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White Light from Dual Intramolecular Charge-Transfer Emission in a Silylene-Bridged Styrylcarbazole and Pyrene Dyad
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White Light from Dual Intramolecular Charge-Transfer Emission in a Silylene-Bridged Styrylcarbazole and Pyrene Dyad
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  • Malgorzata Bayda-Smykaj*
    Malgorzata Bayda-Smykaj
    Faculty of Chemistry, Adam Mickiewicz University in Poznan, Uniwersytetu Poznanskiego 8, 61-614 Poznan, Poland
    Center for Advanced Technology, Adam Mickiewicz University in Poznan, Uniwersytetu Poznanskiego 10, 61-614 Poznan, Poland
    *Email: [email protected]
  • Karolina Rachuta
    Karolina Rachuta
    Faculty of Chemistry, Adam Mickiewicz University in Poznan, Uniwersytetu Poznanskiego 8, 61-614 Poznan, Poland
  • Gordon L. Hug
    Gordon L. Hug
    Radiation Laboratory, University of Notre Dame, Notre Dame, Indiana 46556, United States
  • Mariusz Majchrzak
    Mariusz Majchrzak
    Faculty of Chemistry, Adam Mickiewicz University in Poznan, Uniwersytetu Poznanskiego 8, 61-614 Poznan, Poland
  • Bronislaw Marciniak
    Bronislaw Marciniak
    Faculty of Chemistry, Adam Mickiewicz University in Poznan, Uniwersytetu Poznanskiego 8, 61-614 Poznan, Poland
    Center for Advanced Technology, Adam Mickiewicz University in Poznan, Uniwersytetu Poznanskiego 10, 61-614 Poznan, Poland
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The Journal of Physical Chemistry C

Cite this: J. Phys. Chem. C 2021, 125, 23, 12488–12495
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https://doi.org/10.1021/acs.jpcc.1c00990
Published June 3, 2021

Copyright © 2021 The Authors. Published by American Chemical Society. This publication is licensed under

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Abstract

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Styrylcarbazole linked to pyrene by a dimethylsilyl bridge was synthesized in the search for new charge-transfer active materials for LED applications. In the course of a photophysical study, it turned out that such a donor-bridge-acceptor compound displayed three completely different types of emission depending on solvent polarity. The most attractive emission properties were found in acetonitrile in which two broad emission bands were observed. The resolved mechanism of the excited-state processes in acetonitrile was supported by singular value decomposition with self-modeling treatment of time-resolved emission spectra (ns-TCSPC). The data analysis revealed that there were two excited-state processes, that is, charge transfer within styrylcarbazole and electron transfer from styrylcarbazole to pyrene through a silylene bridge that was responsible for a broad dual emission. The general idea of designing compounds that display emission from more than one excited state covering a wide range of the visible spectrum can be a powerful tool in designing new white-light-emitting materials.

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1. Introduction

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In a previous study, (1) we showed that donor and acceptor moieties separated by monosilylene bridges retain their individual photophysical properties because the Si spacer breaks the π-communication between the linked chromophores. At the same time, intramolecular excited-state processes between the linked chromophores are still feasible (and in many cases even facilitated in the presence of a silicon atom), and they can be a source of the observed emission from this type of compound. Knowing the properties of both chromophores (e.g., electronic structures and ionic potentials), we can predict and control the excited-state processes (such as energy/electron/hole/charge transfer) and subsequently tune the color of the emissions. This way the excited part of a molecule and the emissive center can be specifically targeted. (2) Taking compound 1 as an example (see Chart 1), upon excitation at 300 nm, the “excitation center” in the vast majority is styrylcarbazole and the “emissive center” can be either a pyrene moiety (in a nonpolar solvent) or the formed charge transfer states (in a polar medium).

Figure 1

Figure 1. White-light emission of compound 1 in ACN.

In this study, focusing on intramolecular charge transfer (ICT) type of emission, we show how the idea of using silylene-bridged type of compounds can be applied in designing new white-light-emitting materials (Figure 1).
To accomplish a full-color display in a single molecule, the investigated compound was designed to emit from more than one ICT state. Why ICT emission? It is because this type of emission, having a charge-transfer character, is known to be structureless and broad. From a practical point of view, such a molecule will have two broad emission bands. If these bands are spectrally separated from each other, they can cover the whole range of the visible spectrum. For the sake of the latter, compounds displaying dual ICT emission can be promising candidates for new white-light-emitting materials. (3,4) As will be discussed in the further sections of this study, both the emissive states of a charge-transfer character can be a consequence of two different excited-state processes (in this case, charge transfer vs electron transfer). (5−7) To realize the idea of composing white-light-emitting compounds from chromophores linked by a monosilylene bridge, a styrylcarbazole–SiMe2–pyrene dyad was synthesized, and it was investigated in terms of its unusual emission properties, strongly dependent on the solvent surrounding (for structures of the investigated compounds, see Chart 1; for details regarding synthesis, see Supporting Information).

Chart 1

Chart 1. Structures of the Investigated Compound 1 and Its Reference Compounds 1a and 1b
The components of the chosen dyad (carbazole derivative and pyrene) were selected because they play a pivotal role in many areas of photophysics and optoelectronics. (8−11) These popular polycyclic aromatic hydrocarbons and their various derivatives (including silicon-based carbazole (4,12−14) and pyrene-containing compounds (15−17)) have found application in optoelectronics, solar cells, TADF and organic light-emitting diode (OLED) materials, or fluorescent probes for elucidation of the geometries of various biomolecules. (15,16) There are also interesting studies that report white-light emission from pyrene-based compounds, (18−23) and many of them are related to aggregation-induced emission. Furthermore, there are known examples of carbazole–pyrene hybrid compounds for dye-sensitized solar cell applications, (24) OLEDs, (25−29) and electroluminescent devices. (30)
In addition to the impressive impact of both chromophores in materials science, the major inspiration for our research was the fact that once they are bridged together by a dimethylsilyl linker, they exhibit dual ICT emission that spreads all over the visible range.

2. Experimental Section

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2.1. General Methods

Compounds 1 and 1b were synthesized using the methods described in the Supporting Information, and the synthesis procedure of compound 1a has been reported in our previous study. (31) All of the compounds were purified using high-performance liquid chromatography (HPLC) prior to the photophysical experiments. The HPLC measurements were performed using a Waters 600E multisolvent delivery system pump equipped with a Waters 996 photodiode array UV–vis detector. The HPLC column used was XTerra RP18 of 5 μm (4.6 mm × 250 mm) and the mobile phase was acetonitrile with a flow rate of 0.4 mL × min–1. The solvents used for photophysical measurements (n-hexane, dichloromethane, and acetonitrile) were of spectroscopic grade (from Merck) and were used as received. Tetrahydrofuran (THF) was distilled from sodium under argon (inhibitor-free). UV–vis spectra were recorded using a Cary 100 spectrophotometer, and fluorescence spectra were recorded using a PerkinElmer LS 50B spectrofluorometer. The fluorescence quantum yields of the investigated compounds were determined using quinine sulfate in 1 N H2SO4f = 0.546). (32)

2.2. Nanosecond Time-Resolved Emission Spectroscopy

Time-dependent fluorescence spectra and fluorescence lifetime measurements were recorded using a fluorescence lifetime spectrometer (FluoTime 300 from PicoQuant) with a time-correlated single-photon counting detection system, equipped with 330 nm (510 ps full width at half maximum (FWHM)) and 300 nm (650 ps FWHM) diodes as its excitation sources.

3. Results and Discussion

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3.1. Color-Tuned Emission in Silylene-Bridged Compounds

The concept of this study is to show how the architecture of silylene-bridged compounds can be used to generate white light in a single molecule. The general idea is that a multichromophoric compound (composed of n number of linked fluorescing chromophores) creates n number of the lowest LE states and also the excited states that result from the photoinduced, intrachromophoric interactions (e.g., charge/electron/hole transfer). This means that there are many potential emissive states that can be active in the emission process. For instance, by adjusting the molecular levels of the bridged chromophores, we can specifically direct the energy and electrons from the excited center to the targeted excited states in order to achieve the desired color of emission. Note that in this strategy, because of the broken π-communication between the chromophores caused by the presence of the silylene spacer, we can not only direct the emission to occur in a specific range but also by the proper selection of the chromophores, we can provide the compound with strong absorption over the whole range of absorption. Using as an example the key compound of this study, that is, monosilylene-bridged bichromophoric compound 1 composed of styrylcarbazole and pyrene (for absorption spectra, see Figure 2), we note that the large molar absorption coefficients of the styrylcarbazole chromophore fill the gaps in which the pyrene chromophore shows absorption minima and vice versa. Also note that the absorption spectrum of compound 1 is truly the sum of the spectra of its components (i.e., 1a and 1b), indicating the lack of any ground-state interactions and highlighting that π-isolation between the chromophores is operative. As a consequence, in the mechanistic considerations, all of the excited-state processes rising from the excited states of both chromophores must be included, taking into account the amount of the light being absorbed by each of them. And yet, no matter which chromophore is excited, compound 1 shows the same fluorescence and the fluorescence excitation spectra are in good agreement with the absorption spectrum.

Figure 2

Figure 2. Absorption spectra of compounds 1, 1a, and 1b in ACN.

3.2. On the Way to the White Light

Compound 1 acts as a solvent polarity probe, whose response strongly depends on the solvent used (see top panel of Figure 3). What is important is that the solvent affects only the emission and not the absorption properties (see Figure S1, Supporting Information), and therefore, the environment plays an important role only in the excited states of compound 1. To understand the photoinduced processes responsible for the emission of compound 1, all of the results presented below were analyzed with reference to the single-chromophoric subunits 1a (31,33) and 1b in the respective solvents. Although this work focuses on white-light emission observed in acetonitrile, the results in hexane and dichloromethane are also presented here for a better understanding.

Figure 3

Figure 3. Normalized fluorescence spectra (top panel) and energy diagrams (bottom panel) of compounds 1 (red line), 1a (light gray line), and 1b (dark gray line) in hexane (a,a1), DCM (b,b1), and ACN (c,c1). The colors of the energy diagrams correspond to colors of the spectral contours presented in the top panel.

The three different types of emission, observed for compound 1 in hexane, dichloromethane (DCM), and acetonitrile, can be explained on the basis of the respective energy diagrams calculated for compounds 1, 1a, and 1b (bottom panel of Figure 3). For the sake of clarity of the presented interpretation regarding the phenomenon itself, the detailed quantitative photophysical characteristics of all of the compounds (i.e., absorption and emission maxima, molar absorption coefficients, fluorescence lifetimes, quantum yields, and radiative rate constants) are provided in the Supporting Information (Tables S1–S4).

3.2.1. Emission in Hexane (Local Excited (LE) State Emission from Pyrene)

Compound 1 in hexane exhibited a long-lived fluorescence (τf = 112 ns) irrespective of the excitation wavelength used (for emission spectra at various λexc and excitation spectra at various λem of compound 1 in hexane, see Figure S2, Supporting Information). This emission occurred from the LE state of the pyrene chromophore (Figure 3a), indicating an efficient styrylcarbazole-to-pyrene energy transfer. The energy transfer is driven by the lower energy of the S1 state of 1b compared to 1a (see the energy diagram shown in Figure 3a1). Due to the efficient energy transfer from styrylcarbazole to pyrene, the fluorescence quantum yield of compound 1f = 0.65) matches the fluorescence quantum yield of compound 1bf = 0.58) within the experimental error. Comparing the fluorescence quantum yield of compound 1b with the literature value reported for pyrene (Φf = 0.32), (34) it can be seen that the introduction of a silicon atom significantly (about twice) improved the emission efficiency. The important observation in hexane is that the fluorescence of compound 1 extended only up to 470 nm. This means that, despite its long-lived fluorescence in hexane (about 112 ns), the pyrene chromophore of compound 1 (at the concentration used in the experiment, i.e., 10–6–10–5 M) did not form an intermolecular excimer whose maximum fluorescence is known to be located at 480 nm. (35)

3.2.2. Emission in DCM (CT1 Emission)

In DCM (likewise in THF, see Supporting Information), compound 1 showed a strong (Φf = 0.64), single broad fluorescence band with a maximum at 402 nm, which tends to show a red shift with the increase in solvent polarity. A similar behavior has already been reported for compound 1a in our previous studies (31,33) and is considered to be originating from an ICT state. In this state, the electron density moves toward the styryl part of the styrylcarbazole. (31) On the molecular level, compound 1a undergoes inversion of the excited states induced by the presence of the silicon atom and the polar medium. (31) The charge-transfer emission observed for compound 1 in DCM (called in the subsequent sections as “CT1”) has a much shorter lifetime (16.4 ns) than the lifetimes observed in hexane. However, it is still longer than the fluorescence lifetime observed for 1a (3 ns) in DCM. The latter observation leads us to think that the presence of pyrene stabilizes the formed charge-transfer state.

3.2.3. Emission in Acetonitrile (CT1 + CT2)

White-light emission coming from compound 1 was observed in acetonitrile. This was because there were two broad bands in emission (see Figure 3c) that spread all over the visible spectrum. This finding is of special importance in terms of new white-light-emitting materials. The Commission Internationale de l’Eclairage (CIE) chromaticity coordinates of compound 1 in acetonitrile solution were determined to be (x = 0.33, y = 0.36), which are very close to those of pure white light (x = 0.33, y = 0.33). One of the fluorescence bands (with a maximum at 416 nm), in analogy to compound 1a in ACN, is assigned to the emission from the CT1 state that was already discussed for compound 1 in DCM solution. What is the second broad emission band at longer wavelengths? Since compound 1a in ACN showed no emission in this spectral range, the logic suggests that this must be related to the presence of the pyrene chromophore. The first thought that comes to mind in terms of a long wavelength emission observed in a pyrene-based compound is a pyrene excimer fluorescence. However, the maximum of compound 1 (560 nm) did not match 480 nm that is characteristic of the pyrene excimer emission, and also, the longest lifetime of compound 1 in ACN observed in the 560 nm kinetic trace (15 ns) is way too short to observe any formation of an intermolecular excimer in this concentration range. In addition to the mentioned obstacles, the excimer emission would have to be already visible in other solvents (i.e., hexane, THF, and DCM) and yet that is not that case.
The second thought is that the emission with the 560 nm maximum observed in acetonitrile must originate from another charge-transfer state (called in this study as “CT2”) that has been formed in the excited state of compound 1 in addition to CT1. While CT1 is formed due to a charge transfer within the styrylcarbazole moiety (likewise for compound 1a in ACN), (31) the CT2 state must be formed due to an electron transfer over the silylene bridge from the styrylcarbazole moiety to the pyrene aromatic unit. Comparing the emission spectra of compound 1 in DCM and ACN, it can be concluded that apparently, the formation of such a CT2 state requires a more polar solvent than is needed for the formation of the CT1 state.
What is interesting is how the CT2 state was formed and what was its precursor. Looking at the energy diagram shown in Figure 3c1, there are two possible candidates to be direct precursors of the CT2 state. They are the LE state of pyrene or the CT1 excited state or both. To obtain an insight into the excited-state processes of compound 1 in acetonitrile that are responsible for the two broad emissions, the spectral evolution in time of the emission spectrum was measured with a subnanosecond resolution (time-correlated single-photon counting, TCSPC) using a 330 nm diode as the excitation source (see Figure 4). Although this resolution will cut us off from the detection of the early photophysical events, for the sake of nanosecond lifetimes of both CT emissions, we found this experimental technique suitable to explain the observed phenomena, that is, white-light dual CT emission.

Figure 4

Figure 4. Time-resolved fluorescence spectra of compound 1 in ACN (λexc = 330 nm).

For a proper analysis of the obtained results, it is important to note that at 330 nm, the light is absorbed almost in a 1:1 ratio by both chromophores (48% by 1a and 52% by 1b), see Figure 2. In practice, this means that in the mechanism are involved equally processes resulting from the excitation of both chromophores, that is, styrylcarbazole and pyrene.
To solve the mechanism, the spectra shown in Figure 4 were subjected to principal component analysis (PCA) and singular value decomposition with self-modeling (SVD-SM) analyses (the details regarding these analyses are provided in the refs (36−38)). These techniques are equivalent to a global spectral fitting method. The PCA analysis of all of the spectra shown in Figure 4a–f (i.e., from 0.41 ns delay up to 20 ns) revealed three significant eigenvalues (two major ones and one very weak) linked to the three most significant eigenvectors of the covariance matrix of the time-resolved emission data indicating a three-component system.
A second PCA analysis showed that starting from 1.6 ns (Figure 4d–f), there were only two emitting agents. Therefore, the spectra recorded in this time window (in the range of 1.6–20 ns delay after the pulse) were treated by SVD-SM calculations to resolve the fluorescence spectra of the two pure components, that is, emissions from CT1 and CT2 states (see blue and red curves shown in Figure 5, respectively). The spectrum of the pure third component was the most challenging to resolve because it appeared at very short delay times and its contribution to the total emission was very weak. Based on our experimental findings (i.e., a subtle 380 nm maximum in the steady-state fluorescence spectrum of 1 in ACN (see Figure 3c) but also a small blue shift observed in the initial spectra shown in Figure 4, see also Figure S3, Supporting Information), pyrene was considered to be the third component. Based on this evidence, the steady-state fluorescence spectrum of the substituted pyrene, 1b, served as the spectrum of the third pure component.

Figure 5

Figure 5. Resolved fluorescence spectra of pure CT1 and CT2 states obtained from SVD-SM and experimental calculations and steady-state fluorescence spectrum of 1b (a); combination coefficients (α, β, γ) of the normalized emission spectra presented as points in a 3D vector space spanned by the three most significant eigenvectors of the covariance matrix (b). The colors of the three spheres in (b) correspond to colors of the spectral contours in (a). The corners of the triangle correspond to the spectra of the pure components.

Note that the stoichiometric plane that projects each of the experimental spectra onto the most significant eigenvectors of the covariance matrix (Figure 5b) was determined for all of the spectra shown in Figure 4, whereas the spectra of pure CT1 and CT2 states (Figure 5a) were obtained from the self-modeling performed on a reduced data matrix (i.e., on spectra recorded at longer delay times, starting from 1.6 ns). The fact that pyrene (1b) fits on the stoichiometric plane of the triangle confirms our assignment that pyrene was truly a third component participating in the total emission. However, at this point, we cannot completely dismiss a possibility that these traces of pyrene-like emission may be due to a pyrene-based microimpurity. It can also be seen in Figure 5b that the experimental points from the TCSPC experiment are very close to the corners of the triangle of the stoichiometric plane corresponding to pure CT1 and CT2 emissions and far away from the corner of pure 1b. This indicates a very weak contribution of the LE fluorescence of 1b in the total emission observed.
Now, once all the pure spectra were resolved, it was possible to analyze the time constants obtained from the fluorescence decay traces recorded at the maxima of CT1 (420 nm) and CT2 (560 nm), see Figure 6. Both kinetic traces decay with a triple exponential kinetics and the three time constants are nearly identical comparing those at 420 nm (τ1 = 0.16 ns, τ2 = 2.2 ns, and τ3 = 14.8 ns) with time constants observed at 560 nm (τ1 = 0.11 ns, τ2 = 2.1 ns, and τ3 = 14.2 ns), suggesting their mutual relationship.

Figure 6

Figure 6. Fluorescence decays of compound 1 in ACN, λem = 420 and 560 nm and λexc = 330 nm.

Although the shortest time constant is much too short to be assigned accurately, it can still be associated with a time constant that is considerably shorter than that of the FWHM of the instrumental response function. Moreover, this time constant can be associated with a single third time constant for the system since the PCA spectral analysis found only three pure species. Emission was collected during the time of the laser pulse and thus was part of the global spectral analysis.
At 560 nm, all of the time constants correspond exclusively to the emission of CT2 because in this spectral range, CT2 was the only emitting agent (see Figure 5a). The 560 nm kinetic trace was dominated by a 14 ns decay lifetime and a very short lifetime (0.11 ns) with a negative amplitude indicating a growth (for amplitudes, see Supporting Information). The immediate formation of CT2 coincides with an immediate decay (0.16 ns) observed in the 420 nm kinetic trace, which can be attributed to the decay of the LE state of pyrene (via three possible deactivation channels, that is, fluorescence, energy transfer to CT1, or hole transfer to CT2, see Scheme 1). All of the three competing processes will occur with the same observed rate constant (equal to the sum of the three underlying primary rate constants), but they may differ in yield, which depends on their branching kinetic ratios. PCA and SVD-SM analyses confirmed not only a very fast decay of the pyrene LE emission that shows up only at early delay times but also that this emission is very weak. For sure then, this is not the main pathway for the pyrene excited state deactivation. And also, the fact that pyrene-like emission did not contribute to the total emission at longer delay times indicates that there was no hole transfer back from CT2 to the pyrene excited state, and therefore, these states, by definition, cannot be in equilibrium.

Scheme 1

Scheme 1. Mechanism of the Photoinduced Processes of Compound 1 in ACN (λexc = 330 nm)a

a(LE = emission from the local excited state; ET = energy transfer; HT = hole transfer; = electron transfer; and CT1, CT2 = charge-transfer excited states).

Also, it can be concluded from the SVD-SM analysis (see Figure 5b) that the emission at early delay times is clearly dominated by the CT1 emission. This result points at energy transfer to CT1 to be the main deactivation process for the pyrene excited state. Although thermodynamically a very fast hole transfer to form CT2 is possible (ΔGHT = −0.31 eV; for determination, see Supporting Information), it seems to be less efficient as compared to the ultrafast energy transfer. (1,39) That energy transfer is the dominant channel for Pyr* deactivation, which can be additionally confirmed by the estimated rate constant and efficiency for the Förster energy transfer. (40) These two parameters were calculated assuming a 5.9 Å distance between the center of the donor (1b) and the spatial region of the HOMO and LUMO orbitals involved in the excitation of the acceptor (1a, CT1, see Figure 4 in ref (31)). With this transfer distance, the fluorescence quantum yield Φf(1b) = 0.5, and the fluorescence lifetime τ(1b) = 202 ns, the Förster energy transfer rate constant and its efficiency (relative to local pyrene decay processes) were calculated to be 1.1 × 1010 s (93 ps) and 100%, respectively. A very efficient energy transfer can also be concluded from the very good match between the absorption and fluorescence excitation spectra of compound 1 (see Figure S4, Supporting Information).
Following this logic, electron transfer from CT1 to CT2 must be the main source of the CT2 excited state formation. The negative value of free Gibbs energy for the electron transfer (ΔGel = −0.23 eV, for determination, see Supporting Information) supports this possibility. Note that CT1 decays with the same kinetics as CT2, which occurred in 15 ns as observed in the 420 nm decay trace. This finding (clearly shown in Figure 4f) shows that the CT1 and CT2 states are in equilibrium. In particular, there is an equilibrium between the forward and backward electron-transfer rate constants. The remaining 2 ns lifetime must be then associated with the establishment of this equilibrium, which leaves no doubt that CT1 was one of the CT2 precursors. During this 2 ns necessary to reach the equilibrium, CT1 was fluorescing, as was CT2. The longest time constant (about 15 ns) is the decay time of CT1 and CT2 after the equilibrium is established.
In summary, only the shortest time constants observed at 420 nm and 560 nm kinetic traces can be assigned to the particular emitting species, that is, decay of the pyrene excited state and the formation of the CT2 excited state, respectively. The rest of the time constants result from the establishment of the equilibrium between CT1 and CT2 (about 2 ns), which later on decay together via emission for about 15 ns.
Based on the mechanism shown in Scheme 1, we derived the kinetic equations for CT1 and CT2 (see Supporting Information) for a better understanding of the physical meaning of the experimentally measured lifetimes. Both kinetic equations (for CT1 and CT2) involve three lifetimes with their associated amplitudes/pre-exponential factors, just the way we observe three time constants in the experimental decay traces at 420 nm and 560 nm. Only one out of the three rate expressions is easy to interpret (kPyr = kel + kHT + kf,Pyr), which is the rate of decay of excited pyrene according to our mechanism. The other two expressions (kfast and kslow) are complex mixtures of the other microscopic transition rates between CT1 and CT2 in addition to the emission rates of CT1 and CT2. What is important is that pyrene is not involved in the expressions for kfast and kslow. We observe that if pyrene was not excited, the overall kinetics of CT2 would reduce the concentration profile for CT2 to two components and thus two amplitudes, which are equal but opposite in sign, that is, a growth of CT2 and a decay of CT1, respectively. Such a kinetic model resembles that of an excimer or an exciplex (41,42) in which kfast is the formation rate of the equilibrium between CT1 and CT2 and kslow is the rate of CT1 and CT2 decaying together via emission once equilibrium is achieved.

4. Conclusions

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In summary, the emission of compound 1 in hexane occurred exclusively from the pyrene chromophore indicating a complete energy transfer through the silylene bridge from styrylcarbazole to the pyrene moiety. In solvents of middle polarity (THF and DCM), the emission was governed by the charge-transfer process that takes place within the styrylcarbazole chromophore and resulted in a broad charge-transfer emission (CT1) peaking at 397 nm (in THF) and 402 nm (in DCM). Finally, in polar solvents such as acetonitrile, this CT1 emission with the maximum at 416 nm was accompanied by another charge-transfer emission with a maximum at 560 nm (CT2). In this case, the second CT state was populated by electron transfer from the CT1 state. The two broad bands in emission (CT1 + CT2) formed via different excited-state processes (charge and electron transfer) that covered a wide range of the spectrum make compound 1 an attractive prototype for white-light-emitting materials. SVD-SM calculations applied to the time-resolved fluorescence spectra of the investigated compound in acetonitrile allowed us to formulate the mechanism presented above.

Supporting Information

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The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.jpcc.1c00990.

  • Synthesis procedures and methods used, calculated ΔGel and ΔGHT, absorption and emission properties (λAbs, max, λf, max, τs, and Φf), and kinetic equations derived for CT1 and CT2 (PDF)

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Author Information

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  • Corresponding Author
    • Malgorzata Bayda-Smykaj - Faculty of Chemistry, Adam Mickiewicz University in Poznan, Uniwersytetu Poznanskiego 8, 61-614 Poznan, PolandCenter for Advanced Technology, Adam Mickiewicz University in Poznan, Uniwersytetu Poznanskiego 10, 61-614 Poznan, PolandOrcidhttps://orcid.org/0000-0003-4104-159X Email: [email protected]
  • Authors
    • Karolina Rachuta - Faculty of Chemistry, Adam Mickiewicz University in Poznan, Uniwersytetu Poznanskiego 8, 61-614 Poznan, Poland
    • Gordon L. Hug - Radiation Laboratory, University of Notre Dame, Notre Dame, Indiana 46556, United States
    • Mariusz Majchrzak - Faculty of Chemistry, Adam Mickiewicz University in Poznan, Uniwersytetu Poznanskiego 8, 61-614 Poznan, Poland
    • Bronislaw Marciniak - Faculty of Chemistry, Adam Mickiewicz University in Poznan, Uniwersytetu Poznanskiego 8, 61-614 Poznan, PolandCenter for Advanced Technology, Adam Mickiewicz University in Poznan, Uniwersytetu Poznanskiego 10, 61-614 Poznan, PolandOrcidhttps://orcid.org/0000-0001-8396-0354
  • Author Contributions

    M.B.-S. and K.R. contributed equally.

  • Notes
    The authors declare no competing financial interest.

Acknowledgments

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This work was supported by the Faculty of Chemistry, Adam Mickiewicz University, Poznan, Poland and the National Science Centre (grant no. 2017/25/N/ST4/00299). The document number NDRL-5308 from the Notre Dame Radiation Laboratory is supported by the US Department of Energy Office of Science, Office of Basic Energy Science, under award number DE-FC02-04ER15533.

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  • Abstract

    Figure 1

    Figure 1. White-light emission of compound 1 in ACN.

    Chart 1

    Chart 1. Structures of the Investigated Compound 1 and Its Reference Compounds 1a and 1b

    Figure 2

    Figure 2. Absorption spectra of compounds 1, 1a, and 1b in ACN.

    Figure 3

    Figure 3. Normalized fluorescence spectra (top panel) and energy diagrams (bottom panel) of compounds 1 (red line), 1a (light gray line), and 1b (dark gray line) in hexane (a,a1), DCM (b,b1), and ACN (c,c1). The colors of the energy diagrams correspond to colors of the spectral contours presented in the top panel.

    Figure 4

    Figure 4. Time-resolved fluorescence spectra of compound 1 in ACN (λexc = 330 nm).

    Figure 5

    Figure 5. Resolved fluorescence spectra of pure CT1 and CT2 states obtained from SVD-SM and experimental calculations and steady-state fluorescence spectrum of 1b (a); combination coefficients (α, β, γ) of the normalized emission spectra presented as points in a 3D vector space spanned by the three most significant eigenvectors of the covariance matrix (b). The colors of the three spheres in (b) correspond to colors of the spectral contours in (a). The corners of the triangle correspond to the spectra of the pure components.

    Figure 6

    Figure 6. Fluorescence decays of compound 1 in ACN, λem = 420 and 560 nm and λexc = 330 nm.

    Scheme 1

    Scheme 1. Mechanism of the Photoinduced Processes of Compound 1 in ACN (λexc = 330 nm)a

    a(LE = emission from the local excited state; ET = energy transfer; HT = hole transfer; = electron transfer; and CT1, CT2 = charge-transfer excited states).

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  • Supporting Information

    Supporting Information


    The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.jpcc.1c00990.

    • Synthesis procedures and methods used, calculated ΔGel and ΔGHT, absorption and emission properties (λAbs, max, λf, max, τs, and Φf), and kinetic equations derived for CT1 and CT2 (PDF)


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