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Excimer Emission in J-Aggregates Oleg Petrovich Dimitriev, Yuri P. Piryatinski, and Yuri L. Slominskii J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.8b00481 • Publication Date (Web): 10 Apr 2018 Downloaded from http://pubs.acs.org on April 10, 2018

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The Journal of Physical Chemistry Letters

Excimer Emission in J-Aggregates

Oleg P. Dimitriev1*, Yuri P. Piryatinski2, Yuri L. Slominskii3, 1

V.Lashkaryov Institute of Semiconductor Physics NAS of Ukraine, pr. Nauki 41, Kyiv, Ukraine 2

3

Institute of Physics NAS of Ukraine, pr. Nauki 46, Kyiv, Ukraine

Institute of Organic Chemistry NAS of Ukraine, 5 Murmanska St., Kyiv, Ukraine !

*

Corresponding author, e-mail: [email protected]; tel/fax +38 044 5255530

1 ACS Paragon Plus Environment

The Journal of Physical Chemistry Letters 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Abstract Excimer in J-aggregates has been often considered as a self-trapped exciton originated from the free exciton excited on the same aggregate and relaxed through interaction with vibronic modes. Here we show that other type of excimers due to intermolecular off-diagonal interactions can be observed in J-aggregates of thiamonomethinecyanine dyes. These excimers arise owing to free excitons too, but they possess a longer formation time of more than 100 ps, indicating migration of free excitons to the excimer formation site, where they interact with a guest species in the ground state. Formation of the excimers occurs in solutions as a power law of concentration with exponent of one and a half, showing that an excited aggregate should be twice longer than a ground-state guest species, consistent with the exciton coherence length of four molecules versus one dimer, respectively. Unlike the self-trapped exciton, lower temperatures lead to significant suppression of the observed excimer emission.

Keywords: J-aggregate; excimer; self-trapped exciton; thiamonomethinecyanine dyes

TOC Graphic


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An excimer is normally referred to as an unstable complex composed of a pair of an excited molecule and a ground state molecule, which exists as long as one of the pair components is being in the electronic excited state and which dissociates after photon emission of the pair has occurred [1]. Excimers are normally possess a broad intensive emission band with a large Stokes shift which makes them attractive for application in optoelectronics such as excimer lasers [2] and excimer lamps [3]. On the other hand, a singlet excimer has a relatively short lifetime, of the order of nanoseconds. Although the term excimer or, in other words, excited state dimer is generally limited to cases where a true dimer is formed, a special interest is extended to larger complexes, i.e., those composed of numerous species, which exist in the excited state. For example, binding of a large number of excited atoms forms Rydberg matter clusters, the lifetime of which can exceed many seconds [4,5]. There is some specific to define excimers in the solid state (i.e., crystals, aggregates, films) where species cannot literally dissociate being in the ground state, because of the rigid environment. Nevertheless, the excimeric nature of luminescent emission of different species like rare gas crystals or aromatic pyrene crystals, where intermolecular interactions due to van der Waals forces or π-π stacking are significant, has long been recognized as excimeric in nature as well. In inorganic crystals, composed of metal halides or noble atoms, a strong exciton-lattice coupling can usually result in the covalent bond formation in the excited state of adjacent atoms which do not admit such bonding in their ground state. In molecular crystals or aggregates, it was discussed that the excimer configuration which can be achieved after light absorption, is associated with further approach of the two molecules that were already adjacent [6]. Thus, it was concluded that the molecular excimer in fact arises due to formation of a preassociated dimer [7]. On the other hand, an excimer formation in the solid state, for example, in crystals of noble gases, aromatic molecules of pyrene, etc., has been considered as a particular case of exciton localization or selftrapping [8], which was particularly associated with the deep self-trapping exciton state [9]. The self-trapped exciton (STE) is described as a bound electron-hole pair, where at least one carrier of the pair (electron or hole) 3 ACS Paragon Plus Environment


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experiences a strong coupling to the crystal lattice, owing to which a carrier may be self-trapped as a small polaron in its own lattice distortion field. Thus formed STE may dramatically influence luminescence, showing a broad excimer-like band on the low-energy side of the resonant emission. So far, excimers in molecular Jaggregates have also been classified as a STE originating from a free exciton excited on the same aggregate and relaxed through interaction with vibronic modes [10,11]. That means that the origin of such STE is due to intra-aggregate (diagonal) interactions. Although the excimers due to molecular slip-stacking have been reported [12], their state was recognized to be less stable as compared to a cofacial stacking. Therefore in ordered single J-aggregates representing arrangement of slip-stacked molecules in a quasi-one dimensional direction, formation of even preassociated dimers which could give a classical excimer emission is hardly possible. However, the classical excimer emission should be available in aggregates of higher dimensionality (2D or 3D) or as a result of interaction of different aggregates. For example, it was reported a red-shifted emission in J-aggregates beyond the J-band which was explained by off-diagonal interaction, such as trapping dimeric states [13,14] or dimer-like states formed below the exciton band as a result of disorder [15]. However, such a red emission was not classified as the excimer one. Thus, in the solid state physics, the term excimer confuses with the term of a self-trapped exciton. Here we show that the other type of excimer exists in J-aggregates, which originates from the off-diagonal interaggregate interaction. We believe that in J-aggregates the terms excimer and STE should be separated as they have different properties and origin, as will be demonstrated below. J-band emission of thiamonomethinecyanine dyes (dyes 1 and 2, Scheme S1) in KCl aqueous solutions demonstrates unusual broadening even after some period when the J-aggregate formation process has finished (Fig. 1a). Such a broadening can be explained by the presence of additional components in the solution. Specifically, the above broad band with maximum at ~485 nm can be decomposed into three Gaussians, corresponding to the narrow free resonance exciton (FRE) peaked at 486 nm and FWHM of ca. 500 cm-1, STE centered at 501 nm and FWHM of ca. 1000 cm-1, and dimer emission with maximum at 530 nm and FWHM of


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ca. 1650 cm-1. Details of the assignment of the STE and dimer bands are given in the Supporting Information (Figs. S2-S6). Emission of the monomer can be neglected here as it has very small concentration and quantum yield as compared to the J-band (Fig. S7). It is well-known that STE emission often accompanies the J-aggregate emission in the form of a redshifted and broad band in respect to the narrow FRE emission, because there is no self-trapping barrier in a closed 1D system, where excitons are immediately self-trapped at any exciton-phonon coupling strength. In the real non-ideal 1D J-aggregate, FRE and STE are separated by a self-trapping barrier (insert in Fig.1a) and can co-exist [16,17,18]. However, PL spectrum of the solution of 1 reveals an additional structureless band in the red region of the spectrum at ~620 nm, whose intensity is dependent on the excitation energy, where excitation by higher energy within the 270-405 nm spectral range induces stronger emission band (Figs. 1a and S8). This red band is tentatively assigned to excimer emission and its detailed analysis is given below. PL excitation of the above red band almost perfectly meets the absorption of the J-band (Fig. 1b), therefore, its origin should be assigned to FRE excitation. Comparison of the PL excitation and absorption spectra reveals their difference from the low-energy edge of the J-band. The J-band in the PL excitation spectrum is well fitted by a Gaussian, whereas it has a more heavy tail in the absorption spectrum which is better fitted by a Lorentzian (Fig. S9). The Lorentzian character of the absorption shape evidences in favor of the off-diagonal disorder in J-aggregates [19,20]. A great deal to such a disorder is due to exciton-vibronic interactions which lead particularly to the exciton self-trapping and whose absorption edge can be described as [21]

~ℏ − ± ℏ /

(1),

where is the exciton resonance energy and ℏ the energy of the molecular vibronic mode. On the other hand, the narrower excitation spectrum indicates that exciton-vibronic interactions do not contribute to the excitation of the excimer. The above dependence of the PL spectra on the excitation energy points out that the excimer level is separated from the FRE level by a higher barrier than the barrier between STE and FRE (see insert in Fig.1a). It should be noted that there are additional excitation bands of the excimer in the UV near 230



and 290 nm (Fig. S10), which have an auxiliary role and just help to overcome the above barrier, resulting in the dependence of the excimer intensity of 1 on excitation wavelength.

1,0

1

2 PL excitation, normalized

absorbance. normalized

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0,8

0,6

0,4

0,2

0,0

350

400

450

500

550

wavelength, nm

a

b

Fig.1. (a) PL spectrum of KCl aqueous solution of 1 (2·10-5 M) under excitation at different wavelengths; the red curve is decomposed into Gaussians (dotted curves), and (b) PL excitation spectra (solid curves) of excimers (λreg=620 nm) compared to absorption spectra (dotted curves) of KCl solutions of 1 (2·10-5 M, red lines) and 2 (2·10-5 M, blue lines). All curves have been normalized to the same maximum.

Excimer emission reveals a more rapid increase with concentration than the emission of both J-band and STE band (Fig. 2a). While both J- and STE bands have the same linear tendency of increase with solution concentration (Fig. S5), the excimer band intensity demonstrates a power law dependence on concentration [C] with exponent of about one and a half (Fig. 2b), i.e.,

1.5

(λ) dλ =[C]

(2).

A similar behavior is observed for J-aggregates of 2. Specifically, the dye solution shows development of J-band at ~510 nm with increasing concentration (Fig. 3a). Simultaneously, the excimer band at ~615 nm arises whose origin is due to FRE as can be deduced from the comparison of the excitation and absorption spectra of 2 in Fig.1b, but whose barrier in respect to FRE should be low as compared to 1, as this emission can be easily observed with excitation at 405 nm. The excimer emission again increases more rapidly with


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concentration than the emission of J- and STE bands, and its intensity dependence on concentration has a power law with exponent of about one and a half (Fig. 3b).

1

5.8 12000

5.6

log PL intensity

8000

increasing concentration

PL emission, a.u.

10000

6000

4000

2000

Excimer y=c+dx

5.4 5.2

d/b=1.45±0.1

5.0 4.8 4.6

J-band y=a+bx

4.4 4.2 0.0

0 480

520

560

600

640

680

0.1

0.2

720

wavelength, nm

0.3

0.4

0.5

0.6

0.7

log C

a

0.8

b

Fig.2. (a) PL emission of KCl solution of 1 (λexc=270 nm) with increasing concentration of dye from 1·10-5 to 5·10-5 M and (b) the concentration dependencies of PL intensity of the excimer band (red symbols) and J-band (black symbols) and their linear fits.

5.2

2 5.0

log PL intensity

1500

PL emission, a.u.

1000

increasing concentration

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


500

Excimer y=c+dx

4.8

4.6

d/b=1.40±0.1

4.4

4.2

J-band y=a+bx

4.0 0.0

0

480

520

560

600

640

wavelength, nm

680

720

0.2

0.4

0.6

0.8

log C

760

b

b

Fig.3. (a) PL emission of KCl solution of 2 (λexc=405 nm) with increasing concentration of dye from 1·10-5 to 6·10-5 M and (b) the concentration dependencies of PL intensity of the excimer band (red symbols) and J-band (black symbols) and their linear fits. 7 ACS Paragon Plus Environment


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A similar concentration behavior of the excimer emission for the both dyes suggests the same mechanism of the excimer formation. First, the advancing growth of the excimer emission over the J-band emission with increasing concentration implies that origin of the excimer band is due to off-diagonal intermolecular interaction. In our previous work [22] it was shown that formation of J-aggregates of 1 and 2 proceeds through the changed equilibrium between nucleating J-aggregates and dimers, so that under the dark conditions (which is the condition of these experiments) J-aggregation proceeds as formation of new Jaggregates with a subsequent increase of their amount rather than the increase of the aggregate length. The former mechanism of the J-aggregates formation is confirmed by the practically unchanged shape of the J-band emission with concentration (Fig. 2a). Therefore, the increasing concentration leads to increased interaggregate contacts which may be in the form of short-time aggregate collisions or long-time aggregate stacking, both resulting in off-diagonal inter-aggregate interactions. The evidence of inter-aggregate interactions is clearly indicated through obvious broadening of the J-band bandwidth with dye concentration (Fig. S11). Second, a similar law of the excimer band intensity increase with concentration for the both dyes with the exponent of one and a half (Eq.2) implies that the excited species should be twice longer than the groundstate guest molecule or aggregate. Time-resolved spectroscopy indicates that the excimer emission decays mono-exponentially (Fig.5) with time constant of 2.55 ns for 1 and 1.29 ns for 2. The dynamics of the excimer emission has a significant difference from that of the STE. First, the lifetime of the excimer emission is far larger than that of STE; the latter is being ca. 0.66 ns for 1 and 0.49 ns for 2 (Table 1). Second, the excimer emission has a noticeable rise time of ca. 350 ps for 1 and ca. 160 ps for 2 (Fig. 4), whereas the rise time of STE is within the resolution time of the method and it is estimated to be less than 20 ps. Therefore, while the mechanism of formation of STE is due to vibronic relaxation, the excimer formation time is comparable with the lifetime of FRE. It is known that FRE is able to migrate over the aggregate a relatively large distance during its lifetime [23,24]. Therefore, the excimer can be a result of the free exciton diffusion to the excimer-forming site.


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1

1

Fluorescence intensity, norm.

excimer Fluorescence intensity, norm.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.1

STE

1

FRE

0.01

IRF 483 505 620 1E-3

excimer 0.1

STE

2 0.01

FRE

IRF 505 530 615

1E-3

0

1

2

3

4

0

1

2

Time delay, ns

3

4

Time delay, ns

a

b

Fig.4. Emission time profiles of KCl solutions of (a) 1 and (b) 2 after excitation at 405 nm; the probed emission wavelengths are indicated in the legend. Arrows show emission highs of J-band and excimer band. Table 1. Bi-exponent fluorescence decay fitting of the J-aggregate solutions of 1 and 2 (both at 1·10-5 M).

Dye

Exciton type

1

2

Emission

τ1 (ps)

Amplitude

τ2 (ps)

Amplitude

(%)

wavelength (nm)

τave (ps)

χ2

(%)

FRE

483 nm

160

84

1250

16

334±8

0.936

STE

505 nm

162

63

1505

37

659±10

1.099

FRE

505 nm

160

67

840

34

393±8

1.023

STE

530 nm

150

55

910

45

492±10

1.059

The changes in PL spectra with temperature provide additional evidences on the different origin of STE and excimer emissions. Upon cooling to 77 K, the J-band experiences enhancement, as expected for majority J-aggregates [25] (Fig. 5). The same tendency can be observed for the STE emission, so that the increased contribution of STE results in red shift of the main band from 485 to 491 nm in the spectrum of 1 (Fig. 5a) and from 510 to 522 nm in the spectrum of 2 (Figs. 5b and S12). The increased by-pass from FRE to STE at low temperatures is well known for the other J-aggregate systems [26, 20]. Excimer emission in both dyes shows a 9 ACS Paragon Plus Environment


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blue shift at lower temperature which is a similar trend for excimers observed in other dyes [27]. At the same time, the excimer emission does not increase upon cooling but it becomes suppressed relatively to the J-band. Such a behavior is accompanied by the changed rise time of the excimer formation (insert in Fig. 5b) which increases by a factor of ~1.7 at 77 K as compared to room temperature (from 160 to 270 ps for dye 2) which correlates with the increasing lifetime of FRE from 160 to 250 ps, respectively (Table 1 and insert in Fig. 5b). The increased rise time of the excimer implies increased diffusion length of the exciton towards the excimerforming sites. The increased diffusion length at low temperature is a feature of the FRE, but not the STE, because diffusion coefficient of the former DFRE ~(1/T)1/2

(3),

whereas diffusion coefficient of the latter is governed by temperature activated hopping, DSTE ~ exp(-Ea/kT),

(4),

where Ea is the activation energy [28]. The increase in the diffusion length of FRE thus can be evaluated using the well-known ratio LD=(Dτ)1/2

(5),

which, with account of Eq. (3), yields the relative increase of the diffusion length by 1.83 at 77 K. It should be noted that because an exciton with more effective diffusion will probe more excimerforming sites, the increased diffusion length should be associated with increased excimer emission at low temperature. However, that would be the case if the excimer is due to diagonal intra-aggregate interaction. Because intermolecular diffusion and therefore intermolecular contacts are suppressed in the rigid matrix, suppression of the excimer emission at low temperature is associated with its origin due to the off-diagonal inter-aggregate interactions.


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1500

3

1000

0.1

0

1

2

3

4

time delay, ns

1

0.7 0.6 0.5

0.4

0.3

0.00 0.25 0.50 0.75 1.00 1.25 1.50 1.75 2.00

3

STE

time delay, ns

0

500

600

700

450

wavelength, nm

FRE

STE

FRE

0.8

0.2

500

1

500

0

2

1000 IRF 485 nm, as-prepared 485 nm, after thawing 620 nm, as-prepared 620 nm, after thawing

0.9

PL emission, norm.

2

506 nm, 300 K 620 nm, 300 K 506 nm, 77 K 605 nm, 77 K

1

PL intensity, a.u.

PL emission, norm.

1

2000

PL intensity, a.u.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


500

550

600

650

700

750

wavelength, nm

a

b

Fig. 5. PL emission (λexc=270 nm) of KCl solutions (2·10-5 M) of (a) 1 and (b) 2 at 300 K (curves 1 and 3), and at 77 K (curves 2). Solution (1) is the as-prepared solution and (3) the same solution after the freezing-thawing cycle. Inserts show emission time profiles of FRE and excimer after excitation at 405 nm.

Heating of the frozen solution back to room temperature, however, does not completely recover excimer emission, which is especially pronounced for the solution of 1 (Fig. 5a). This effect can be related to the solution ‘ageing’, suggesting the irreversible aggregate structural changes, because a similar partial or complete suppression of the excimer emission was found for the aged solutions as well (Fig. 6). Suppression of the excimer is also accompanied with increasing intensity of the J-band along with increasing lifetime of the FRE by ~70% depending on the relative suppression of the excimer, while the lifetime of the excimer emission remains virtually the same (Insert in Fig. 5a and Fig. 6). As exciton communicates to excimer-forming sites through diffusion, the increased lifetime of the exciton at the same temperature implies decrease of concentration of the excimer-forming sites which equivalent to the suppression of the excimer emission.



700

τ1

600

PL emission, a.u.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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τ2

106 ps (80%) 1264 ps (20%) 121 ps (72%) 1254 ps (28%) 160 ps (84%) 1250 ps (16%) 172 ps (83%) 1200 ps (17%)

500

400

300

200

100

0

450

500

550

600

650

700

750

wavelength, nm

Fig. 6. PL emission of KCl solutions of 1 (λexc=405 nm) at 300 K prepared under different conditions (asprepared, after the freezing-thawing cycle, and aged) along with corresponding time constants and their amplitudes obtained from bi-exponent PL decay fitting at 483 nm.

Increasing emission of the excimer is accompanied also by broadening of the J-band from the lowenergy side (Figs. 6 and S13) which corresponds to development of the emission component near 530 nm assigned to dimers (Fig. S2). On the contrary, suppression of the excimer emission due to the ageing effect above is associated with complete expense of the dimers to build up J-aggregate structure, respectively. Therefore, some amount of dimers is necessary to support the excimer emission, and the formation of excimer itself can be considered due to interaction of the excited J-aggregate and a ground-state dimer next to the aggregate. Such a suggestion is intuitively clear as J- aggregate growth proceeds through consumption of dimers which are in the thermodynamic equilibrium with the growing aggregate [22]. This suggestion is also in agreement with the above ‘one and a half’ power law (see Figs. 2b, 3b, and Eq.(2)), where formation of the excimer is due to the excited species which is twice longer than the ground-state counterpart; that is consistent with the coherent length of the exciton to be approximately four molecules (namely, 4 molecules for 1 and 4 to


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6 molecules for 2, see SI). Slightly longer coherent length in J-aggregates of 2 is consistent with slightly lower exponential coefficient in Eq. (2) as can be seen from Fig. 3b. In conclusion, it has been found that although both excimer and STE emissions originate from the same excitation source, i.e., from free resonance excitation, the excimer emission in J-aggregates demonstrates certain specific properties which allow one to separate this emission from that of STE. First, the excimers have significantly higher formation time associated with migration of the free exciton to the excimer-formation sites; second, they are suppressed at low temperatures; and, third, their emission have an advance increase with concentration over the FRE emission. Whereas formation of STE is governed by diagonal intra-aggregate disorder and interaction of free exciton to vibronic modes, the excimer formation is due to off-diagonal interaggregate interaction of an exciton with a coherence length of four molecules and a ground-state guest dimer, respectively. Supporting Information. SI contains description of dye synthesis and experimental methods, comparison of J-aggregate, dimer and monomer spectra, PL excitation spectra of dimer and excimer in the UV, multipeak fitting of PL, concentration dependencies of the PL emission of STE and J-bands, time-resolved PL emission spectra at different delay times, dependence of FWHM of J-band on concentration, and calculation of the exciton coherent length. This material is available free of charge via the Internet at http://pubs.acs.org.

References 1

Birks, J.B. Excimers. Rep. Prog. Phys. 1975, 38, 903–974. Rhodes, Ch.K. ed., Excimer Lasers; Springer: Berlin, Germany, 1984. 3 Lomaev, M.I.; Skakun, V.S.; Sosnin, E.A. ; Tarasenko, V.F.; Shitts, D.V.; Erofeev, M.V. Excilamps: Efficient sources of spontaneous UV and VUV radiation. Phys.-Usp. 2003, 46, 193–209. 4 Wang, J.; Holmlid, L. Rydberg Matter clusters of hydrogen with well-defined kinetic energy release observed by neutral time-of-flight. Chem. Phys. 2002, 277, 201–210. 5 Yarygin, V.I.; Sidel’nikov, V.N.; Kasikov, I.I.; Mironov, V.S.; Tulin, S.M. Experimental Study on the Possibility of Formation of a Condensate of Excited States in a Substance (Rydberg Matter). JETP Letters 2003, 77, 280–284. 6 Han, G.; Kim, D.; Park, Y.; Bouffard, J.; Kim, Y. Excimers beyond pyrene: A far-red optical proximity reporter and its application to the label-free detection of DNA. Angew. Chem. Int. Ed. 2015, 54, 3912–3916. 2



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