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Blue-shifting intramolecular charge transfer emission by nonlocal effect of hyperbolic metamaterials Kwang Jin Lee, Yeon Ui Lee, Frédéric Fages, Jean-Charles Ribierre, Jeong Weon Wu, and Anthony D'Aleo Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.7b05276 • Publication Date (Web): 25 Jan 2018 Downloaded from http://pubs.acs.org on January 27, 2018

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Nano Letters

Blue-shifting intramolecular charge transfer emission by nonlocal effect of hyperbolic metamaterials Kwang Jin Lee,†$ Yeon Ui Lee,†$ Frédéric Fages,‡ Jean-Charles Ribierre,#* Jeong Weon Wu,†* Anthony D’Aléo,†‡¶*

† Department of Physics, Quantum Metamaterial Research Center, Ewha Womans University, Seoul 03760, South Korea

‡ Aix Marseille Univ., CNRS, CINaM UMR 7325, Campus de Luminy, Case 913, 13288 Marseille, France #

¶

College of Optical Science and Engineering, Zheijiang University, Hangzhou, 310027, China

Center for Quantum Nanoscience, Institute for Basic Science (IBS), Seoul 03760, Republic of

Korea Keywords: intramolecular charge transfer, hyperbolic metamaterials, nonlocal effect, Purcell effect, Lippert-Mataga formalism

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Abstract: Metallic nanostructures permit controlling various photophysical processes by coupling photons with plasmonic oscillation of electrons confined in the tailored nanostructures. One example is hyperbolic metamaterial (HMM) leading to an enhanced spontaneous emission rate of emitters located nearby. Noting that emission in organic molecules is from either π-π* or intramolecular charge-transfer (ICT) states, we address here how HMM modifies ICT emission spectral features by comparing them with a spectral shift dependent on the local polarity of the medium. 7.0 nm blue-shift is observed in ICT emission from 4-dicyanomethylene-2-methyl-6-(pdimethylaminostyryl)-4H-pyran dispersed into a polymer matrix prepared on HMM multilayered structure, while no spectral shift is observed in π-π* emission from perylene diimide. In the frame of the Lippert-Mataga formalism, the blue-shift is explained by the HMM nonlocal effects resulting from 8% decrease in refractive index and 18% reduction in dielectric permittivity. This phenomenon was also shown in a hemicurcuminoid borondifluoride dye yielding 15.0 nm blueshift. Such a capability of spectral shift control in films by HMM structure opens new prospects for engineering organic light-emitting devices. Text: Tuning of the photophysical properties of light-emitting dyes (i.e. spectral shift, photoluminescence quantum yield (PLQY), radiative and nonradiative decay rates) is essential for a range of organic optoelectronic devices that are readily commercialized. To this end, plasmonic nanostructures have been extensively employed to control photophysical processes such as spontaneous emission and energy transfer.1-7 For instance, since the pioneering work of Drexhage, it is well known that the spontaneous emission rate of a light-emitting molecule located close to a metallic surface can be controlled by tuning the distance between the emitter and the metallic mirror.8,9 This effect is due to a modification of the photonic density of states by coupling optical fields of emission with plasmonic oscillations. Among the plasmonic


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nanostructures, hyperbolic metamaterials (HMMs) exhibiting hyperbolic dispersion (HD) have been studied intensively in the last few years for emission rate control by making use of high photonic density of states over a broad spectral range.10-17 For example, Lu et al. demonstrated that the photoluminescence (PL) decay rate of light-emitting molecules dispersed into a polymer host could be greatly enhanced using nanostructured multilayer HMM substrates.14 They also carried out theoretical calculations providing evidence that these enhancements could be controlled to the appropriate wavelengths by modifying HD via varying the filling ratio of HMMs. Recently it has also been demonstrated that Förster resonance energy transfer (FRET) and photochemical processes are affected by nonlocal dielectric permittivity of HMM substrate.18,19 Furthermore the nonlocal effect of HMM structures has been applied to tune the photo-induced electron transfer (PET) dynamics via image dipole interaction (IDI).20 In that latter work, slowing down of the PET dynamics was attributed to a modification in nonlocal dielectric permittivity determining charge separation and recombination rates driven by the thermodynamic Gibbs free energy. This mechanism was fully rationalized in terms of Marcus theory framework extended by employing IDI. Intramolecular charge transfer (ICT) dyes have been widely investigated over the past few decades for fundamental purposes leading to applications such as organic light-emitting diodes (OLEDs), organic photovoltaics (OPVs), artificial photosynthesis, sensing and molecular electronics.21-33 Noticeably, the third generation of OLEDs is based on highly efficient thermallyactivated delayed fluorescent dyes, many of them showing a strong ICT character.31 Photophysical properties of ICT emitters in solution are characterized by a strong dependence on the local solvent polarity. In particular, solvatochromism arises from local reorganization of solute and solvent dipoles to minimize the total energy. Similarly, solvatochromic shift can take


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place in organic thin-films, depending on the local polarity of the light-emitting medium and the ICT character of the emitter.34 In this context, it is essential to examine the nonlocal effect of HMM on the emission spectra of ICT emitters and the Purcell factor enhancement related to their emission decay rates in solid thin films. In this letter, we demonstrate that ICT dyes doped in polymer thin films deposited on HMM multilayered structure exhibit a blue-shifted emission due to the nonlocal changes in dielectric permittivity and refractive index. In addition, their fluorescence radiative and non-radiative decay rates and their PLQYs are found to undergo significant changes due to HMM. LippertMataga (L-M) formalism turns out to be very useful to distinguish and describe local and nonlocal effects of the dielectric permittivity and refractive index in evidencing the appearance of spectral changes in ICT emission. Two ICT dyes are employed, 4-dicyanomethylene-2-methyl-6-(p-dimethylamino-styryl)-4Hpyran (DCM) laser dye35-41 and a dipolar hemicurcuminoid borondifluoride (HTPA) dye used recently in far red/NIR OLEDs.42 While DCM consists of a dimethylamino- dicyanomethylene donor-acceptor pair (Figure 1a), in HTPA a triphenylamino group and the acetylacetonate borondifluoride act as donor and acceptor (Figure S2a), respectively. Such systems undergo a photoinduced ICT with ground state dipole moment of 6.1 D and excited state dipole moment of 26.3 D for DCM35 (7.3 D and 18 D for HTPA), respectively. In those dyes, ICT is mediated by the conjugated bridge composed of two aromatic rings and alternating single and double bonds that extend all the way from the donor to the acceptor units. The HMM structure has the configuration presented in Figure 1b and consists of 10-nm thick Ag and Al2O3 alternating thin films deposited on fused silica. In the following, p is defined as the number of Ag/Al2O3 pairs. 140 nm thick films of DCM blended into a poly(methyl-methacrylate) (PMMA) host with


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different doping concentrations (1, 2, 3, and 5 wt%) were spin-coated on top of the HMM substrates. First of all, in order to compare the local effect of the medium polarity and the nonlocal effect of the HMM substrate in giving rise to a spectral shift of the emission, we compare steady-state photophysical properties of two sets of samples, DCM:PMMA films with 2 wt% and 5 wt% on 0p (fused silica) as well as DCM:PMMA films with 2 wt% on 0p and 4p HMM substrates. In Figures 1c and 1d are displayed the normalized steady-state photoluminescence (SSPL) spectra of two sets of samples. Figure 1c shows a red-shift of 9.4 nm (35 meV) when the concentration of DCM is increased from 2 wt% to 5 wt% in the first set of samples. This is attributed to the increased local medium polarity, resulting from the increased local dielectric permittivity when increasing the concentration of polar DCM dye in the PMMA host.35 In contrast, the blue-shift of 6.0 nm (23 meV) in the emission from 2 wt% DCM:PMMA on 4p HMM structure (Figure 1d) compared to that on 0p in the second set of samples indicates that the HMM structure produces a modulation of the ICT excited state and ground state. As will be discussed further below, this is related to changes of the nonlocal dielectric permittivity and refractive index in the vicinity of ICT emitter when HMM substrate is present. We then measured the SSPL spectra of 2 wt% DCM:PMMA films spin-coated on top of 0p, 1p, 2p, 3p, 4p and 8p HMM substrates (Figure S1d). The emission energy is found to increase almost linearly as the number of pairs p increases up to 4p followed by a saturation for 8p (Figure S1d, Table S2), for which a maximum of 6.8 nm (25.9 meV) blue-shift was observed. The saturation can be attributed to the validity of an effective medium description of electromagnetic response in an infinite number of pairs HMM. For the PLE spectrum, 1.2 nm redshift is observed for 5 wt% DCM:PMMA when compared with 2 wt% DCM:PMMA in the


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first set of samples (Figure S3a), while no shift is shown on 4p when compared with 0p in the second set of samples (Figure S3b). For the HTPA blend, an even more pronounced blue-shift of 15.2 nm (43.3 meV) is measured in the SSPL spectra, which indicates that the nonlocal effects of HMM substrate observed in DCM dye also takes place in other ICT emitters (Figure S2b). As a counter example of no spectral shift, SSPL spectrum is measured for a film of perylene diimide (PerDi) dispersed in PMMA deposited on 4p HMM substrate (Figure S4). The emission of PerDi:PMMA with 2 wt% on 4p HMM is mainly of π-π* nature while its emission spectral range is in the red region, similar to that of DCM dye. In this case, no spectral shift is observed in the presence of HMM structure. To illustrate the difference between ICT and π−π* dyes, the steady state emission spectra of two DCM and HTPA (ICT) and PerDi (π−π*) are shown in Figure 2. While spectral emission blue-shifts were observed in DCM and HTPA, no shift was observed for PerDi. Hence, the spectral shift occurring in ICT dye cannot be attributed to Purcell effect in the current thin film sample. Indeed, we note that

HD of HMM is known to enhance the Purcell factor of an emitter located nearby HMM substrate, leading to an increased spontaneous emission rate.14 In addition to altering the temporal behavior of the emission, the HD yielding the broadband nature of the Purcell factor can also give rise to a spectral shift of emission in a sandwiched cavity structure, irrespective of ICT or π-π* character.11 However, the absence of shift for the π−π* emission of the perylene bisimide, which emits in the spectral region similar to DCM ICT dye, is a clear evidence that the observed behavior is not attributed to the photonic density of states. Change in Purcell factor is negligible in the emission spectral region of DCM and PerDi, as shown in figure S16. If the change in Purcell factor is responsible for the spectral shift of ICT emission in the thin film sample structure, the same kind of spectral shift would be observed for PerDi dye as well as DCM, HTPA and PerDi dyes, which is not


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the case. Therefore, the spectra in Figure 2 support the notion that the blue-shifts of DCM and HTPA dyes are certainly associated with the nonlocal effects of the HMM structure owing to their ICT character.

Time-resolved photoluminescence (TRPL) was carried out to examine the influence of the Purcell factor on ICT emission. Figure S6 shows the PL decays obtained in 2 wt% DCM:PMMA blend spin-coated on top of HMM substrates with different p. The PL dynamics show an emission wavelength dependence and are characterized by three time constants, which are listed in Table S2. These results imply that three emitting states are involved in the emission process of DCM in PMMA blend. We attribute these time constants to the decay time of the local excited (LE) state and the decay times of two ICT excited states with monomeric and dimeric structures, similarly to what has been previously reported for HTPA in 4,4’-bis(N-carbazolyl)–1,10biphenyl matrix.42 Furthermore, an evidence of the presence of the LE state is also provided in Figure S8. Noticeably, these results also indicate that a relaxation takes place from LE to ICTs and an energy transfer takes place from the monomer to the dimer ICT. Such behavior was observed for all samples used in this study (Figures S8-S10). To identify how spectral shift and spontaneous decay rate are related with local medium polarity and nonlocal HMM effects, TRPL spectra of two films of 5 wt% DCM:PMMA on 0p and 2 wt% DCM:PMMA on 4p HMM substrate are then compared with those of 2 wt% DCM:PMMA on 0p. As shown in Figure 3a, while there are differences in the weight of monomer and dimer contributions to red-shift in 5 wt% DCM:PMMA on 0p and blue-shift in 2 wt% DCM:PMMA on 4p HMM substrate in TRPL spectra, an increased spontaneous emission rate is strikingly


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pronounced in 2 wt% DCM:PMMA on 4p HMM substrate. Figure 3b shows the integrated PL spectra along with time axis. Upon time gating the emission (after ca. 4.2 ns for 0p and 3.6 ns for 4p), the spectrum of the long-lived species corresponding to dimer can be recorded, which allows to deconvolute the spectrum of short-lived species corresponding to monomer from the SSPL spectrum. PL spectrum for 2 wt% DCM:PMMA on 4p (5 wt% DCM:PMMA on 0p) shows a blue-shift (red-shift) of 2.4 nm (7.2 nm) for monomer and 3.6 nm (5.4 nm) for dimer compared to those for 2 wt % DCM:PMMA on 0p. This indicates that the spectral shifts of both monomer and dimer result from local medium polarity in 5 wt% DCM:PMMA on 0p and from nonlocal HMM effect in 2 wt% DCM:PMMA on 4p HMM. Interestingly, from the comparison of PL spectra of 2 wt% DCM:PMMA on 0p and 2 wt% DCM:PMMA on 4p shown in Figure 3b, it can be seen that the ratio of monomer emission intensity to dimer emission intensity increases in the presence of HMM substrate. This is also supported by the data shown in Figure S5b and Table S2. Such a behaviour is certainly due to a less efficient FRET process in the presence of HMMs as noticed previously by Noginov et al.18 This statement is well supported by the analysis of the PLQY, fluorescence radiative and nonradiative decay rates, provided in SI. (Figure S11 and table S5) As shown in Table S5b, the PLQYs of the monomeric and dimeric structures decrease as the number of pair increases, which is due to the volume plasmonic polariton coupling in HMM structures resulting also in an increased nonradiative decay rate. In order to find whether local medium polarity and nonlocal HMM effects are superimposable in giving rise to a spectral shift of ICT emitter, we compare SSPL spectra, streak images, and spontaneous emission decays of two films of 5 wt% DCM:PMMA on 0p and 4p HMM substrate with that of 2 wt% DCM:PMMA on 0p as shown in Figures 4a~4c. 9.4 nm red-shift in 5 wt%


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DCM:PMMA on 0p by local medium polarity is compensated by 6.6 nm blue-shift in 5 wt% DCM:PMMA on 4p HMM by nonlocal effect without noticeable spectral distortion. The resulting PL spectrum is almost the same as that of 2 wt% DCM:PMMA on 0p (Fig. 4a), while the spontaneous emission rate is substantially increased in 5 wt% DCM:PMMA on 4p, when compared with 2 wt% DCM:PMMA on 0p, owing to the enhanced Purcell factor of emitters located nearby HMM substrate (Fig. 4b and 4c). This observation clearly elucidates the two-fold role of HMM substrate in the spectral shift and the Purcell factor enhancement for ICT emission. Differently, only the Purcell factor enhancement takes place for π-π* emission with no spectral shift, as discussed above in the emission of PerDi :PMMA on 4p HMM. To rationalize and quantify the blue-shift of DCM monomer emission observed on HMM structures, L-M formalism was employed to unravel the effects of HMM structures inducing the shift.43,44 To relate the ground and excited state dipole moments with the Stokes shift, we decided to focus mainly on the monomeric species. L-M formalism relates the Stokes shift (∆ν) of a solvated ICT emitter with the medium polarity via the orientation polarizability ∆f’(ε, n) of the medium with the dielectric permittivity and n the refractive index, implying that both ε and n contribute to a change in ICT state energy level. For DCM:PMMA films with 1 wt% and 5 wt% on 0p the refractive index n was extracted (see Methods) to be 1.599 and 1.601 from the reflection spectra at the emission peak (λ=580 nm), respectively (shown in Figure S12), indicating that the contribution of refractive index change can be neglected while an increase in the local dielectric permittivity is mainly responsible for a red-shift. For DCM:PMMA films with 2 wt% on 0p , 1p, 2p, 3p, 4p, and 8p HMM substrates, nonlocal refractive indices are extracted from the Purcell factors determining fluorescence


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lifetimes (shown in Figure S13). We find that n decreases by up to 8% for 8p HMM substrates based on the basic relation between the Purcell factor and refractive index,45,46 which has been explained in SI-sections of ‘Discussion on the nonlocal refractive index and Discussion on the Purcell effect on the refractive index’. We suggest that a decreasing behavior of n is also supported by the Strickler-Berg relation. We suggest that a decreasing behavior of n is also supported by the

Strickler-Berg relation. In L-M plot the slope of a straight line is associated with the Onsager radius of the dye and the difference between the ground and excited state dipole moments, which is an intrinsic molecular property of ICT emitter.43,44 Black solid squares in Figure 5a are experimental data obtained in various solvents with different ∆f’, which yield the slope value of 8414.5 (cm-1), similar to that reported in a previous study .35 By use of this slope we determine the orientation polarizability values ∆f’ of the films associated with local medium polarity and nonlocal HMM effect from the measured Stokes shift ∆ν of monomer and dimer. By using ∆f’ of medium, we obtained ε(wt%) for local medium polarity and ε(p) for nonlocal HMM effect by taking into account the extracted n (see Methods), which are plotted as a function of wt% and p in Figure 5b. ε(wt%) is found to increase from 4.34 (1 wt%) to 5.22 (5 wt%) with 20% increase, while ε(p) varies from 4.50 to 3.71, indicating that 18% reduction of dielectric permittivity (compared to the value obtained for 0p) results from HMM structure. In addition, in order to confirm the validity of L-M plot for red-shifted DCM emission in thin film, we examined ICT emission of DCM dye dispersed in polyvinylpyrrolidone (PVP, = 1.527 in literature) matrix, possessing a higher dielectric permittivity than PMMA (= 1.491 in literature). As expected from ∆f’, the emission of the DCM:PVP film on 0p is 45 nm more redshifted than that of DCM:PMMA film on 0p (Figure S14).


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A blue-shift (by ca. 7.0 nm) is also found when the DCM:PVP film is prepared on top of 4p HMM, similar to what was observed in DCM:PMMA film (Figure S14d). The same amount of spectral shift observed in PVP stems from a simultaneous ~24 % reduction of nonlocal dielectric permittivity and 8.1 % decrease of nonlocal refractive index (Figure S15). The larger amount of change in nonlocal dielectric permittivity when using PVP host is due to the fact that the redshifted emission in PVP falls in the spectral range where the real part of dielectric permittivity of HMM is more negative, leading to a smaller nonlocal refractive index.. As mentioned above, the effect of the HMM on the broadband nature of the Purcell factor can lead to a shift of the emission spectra in a sandwiched cavity structure.11 In order to examine the effect of the Purcell factor on a spectral shift, first we calculated by FDTD simulation the Purcell factor dispersion for a series of different number of p HMM as displayed in Figure S16a. Differently from the previous work,11 in which a cavity structure has been used to obtain a strongly dispersive Purcell factor, Purcell factor variation in the emissive layer on top of HMM (Figure 1b) results in a null spectral shift in the range of dye emissions. Subsequently, we compare SSPL

spectrum of 2 wt% DCM:PMMA film on 0p corrected by the Purcell factor of Figure S16a with two SSPL spectra of the same film on 0p and 4p, as shown in Figure S16b. Purcell factor correction yields a very small amount (less than 0.2 nm) of spectral red-shift while the DCM:PMMA film on 4p reveals a blue-shifted emission by 7.0 nm. These results demonstrate that the broadband Purcell effect is not responsible here for the ICT emission spectral shift observed on top of HMM. In Fig. 5c and 5d are shown schematically how the local and nonlocal effects stabilize and destabilize ICT energy level to result in red and blue spectral shift, respectively.


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Finally, we have examined the influence of energy-momentum dispersion of HMM on the spectral shift in ICT emission by using HMM substrate with a lower fill factor. A similar amount of blue shift of DCM emission in the presence of HMM substrate with a lower fill factor was observed (See Figure S18), which results from a smaller decrease of the dielectric permittivity for a lower fill factor (See Table S6). This can be understood in terms of the difference in the change of nonlocal dielectric constant of medium surrounding ICT emitter based on that IDI is inversely proportional to dipole-dipole distance. In addition, we addressed how IDI in a HMM structure depends on the elliptic and hyperbolic dispersion relations by comparing two different HMM structures having different fill factors (see figure S19). A similar spectral blue shift of DCM monomer observed in elliptic dispersion region indicates that nonlocal effect of HMM based on the IDI is dependent on the energy-momentum dispersion of HMM. Instead, the negative dielectric permittivity of Ag layer provides the structure for the IDI. All the data and detail discussion are presented in Table S6 and SIsection ‘Discussion on the influence of fill factor on the spectral shift: comparison of elliptic and hyperbolic dispersions’.

In summary we have investigated a blue-shift of emission from ICT emitter in thin film prepared on top of multi-layered HMM. Noting that emission spectrum of ICT emitter, possessing large ground and excited state dipole moments, is strongly affected by the medium polarity, the relation between the spectral shift and the number of dielectric/metallic layer pairs of HMM is analyzed in terms of Lippert-Mataga formalism. It is shown that HMM alters both dielectric permittivity and refractive index of environment of ICT emitter, though via very different mechanisms. Image dipole interaction of charge transfer state present in multi-layered HMM structure is accounted for by nonlocal dielectric permittivity,


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while the Purcell factor enhancement from HD of HMM allows to introduce nonlocal refractive index. 4 pair HMM resulted in 18% and 8% decreases in dielectric permittivity and refractive index, respectively, leading to 7.0 nm blue-shift of emission spectrum from DCM in PMMA matrix. We note that a blue-shift occurs in DCM since it possesses dipole moment of the excited state µe larger than that of the ground state µg. A red-shift is expected for ICT emission from dyes with µg larger than µe since ground state would be more affected than excited state in the presence of HMM. Red-shift of emission in high-doping concentration is studied to examine the compatibility of the local and nonlocal effects on dielectric permittivity and refractive index, and it is found that the nonlocal effect of HMM and the local effect of medium polarity are superimposable in giving rise to a spectral shift. The equivalence of local and nonlocal effects for spectral shift is an important basic feature that gives a guideline when a wide tunability of spectral emission is desired. Multi-layered HMM is a straightforward simple structure when integrated in the fabrication process of display and light-harvesting devices. One advantage of HMM is that spectral dispersion of VPP47 and spatial distribution of the Purcell factor enhancement can be tailored independently, allowing for a device design to overcome PLQY decrease observed in the presence of HMM.48 The capability of controlling spectral shift non-locally by HMM serving as substrate opens a future research area for high-performance display and light-harvesting devices, when metamaterials are incorporated in device architecture.49


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Figure 1. Spectral shift of the ICT emission (a) Molecular structures of DCM dye with dipole moment values in the ground and excited states. (b) Sample architecture with HMM substrate (c) Steady-state photoluminescence (SSPL) spectra (excited at 470 nm) of DCM:PMMA film with 2 wt% (black curve) and 5 wt% (red curve) on 0p (fused silica) (d) SSPL spectra of DCM:PMMA film with 2 wt% in the absence (0p, black curve) and presence of 4p (blue curve) HMM substrate. In case of 4p substrate, both spectra were corrected by taking into account the reflectance of substrate.


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Figure 2. Steady state emission spectra of the two ICT dyes (DCM and HTPA) and π-π* dye (PerDi) on the fused silica substrate (black) and 4p HMM substrate (blue).


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Figure 3. Spectral and temporal behavior of ICT emission from DCM blends. (a) Streak camera images of DCM:PMMA film with 2 and 5 wt% on 0p and 2 wt% on 4p substrates (excited at 470 nm). (b) Integrated photoluminescence spectra for the entire time range (black), the dimer (red) emission integrated from 4.2 ns to the end, and the monomer (blue) emission obtained by the deconvolution of the entire PL spectra with the dimer spectra


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Figure 4. Change in nonlocal HMM effect vs local medium polarity (a) SSPL spectra of 2 wt% DCM:PMMA on top of 0p substrate and 5 wt% DCM:PMMA on top of 0p substrate and 4p HMM. The 9.4 nm red-shift and 6.4 nm blue-shift observed between these spectra enable to compare the local and nonlocal effects on ICT emission spectra. (b) Streak images of 2 wt% DCM:PMMA on 0p substrate and 5 wt% DCM: PMMA on 4p HMM. (c) PL dynamics of these two samples, that emit with the same emission spectral range. The faster decay rate for the 5 wt% blend on 4p indicates that the shortening of the fluorescence lifetime results mainly from an increase in the Purcell factor.


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Figure 5. Nonlocal effect of HMM structure as solvatochromism analogue. (a) Lippert-Mataga plot of DCM in solvents with different polarities, 2 wt% DCM:PMMA films on top of 0,2,4 and 8p substrates and 5 wt% DCM:PMMA on 0p substrate for ICT monomer emission.. (inset: magnification of circled region) (b) Evolution of the nonlocal dielectric constant as a function of the doping concentration (ε(wt%)) and p (ε(p)) (c),(d) Description of energy parabolas as a function of the nuclear coordinates of solute; after photo-excited from ground (black parabola) to local excited states (brown parabola), relaxation into ICT state occurs by reorientation of solute and environment molecules to minimize the total energy (yellow parabola). Then ICT energy level is (c) stabilized owing to increase in the local dielectric constant by enhancing the local polarization field with increasing dye concentrations. (d) destabilized due to decrease in dielectric constant based on IDI; the colored solid arrows are associated with the excitation and relaxation and colored dashed arrows show stabilization/destabilization.


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ASSOCIATED CONTENT Supporting Information. Detailed experimental data and analysis based on the steady-state and time-resolved fluorescence measurements, description of ICT process in DCM dye, behavior of quantum yield and fluorescent radiative decay rate and non-radiative decay rate, discussion on the Purcell effect on the refractive index, spectral shift behavior in different polymer hosts, Numerical simulation of Purcell factor, Analysis of spectral shift of ICT emission using new HMM with different fill factor and another ICT dye (Coumarin500). This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected], [email protected], [email protected] Author Contributions KJL, JCR, JWW and ADA conceived and designed the experiments. KJL and ADA prepared samples and KJL completed all experimental measurements. KJL, YUL, JWW and ADA discussed the results and analyzed the data with feedbacks from FF and JCR. YUL completed the Purcell factor calculations based on FDTD. KJL, JWW and ADA wrote the manuscript. All authors commented the manuscript. $ These authors contributed equally. Funding Sources


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Any funds used to support the research of the manuscript should be placed here (per journal style). Notes Any additional relevant notes should be placed here. ACKNOWLEDGMENT This work is supported from the Ministry of Science, ICT and Future Planning, Korea (2015001948, 2014M3A6B3063706) and ADA would like to thanks CNRS for “mise à disposition”. REFERENCES (1) Pelton, Matthew, Nat. Photon. 2015, 9, 427-435.. (2) Russell, K. J.; Liu, T. L.; Cui, S.; Hu, E. L. Nat. Photon. 2012, 6, 459-462. (3) Belacel, C.; Habert, B.; Bigourdan, F.; Marquier, F.; Hugonin, J. P.; Michaelis de Vasconcellos, S.; Dubertret, B. Nano Lett. 2013, 13, 1516-1521. (4) Ghenuche, P.; de Torres, J.; Moparthi, S. B.; Grigoriev, V.; Wenger, J. Nano Lett. 2014, 14, 4707-4714. (5) De Torres, J.; Mivelle, M.; Moparthi, S. B.; Rigneault, H.; Van Hulst, N. F.; García-Parajó, M. F.; Margeat E.;Wenger, J. Nano Lett. 2016, 16, 6222-6230 (6) Podolskiy, V. A.; Ginzburg, P.; Wells, B.; Zayats, A. V. Farad. Discuss. 2015, 178, 61-70.


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