coherent exciton transport. However, incoherent hopping of excitons over adjacent delocalization segments is * To whom correspondence should be addressed. E-mail: sorokin@ isc.kharkov.com.
supposed as well.6,9,12,16,19 For example, such experimental results as nonexponential luminescence decay, a strong dependence of fluorescence lifetimes on detection wavelength and a timedependent Stokes shift observed for Davydov splitted Jaggregates are not explained within the coherent exciton migration model.9 To provide a correct description of the observed results, authors assumed the mechanism of a predominant incoherent energy migration.9 In ref 12 for THIATS J-aggregates a strong nonmonotonic temperature dependence of the exciton-exciton annihilation (EEA) rate, which is considered to be proportional to the exciton diffusion coefficient, was reported. At the low temperature range (T < 20 K), the model of the incoherent exciton transport was used to explain the exponential growth of EEA rate temperature dependence.12 At high temperatures (30 K < T < 70 K), a very fast coherent exciton migration over more than 106 molecules was supposed.12 However, the theoretical analysis of the low-temperature diffusion of 1D Frenkel excitons in J-aggregates with a moderate diagonal disorder16 prejudiced the results presented in ref 12. The motion of excitons, which are localized over relatively large segments of typical size Ndel was concluded to be incoherent hops over localized states.16 The same conclusion was made in ref 6 under the analysis of the EEA in PIC J-aggregates. To explain an efficient exciton migration to acceptor molecules in 2D J-aggregates formed in monolayers (50% energy transfer occurs at the acceptor-to-donor ratio ) 1:104), the model of unbiased random walks of an exciton toward an acceptor molecule was used, which is caused by the attraction of acceptor shallow energy levels.11 Such a model allowed authors to estimate the exciton migration length to be equal to 100 molecules.11 Moreover, despite the model of Gaussian wave packet spreading that was involved to describe the exciton migration, the statement about the increase of exciton mobility with temperature increasing7,11 implies the incoherent exciton migration. Similar conclusion was made in ref 19, where experimental data obtained in ref 7 were theoretically described.
10.1021/jp903328r CCC: $40.75 2009 American Chemical Society Published on Web 06/15/2009
12884
J. Phys. Chem. C, Vol. 113, No. 29, 2009
Lebedenko et al.
CHART 1: Structural Formulas of the Dyes Investigated: (a) amphi-PIC and (b) DiD
It is known that the main reason of the exciton localization, which is a key factor governing the exciton transport in J-aggregates, is an energetical and topological static disorder in a molecular chain of the J-aggregate.2,4,5 To clear up the predominant mechanism of the exciton migration, optical properties of J-aggregates of 1-methyl-1′-octadecyl-2,2′-cyanine perchlorate (amphi-PIC, Chart 1), that reveal one of the largest disorder degree among J-aggregates,24 have been studied. The amphi-PIC J-bandwidth (fwhm) at low temperatures (1.5-5 K) is 380 cm-1 (ref 24) that is the largest value among J-aggregate family: 34 cm-1 for PIC,25 85 cm-1 for THIATS,12 and 160 cm-1 for TDBC.26 Furthermore, the energetic disorder degree ∆/J, where ∆ is the energetic disorder and J is the dipole-dipole interaction strength, for amphi-PIC J-aggregates (0.2)24 is two times larger than that for PIC J-aggregates (0.1).5 The decrease of the water content in a dimethylformamide/water (DMF/W) binary solution down to 50% causes a significant topological disorder in a molecular chain of the amphi-PIC J-aggregate.24 In this case, at low temperatures the long-wavelength edge of the J-band changes its form from Gaussian to Lorentzian.4,24 To clear up the mechanism of the exciton transport in amphiPIC J-aggregates, special energy traps (organic dye molecules) were embedded into the J-aggregate chain and a temperature dependence of the exciton diffusion coefficient has been analyzed. As an exciton trap, amphiphilic cyanine dye 1,1′dioctadecyl-3,3,3′,3′-tetramethylindodicarbocyanine perchlorate (DiD, Chart 1) was used. This dye is an effective energy trap that has been already used in our previous studies.10,27 Experimental Section The amphi-PIC dye was obtained from the dye collection of Dr. I.A. Borovoy (Institute for Scintillation Materials) with purity controlled by thin layer chromatography. DiD dye was purchased from Sigma-Aldrich and used as-received. Sample solutions containing amphi-PIC J-aggregates with traps were prepared as follows. DiD and amphi-PIC dyes were dissolved in the organic solvent DMF to form a mixture at the ratio 1:50. Then doubly distilled water was added to obtain a binary solution DMF/W with 50% or 75% water content. The concentration of the amphi-PIC dye in the solution was 2 × 10-4 mol/L. At temperatures below the vitrifying temperature (Tvitr ) 265 K), this binary solution forms a homogeneous vitrescent matrix. Luminescence spectra and luminescence excitation spectra were recorded using a spectrofluorimeter based on two grating monochromators MDR-23 and a xenon lamp. One of the monochromators was used to select a required wavelength (fwhm ≈ 0.5 nm), and the other one was used for the
Figure 1. (a) Absorption (solid line), trap luminescence excitation (λreg ) 680 nm, dashed line) and (b) luminescence (λexc ) 530 nm) spectra of amphi-PIC J-aggregates with DiD trap (amphi-PIC/DiD ) 1:50) in a DMF/W ) 1:1 solution at room temperature.
luminescence collection. Absorption spectra was registered using a microspectrometer USB4000 (Ocean Optics) supplied with an incandescent lamp. Solutions were placed in a small cell of 1 mm thickness into a cryostat and cooled down to liquid nitrogen temperature. Luminescence intensity was measured in the temperature range of 80-240 K with a step of 5 K. The temperature was controlled within 1 K. Results and Discussion DiD molecules are known to form unordered associates in aqueous solutions that leads to their luminescence quenching.10,27 Due to long hydrophobic tails, DiD molecules are incorporated into the amphi-PIC J-aggregate chains and at the excitation within the J-band an intense sensitized DiD luminescence with λmax ) 675 nm (Figure 1b) is observed. As the concentration of DiD molecules is small (4 × 10-6 M), in the absorption spectrum of the solution containing J-aggregates with traps, the trap absorption band with λmax ) 650 nm can not be separated (Figure 1a). In the DiD luminescence excitation spectrum (λreg ) 680 nm), the band corresponded to the absorption of J-aggregates (so-called J-band, λmax ) 585 nm, ∆νfwhm ) 625 cm-1) is observed (Figure 1a). Hereafter this band will be referred to as “excitation J-band” by comparison with the J-band. The analysis of the absorption and luminescence excitation spectra reveals a discrepancy between the long-wavelength edges of the J-band and the excitation J-band near λ ) 615 nm (Figure 1a). The long-wavelength tail of the excitation J-band is steeper than that of the J-band. The temperature decrease down to 80 K causes the change of the absorption, luminescence, and trap luminescence excita-
Exciton Transport in Disordered J-Aggregates
J. Phys. Chem. C, Vol. 113, No. 29, 2009 12885
Figure 3. Absorption (solid line) and trap luminescence excitation (λreg ) 680 nm, dashed line) spectra of amphi-PIC J-aggregates with DiD trap in a DMF/W ) 1:3 solution at T ) 80 K.
Figure 2. (a) Absorption (solid line), trap luminescence excitation (λreg ) 680 nm, dashed line) and (b) luminescence (λexc ) 530 nm) spectra of amphi-PIC J-aggregates with DiD trap in a DMF/W ) 1:1 solution at T ) 80 K.
tion spectra of the solution containing J-aggregates with a trap (Figure 2). The maximum of the J-band is blue-shifted (λmax ) 583 nm), and the band become narrower (∆νfwhm ) 430 cm-1) as compared to the room temperature (Figure 1a) due to a decrease of exciton-phonon scattering25 (Figure 2a). The longwavelength edge of the J-band is fitted by the Lorentz contour as at room temperature that points to the significant topological disorder in J-aggregates.4,24 However, the long-wavelength edge of the excitation J-band reveals Gauss-like form (Figure 2a) similar to that of the J-band at low temperatures in absence of topological disorder.1-5,24 So, the discrepancy between the longwavelength edges of the J-band and the excitation J-band become more pronounced (Figure 2a). At high water content (75%) in a binary solution, the long-wavelength edges of the J-band and the excitation J-band appear to be similar and are well-fitted by Gaussian (Figure 3). The discrepancy between the bands in the solution with a low water content (50%) (Figure 2a) was attributed to the formation at the low-energy tail of the J-band strongly localized exciton states (practically motionless).28 The characteristic feature of amphi-PIC J-aggregates is exciton self-trapping that takes place at low temperatures at high disorder degree in a molecular chain.28-30 At 80 K in the luminescence spectrum of amphi-PIC J-aggregates with a DiD trap a new band (λmax ) 630 nm, Figure 2b) that corresponds to self-trapped excitons appears.28-30 This band is red-shifted as compared to the free exciton band (λmax ) 584.5 nm, Figure 2b). The laser excitation into the long-wavelength edge of the J-band shown that there is no self-trapped exciton luminescence,
Figure 4. J-band (solid line) from Figure 2a represented in a semilogarithmic scale. Dashed line, approximation by Loretzian; dotted line, approximation by exponent. T ) 80 K.
i.e., only delocalized (free) excitons forming J-band maximum are the source for self-trapping states.28 It is well known that the exciton self-trapping is a result of strong exciton-phonon coupling that leads to the exponential contour of the exciton long-wavelength edge according to the Urbach rule.31 However, the plot of the J-band in a semilogarithmic scale clearly shows that the long-wavelength edge of the J-band is well fitted by the Lorentz contour and the Urbach exponential tail makes a minor contribution to it (Figure 4). So, we can conclude that topological disorder provides the main contribution to the non-Gaussian red tail of the J-band. The comparison of luminescence spectra of J-aggregates in presence and absence of the traps revealed that only luminescence of free excitons are significantly quenched by the traps contrary to self-trapped exciton luminescence.10 Therefore the exciton self-trapping will not be taken into consideration at the exciton diffusion analysis. Thus, in amphi-PIC J-aggregates with a high degree of topological disorder in molecular chain (binary solutions with 50% water content) the J-band is characterized by the complex structure: the long-wavelength edge is formed by localized excitons; the band maximum and the short-wavelength edge are formed by delocalized ones. The question arises whether the mechanism of the exciton transport for these two types of exciton states similar or not. To answer this question the temperature dependence of the trap luminescence, which is defined by the temperature dependence of the exciton diffusion coefficient,32 has been studied at the excitation to the different sites within the J-band (Figure 5). A subtraction of the excitation
12886
J. Phys. Chem. C, Vol. 113, No. 29, 2009
Figure 5. (a) Localized exciton band (dashed line) within the J-band (solid line), arrows point to different excitation sites. (b) Curves of temperature dependence of exciton trap luminescence intensity (λreg ) 680 nm) at the different excitation wavelengths: λexc ) 583 nm (solid line); λexc ) 587 nm (dashed line); λexc ) 592 nm (dash-dotted line); λexc ) 610 nm (dotted line). Data are normalized to the value at T ) 80 K for a clear presentation.
J-band from the absorption J-band gives us the absorption band of localized excitons with a maximum at λmax ) 591 nm (Figure 5a, dashed line). Therefore, the trap luminescence was excited at different sites within the J-band and the localized exciton band, correspondently (Figure 5). At the excitation at the J-band maximum (λexc ) 583 nm, delocalized excitons) the trap luminescence intensity reveals a monotonic decrease within the 80-240 K temperature range (Figure 5b, solid line). Such a temperature dependence of the diffusion coefficient (D ≈ T-1/2) is characteristic for the coherent exciton motion and is governed by the exciton scattering on phonons.5,32 The temperature dependence of the localized exciton migration (excitation at the maximum of localized exciton band, λexc ) 592 nm) is more complicated (Figure 5b, dot-dashed line). In the temperature range of 80-150 K, the intensity of the trap luminescence increases that is characteristic feature of the incoherent hopping exciton migration (D ≈ exp-Ea/kT) and is governed by the thermal activation of the localized excitons.5,32 The curve fitting by the exponential law gives the hopping activation energy Ea ) 520 K ) 365 cm-1, which is proportional to the energetic disorder ∆.12 Using J ) 750 cm-1 defined from the absorption spectrum as (λmaxJ - λmaxmon)/2.4 taking into account long-range interactions,5 we obtain the energetic disorder degree ∆/J ≈ 0.5 for localized excitons that is 2.5 times larger than that for delocalized excitons (0.2).24 At T ∼ 155 K the curve of the temperature dependence of the trap luminescence exhibits a maximum and then the luminescence intensity
Lebedenko et al. decreases sharply within the 160-240 temperature range (Figure 5b, dot-dashed line). The competition of two processes affected localized exciton motion is supposed. On the one hand, thermoactivation of localized excitons promotes their mobility, but on the other hand, scattering on phonons restricts the exciton motion. It should be noted that the temperature dependence of the diffusion coefficient for Frenkel excitons hopping in conjugated polymers does not reveal a drop up to room temperature.33 Scattering on phonons for localized excitons in amphi-PIC J-aggregates assumed to be determined by a strong exciton-phonon coupling which is characteristic for these J-aggregates.10,24,28-30 The temperature dependence of the trap luminescence excited at λexc ) 587 nm, between delocalized and localized exciton states, reveals similar to a localized exciton states character (Figure 5b, dashed line). However, this dependence is less pronounced that points to the contribution of delocalized excitons. The excitation at the red tail of the localized exciton band (λexc ) 610 nm) gives a temperature dependence similar to that for delocalized excitons (Figure 5b, dotted line). In the solution with 75% water content, where the J-band longwavelength edge is fitted by Gaussian (Figure 3), the excitation at the sites corresponding to the delocalized (λexc ) 583 nm) and localized (λexc ) 592 nm) exciton band maxima also gives the temperature dependence of the trap luminescence that is characteristic for the coherent exciton transport (not shown). To summarize experimental results obtained we can conclude that the main mechanism of exciton migration in amphi-PIC J-aggregates is coherent. Incoherent exciton hopping is observed only for localized excitons, which is formed on long-wavelength edge of the J-band under the special conditions (low water content in a binary DMF/W solution). The contribution of the incoherent mechanism into the exciton transport in amphi-PIC J-aggregates is negligible. We should to note that the term “delocalized” for excitons in amphi-PIC J-aggregates is quite relative, because exciton delocalization length Ndel is very small due to the influence of a moderate static disorder and strong exciton-phonon coupling. We can estimate Ndel using the following equation:5,34
Ndel )
mon 3(∆νFWHM )2 J 2(∆νFWHM )2
-1
(1)
mon J where ∆νFWHM and ∆νFWHM are full widths at half-maximum (fwhm) of monomer and J-aggregate absorption bands, respectively. The eq 1 gives us only the lower limit of Ndel value due mon value determination.34 As a rule, the to uncertainty in ∆νFWHM mon value is determined in a dilute solution of the dye, ∆νFWHM mon where no aggregation occurs.34 Taking ∆νFWHM ) 1200 cm-1 from ref 24, we obtain Ndel ) 4 monomers at room temperature and Ndel ) 11 monomers at T ) 80 K. The comparison of these values and Ndel ) 25 monomers obtained using luminescence decay time measurements at T ) 1.5 K24 reveals that the values of exciton delocalization length obtained using eq 1 are quit valid. Recently, it was shown that even at room temperature excitons in amphi-PIC J-aggregates can migrate over 80 monomers and more.20 Coherent spreading along molecular chains of excitons delocalized on such small segments is not evident16 and needs further theoretical and experimental research.
Conclusions The temperature dependence of an exciton transport in amphiPIC J-aggregates has been investigated in the temperature range
Exciton Transport in Disordered J-Aggregates of 80s240 K using luminescent traps. This temperature dependence is proportional to that for the exciton diffusion coefficient and allows the predominant mechanism of the exciton transport in J-aggregates to be determined. Amphi-PIC Jaggregates reveal a moderate static disorder (∆/J ≈ 0.2) and a strong exciton-phonon interaction that allowed us to expect incoherent exciton hopping, as was predicted in ref 16. However, experiments show a drop of exciton diffusion coefficient (D ≈ T -1/2) due to scattering on phonons, similar to well-ordered (∆/J < 0.02) THIATS J-aggregates,12 which is evidence of the coherent exciton transport. The incoherent exciton hopping have been observed only for strongly localized by the significant topological disorder (∆/J ≈ 0.5) excitons which form Lorentz long-wavelength tail of the J-band. Due to much smaller intensity of the localized excitons band as compared to the delocalized one and special conditions of localized excitons appearing (strong topological disorder), their influence on the exciton transport in amphi-PIC J-aggregates can be ignored. So, we can conclude that despite excitons confinement within small delocalization segments (11 monomers at 80 K) they migrate along a molecular chain coherently as a wave packet. Acknowledgment. We are grateful to Dr. I. A. Borovoy (Institute for Scintillation Materials, NAS of Ukraine) for providing the amphi-PIC dye. References and Notes (1) Mobius, D. AdV. Matter. 1995, 7, 437. (2) Kobayashi, T., Ed. J-Aggregates. World Scientific Publishing: Singapore, New Jersey, London, Hong Kong, 1996. (3) Shapiro, B. I. Russ. Chem. ReV. 2006, 75, 433. (4) Fidder, H.; Knoester, J.; Wiersma, D. A. J. Chem. Phys. 1991, 95, 7880. (5) Knoester, J. and Agranovich, V. M. In Electronic Excitations in Organic Based Nanostructures. Thin Films and Nanostructures; Agranovich, V. M., Bassani, G. F., Eds.; Elsevier: Amsterdam, Oxford, 2003; Vol. 31. (6) Sundstro¨m, V.; Gillbro, T.; Gadonas, R. A.; Piskarskas, A. J. Chem. Phys. 1988, 89, 2754. (7) Mo¨bius, D.; Kuhn, H. J. Appl. Phys. 1988, 64, 5138. (8) Sato, T.; Kurahashi, M.; Yonezawa, Y. Langmuir 1993, 9, 3395. (9) De Rossi, U.; Da¨hne, S.; Gomez, U.; Port, H. Chem. Phys. Lett. 1998, 287, 395.
J. Phys. Chem. C, Vol. 113, No. 29, 2009 12887 (10) Malyukin, Yu.V.; Tovmachenko, O. G.; Katrich, G. S.; Efimova, S. L.; Kemnitz, K. Mol. Cryst. Liq. Cryst. 1998, 324, 267. (11) Tuszyn˜ski, J. A.; JLrgensen, M. F.; Mo¨bius, D. Phys. ReV. E 1999, 59, 4374. (12) Scheblykin, I. G.; Sliusarenko, O.Yu.; Lepnev, L. S.; Vitukhnovsky, A. G.; Van der Auweraer, M. J. Phys. Chem. B 2001, 105, 4636. (13) Sakomura, M.; Takagi, T.; Nakayama, H.; Sawada, R.; Fujihira, M. Colloids Surf., A 2002, 198-200, 769. (14) Lu, L.; Helgeson, R.; Jones, R. M.; McBranch, D. W.; Whitten, D. G. J. Am. Chem. Soc. 2002, 124, 483. (15) Lu, L.; Jones, R. M.; McBranch, D.; Whitten, D. Langmuir 2002, 18, 7706. (16) Malyshev, A. V.; Malyshev, V. A.; Domı´nguez-Adame, F. Chem. Phys. Lett. 2003, 371, 417. (17) Kirstein, S. ; Da¨hne, S. Int. J. Photoenergy 2006, 2006, article ID 20363. (18) Spitz, C. and Daehne, S. Int. J. Photoenergy 2006, 2006, article ID 84950. (19) Lemaistre, J. P. Chem. Phys. 2007, 333, 186. (20) Grynyov, R. S.; Sorokin, A. V.; Guralchuk, G.Ya.; Yefimova, S. L.; Borovoy, I. A.; Malyukin, Yu.V. J. Phys. Chem. C 2008, 112, 20458. (21) Kim, O.-K.; Melinger, J.; Chung, S.-J.; Pepitone, M. Org. Lett. 2008, 10, 1625. (22) Sundstro¨m, V.; Pullerits, T.; van Grondelle, R. J. Phys. Chem. B 1999, 103, 2327. (23) van Amerongen, H.; Valkunas, L.; van Grondelle, R. Photosynthetic excitons; World Scientific Publishing: Singapore, 2000. (24) Malyukin, Yu.V.; Tovmachenko, O. G.; Katrich, G. S.; Kemnitz, K. Low Temp. Phys. 1998, 24, 879. (25) Fidder, H.; Terpstra, J.; Wiersma, D. A. J. Chem. Phys. 1991, 94, 6895. (26) Moll, J.; Da¨hne, S.; Durrant, J. R.; Wiersma, D. A. J. Chem. Phys. 1995, 102, 6362. (27) Grynyov, R. S.; Sorokin, A. V.; Guralchuk, G.Ya.; Yefimova, S. L.; Borovoy, I. A.; Malyukin, Yu.V. Func. Mater 2008, 15, 475. (28) Malyukin, Yu. Phys. Stat. Sol. C 2006, 3, 3386. (29) Malyukin, Yu., V.; Seminozhenko, V. P.; Tovmachenko, O. G. JETP 1995, 80, 460. (30) Katrich, G. S.; Kemnitz, K.; Malyukin, Yu.V.; Ratner, A. M. J. Lumin. 2000, 90, 55. (31) Song, K. S.; Williams, R. T. Self-trapped excitons, 2nd ed.; Springer-Verlag: Berlin, Heidebelberg, NY, 1996. (32) Agranovich, V. M. ; Galanin, M.D. Electronic Excitation Energy Transfer in Condensed Materials; North-Holland Publishing Company: Amsterdam, 1982. (33) Mikhnenko, O. V.; Cordella, F.; Sieval, A. B.; Hummelen, J. C.; Blom, P. W. M.; Loi, M. A. J. Phys. Chem. B 2008, 112, 11601. (34) Bakalis, L. D.; Knoester, J. J. Lumin. 2000, 87-89, 66.
JP903328R
