Letter Cite This: J. Phys. Chem. Lett. 2019, 10, 3080−3086
pubs.acs.org/JPCL
Intramolecular Long-Range Charge-Transfer Emission in Donor− Bridge−Acceptor Systems Jason T. Buck,† Reid W. Wilson,† and Tomoyasu Mani*,†,‡ †
Department of Chemistry, University of Connecticut, Storrs, Connecticut 06269-3060, United States JST, PRESTO, 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan
‡
Downloaded by UNIV OF ROCHESTER at 10:05:39:448 on May 23, 2019 from https://pubs.acs.org/doi/10.1021/acs.jpclett.9b01269.
S Supporting Information *
ABSTRACT: Charge recombination to the electronic ground state typically occurs nonradiatively. We report a rational design of donor−bridge−acceptor molecules that exhibit charge-transfer (CT) emission through conjugated bridges over distances of up to 24 Å. The emission is enhanced by intensity borrowing and extends into the near-IR region. Efficient charge recombination to the initial excited state results in recombination fluorescence. We have established the identity of CT emission by solvent dependence, sensitivity to temperature, femtosecond transient absorption spectroscopy, and unique emission polarization patterns. Large excited-state electronic couplings and small energy gaps enable the observation of intramolecular long-range CT emission over the unprecedented long distance. These results open new possibilities of using intramolecular long-range CT emission in molecular electronic and biomedical imaging probe applications.
E
lectron transfer is one of the most fundamental chemical reactions.1 Detailed systematic studies of photoinduced electron transfer (ET) reactions in donor−bridge−acceptor (D−B−A) molecules and molecular assemblies2−4 have led to significant insights into the molecular factors controlling the process such as Gibbs energy change and distance. A major focus of these efforts has been on nonradiative charge separation and recombination reactions.5−7 This is not surprising because many ET reactions do not result in the emission of photons, yet certain molecular systems can exhibit radiative charge recombination from charge-separated (CS) states. This is generally referred to as charge-transfer (CT) emission (Figure 1a). CT emission is due to the recombination of electrons and holes that are spatially apart from each other, making it distinct from local excited-state emission and exciplex emission.8,9 In condensed media, it leads to broad spectral line shapes, large Stokes shifts, and a profound sensitivity to the environmental conditions of polarity and polarizability. Taking advantage of these properties, CT emission has been employed in technologies such as organic light-emitting diodes, temperature sensors, and responsive biomedical optical imaging probes.10−13 The current molecular designs for these applications rely on emission from shortrange (near contact D−A distance; typically rDA < 5 Å) systems. Reports of the CT emission from long-range intramolecular analogues are scarce. Among the few previously reported are some with donor−acceptor distances remarkably up to rDA ≈16 Å (center-to-center distance) using nonconjugated bridges.14−16 CT emission of >30 Å was reported in semiconductor physics17−19 through the use of cryogenic temperatures to prevent thermal escape from shallow traps. The molecular systems described here giving CT emission at © XXXX American Chemical Society
Figure 1. Radiative charge recombination of charge-separated states. (a) Abbreviated photophysical pathway upon photoexcitation of acceptor moiety in D−B−A molecules. Nonradiative decay pathways from the local singlet excited state and the CS state as well as triplet manifolds are not shown for brevity. (b) General structure of the molecules investigated in this study; R = H or Et. Bridge molecules include no bridge (NA), phenyl (Ph), fluorene (F1), indenofluorene (iF), and fluorene dimer (F2). The numberings are for the molecules with R = H.
Received: May 3, 2019 Accepted: May 20, 2019
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DOI: 10.1021/acs.jpclett.9b01269 J. Phys. Chem. Lett. 2019, 10, 3080−3086
Letter
The Journal of Physical Chemistry Letters Table 1. Select Emission and Photophysical Properties of the D−B−A Compoundsa 1 2 3 4 5
BDH−TPA BDH−Ph−TPA BDH−F1−TPA BDH−iF−TPA BDH−F2−TPA BDH
rDA (Å)b
ΔG0CS (meV)c
7.3 11.6 15.6 19.8 24.0
−106 −72 −68 −67 4.1
Φfld,e 0.015 0.087 0.077 0.068 0.44 0.68
ΦCTe,f
krad (s−1)g
V* (cm−1)h
0.11 0.15 0.078 0.036 0.006
× × × × ×
1000 1800 2300 1700 700
2.0 2.4 2.1 1.4 1.7
7
10 107 107 107 106
a Reported in chloroform at 20 °C. bDonor−acceptor distance defined by the center-to-center length based on the optimized structures. cEstimated based on the fittings of biphasic fluorescence lifetime decays. See SI Section 3 for details. The energy of S1BDH* is 2.44 eV, determined based on the crossing of the absorption and emission spectra. dQuantum yield of fluorescence. Determined by the absolute method. eErrors are typically ±10%. fQuantum yield of CT emission, determined by referencing to Φfl of the same molecule. gkrad is the radiative rate constant of CT emission; krad = ΦCT × kdecay (CT). kdecay (CT) is reported in Table S1. hElectronic coupling between the local excited state of BD and the CS state. See SI Section 2 for details.
Figure 2. CT emission recorded from the D−B−A compounds dissolved in chloroform. (a) Absorption spectra of 1−5 and BDH (λmax ≈ 503 nm). Absorption bands in the 300−400 nm region in longer bridges come from the contribution of the bridge moiety. (b) Emission spectra of 1−5 and BDH (λex = 480 nm). The spectra are normalized to the emission maxima of BD fluorescence for clarity. The dominant fluorescence emission in the normalized emission spectrum of 5 comes from high Φfl. (See the main text.) (c) Comparison of 5 and BDH in a zoom-in figure that shows CT emission from 5. (d) Decay profiles of fluorescence (525 nm) and CT emission (725 nm) of 5 in chloroform (λex = 506 nm). Black lines are fitted curves.
couplings, we can express the ground state (GS) and the CS state as
distances up to 24 Å operate at room temperature. We present a generally applicable design principle of intramolecular longrange CT emission and recombination fluorescence. Our molecular design to realize efficient CT emission over long distance takes advantage of intensity borrowing from the oscillator strength of the local excited state.20,21 The intensity borrowing of CT emission can be understood through the interaction of the CS state (D•+BA•−) and the emissive local excited state (here S1A*). Following the original treatment of Murrell using the perturbation theory,20 Bixon, Jortner, and Verhoeven (BJV),21 showed that CT emission can be accounted for in terms of a contribution of S1A* and D•+BA•−, involving intensity borrowing from local S1A* electronic excitation. The zeroth-order electronic states are | DBA⟩, |DBS1A*⟩, and |D•+BA•−⟩, with their respective energies 0, ES1, and ECS. Two electronic couplings between |D•+BA•−⟩ and the other two states (|DBA⟩ and |DBS1A*⟩) are defined as V and V*, respectively. Assuming that the energy gaps between the ground and excited states are large relative to the electronic
|GS⟩ = |DBA⟩ +
V •+ •− |D BA ⟩ ECS
|CS⟩ = |D•+BA•−⟩ −
(1)
V V* |DBA⟩ + |DBS1A*⟩ ECS ES1 − ECS (2)
The transition dipole moment (μ) between |GS⟩ and |CS⟩ is then μ = μ0 +
V V* Δμ + μ* ECS ES1 − ECS
(3)
where μ0 is the transition dipole moment between |DBA⟩ and | D•+BA•−⟩, Δμ is the difference between the permanent dipole moments of |D•+BA•−⟩ and |DBA⟩, and μ* is the transition dipole moment between |DBA⟩ and |DBS1A*⟩. μ0 is usually 3081
DOI: 10.1021/acs.jpclett.9b01269 J. Phys. Chem. Lett. 2019, 10, 3080−3086
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The Journal of Physical Chemistry Letters
Figure 3. Efficient charge-separated formation upon photoexcitation of BD. (a) Transient absorption spectra of 4 at 1, 350, and 6000 ps after the femtosecond laser pulse (λex = 500 nm) in chloroform. (b) Decay kinetics of 4 monitored at respective wavelengths, as indicated by vertical colored bars.
in Figure 2c. Biphasic decays of fluorescence were observed from 1−5. (See Figure 2d for compound 5 and Figure S3 for 1−4.) The faster component (kfast) of the fluorescence is an order of magnitude or more faster than the fluorescence lifetime of BDH (Table S1), indicating that a significant deactivation route exists, which is the CS pathway. The slower of the two components of the biphasic decay (kslow) is due to the recombination fluorescence of S1BDH* formed upon charge recombination from the CS state (bCR). ΔG0CS in chloroform was estimated from fitting the biphasic fluorescence lifetimes, as previously described26 (Table 1). Efficient charge recombination to the initial excited state and the resulting emission are enabled by the proximity in energy of S1BDH* and the CS state. Varying temperatures cause a change in the intensities of the BD fluorescence and the CT emission (Figure S4). The observed temperature sensitivities are greater than or comparable to those of temperatureresponsive fluorophores previously developed based on CT emission.13 This temperature dependence of the emissions shows that S1BDH* and the CS state are in thermal equilibrium. A more detailed analysis of temperature dependence and its impact on ET dynamics is currently underway in our laboratory. In Table S2, we report reduction and oxidation potentials with estimates of Coulomb corrections that predict that the charge separations are energetically very favorable in a polar solvent (ΔG0CS ≈ −0.3 to 0.4 eV). In the polar solvent, N,Ndimethylformamide (DMF), CT emission becomes much less prominent (Φfl ≈ 0.005 for 1) or disappears completely (Φfl < 10−4 for 2−5), as lowering the energy of the CS state reduces the strength of mixing between the singlet excited state and the CS state (Figure S5). ΔG0CS is less negative in the less polar solvent, chloroform (ΔG0CS > ∼−0.1 eV, Table 1), leading to increased intensity borrowing that enhances CT emission and at the same time enabling recombination fluorescence (see eq 3). This signals the importance of intensity borrowing to realize radiative charge recombination. In addition to these two solvents, we observed the clear dependence of the broad peaks on solvent polarity (Figure S6). Reduction and oxidation potentials of BDH and TPA are almost independent of the bridge lengths, arguing that charges are expected to be localized in each moiety (Table S2 and Figure S7). Electronic structure calculations of select Bridge−TPA fragments further support that the positive charge is localized to TPA (Figure S8). These results justify our use of the center-to-center distance as a measure of charge separation (rDA in Table 1). In contrast with the BDH series, BDEt analogues of 1−3 (6, 9, and
small, and when ECS is much larger than V, the third term in eq 3, the contribution from the local excited state plays a major role in enhancing the CT emission, and thus this is called intensity borrowing. As a radiative rate constant, krad is proportional to the square of μ (krad ∝ |μ|2); increasing μ will enhance krad and the quantum yield of CT emission.22 Our molecular systems are based on boron dipyrromethene (BODIPY or BD), linear conjugated bridges, and triphenylamine (TPA) (Figure 1b). BD serves as a photon absorber in the visible range and the electron acceptor, whereas TPA is the electron donor. The combination of BD and TPA enables us to tune the energy gap of the singlet excited state of BD (S1BD*) and the CS state (BD•−−Bridge−TPA•+) to generate CT emission from what are usually nonradiative CS states. Linear conjugated bridges serve as an efficient electron conductor. This combination contrasts with the previous single-molecular systems that use nonconjugated molecules as the bridge/spacer and with chromophore absorption only in the UV region ( 75%. The use of π-conjugated bridges enables such efficient charge separation over the long distance even without large driving forces. Therefore, in this series, ΦCT is limited not by the efficiency of the initial charge separation, unlike nonconjugated bridges,14,16 but instead by branching of the charge recombination pathways of the CS states (Figure 1a). Interestingly, Φfl of 5 is much higher than that of 1−4 and comparable to that of the parent BDH despite the efficient initial charge separation. This unexpected behavior results from efficient bCR, likely facilitated by a smaller ΔG0CS as well as slower radiative charge recombination in 5 compared with the others (Table S3). A similar behavior of recombination fluorescence was observed in dendrimer-type architecture.26,32 Most of the emission comes from recombination fluorescence (∼80% of Φfl). Time-resolved anisotropy measurements (see SI Section 1.3 for details)33,34 reveal unique emission properties of these compounds and further corroborate the identity of CT
13) do not exhibit CT emission in the same solvent (chloroform, Figure S9). Reduction potentials of the BDEt compounds are more negative than those of BDH by ∼60−70 meV (Table S2), which, together with the lower singlet excited state energy of BDEt (S1BDEt* = 2.36 eV), is large enough to make the energies of the CS states too high relative to S1BDEt*. These combined observations strongly support the notion that the broad peaks observed in 1−5 are the result of the radiative charge recombination of the CS state. In addition to the biphasic decays of BD fluorescence, we observed clear rises of the CT emission (Figure 2d and Figure S3). As a trend, the rise time (krise) becomes slower in the compounds with longer bridges (Table S1). Transient absorption measurements provide the dynamics of the ET processes. The rise and decay times of CT emission correspond well to those of the transient absorptions of radical ions in the CS states (BDH•−−Bridge−TPA•+) probed by femtosecond transient absorption (fsTA) spectroscopy (Table S3). The spectra are shown in Figure 3 for 4 and Figure S10 for the others. They show that the CT emissions correlate well with the dynamics of the CS state. We assigned the transient absorption bands based on the spectral signature of the BD radical anion BDH•− previously reported27 and those of electrochemically produced radical cation BDH−Bridge−TPA•+ (Figure S11). The absorption spectra of BDH−Bridge−TPA•+ depend on the bridge lengths, but the oxidation potentials, which give a more direct readout of the energies of charges, tell us that the spectral changes do not mean that the positive charge is delocalized over the bridge moieties. Instead, it means that the upper or lower states of these electronic transitions involve the bridge segments; they are CT transitions. No spectral features corresponding to the reduction or oxidation of the bridge moieties were observed clearly, indicating that bridge moieties may not be directly involved in ET processes (no hopping),28 3083
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electronic couplings are significantly larger than the D−B−A molecules with bridges of similar distance that do not possess CT emission31,38 but are comparable to the molecules with shorter distances that possess CT emission.21,39 A lack of clear distance dependence of the electronic couplings among this series likely comes from the effect of different bridge moieties (varying distance attenuation factors)5,40 and a possible breakdown of the assumptions made for the current formalism of intensity borrowing in these molecules such as strong vibronic mixings because of small ΔG0CS.21,41 Therefore, the values we obtained are considered rough estimates. Nevertheless, we can conclude that using the concept of intensity borrowing allows us to design the molecules that exhibit intramolecular long-range CT emission over the long distance. Among the factors involved, a large transition dipole moment (μ*), a small ΔG0CS, and large electronic couplings are the three important factors that we can control by chemical synthesis. Given that many chromophores used and developed in the past typically have large μ* values, we expect that many such molecules will serve as a suitable platform. In conclusion, we present a rational design of D−B−A systems that takes advantage of intensity borrowing between states to exhibit intramolecular long-range CT emission. Moreover, the systems possess the highly desired photophysical properties of visible absorption with high extinction coefficients. A series of evidence is presented to unambiguously support the origin of CT emission. A rigid D−B−A motif using conjugated bridges and a general concept of intensity borrowing applied here can be easily adopted to other types of chromophores and molecular systems. We envision that these results will allow chemists and others to design molecules that utilize long-range CT emission in applications, augmenting the commonly used molecular systems associated with short-range CT emission and thus providing new opportunities.
emission. The transition dipole moment (μ*) of the initial BD excitation (S0 → S1) aligns with the longest axis of the BD molecule (Figure 4a), as previously shown by electronic structure calculations.25 The initial anisotropy of the fluorescence (rfl0) is therefore expected to be close to 0.4. The initial anisotropy of small molecules such as BD usually decay rapidly with time in nonviscous solutions because of fast rotational correlation times (τrot).33 Our own measurement of BDH in chloroform gives rfl0 ≈ 0.4 and τrot ≈ 120 ps (Figure S12). The initial anisotropies of the BD fluorescence in 1−5 follow the same trend as BDH. (See Figure 4 for 4 and Figure S12 for the others.) The values of τrot are discussed in Table S4. In contrast with the BD fluorescence, the anisotropy of the CT emission exhibits the opposite anisotropy with rCT ≈ −0.2 because of the orientation of the transition dipole moments. (See Figure 4b for 4 and Figure S12 for the others.) The direction of the ET reaction, and thus the transition dipole moment of the CT emission, is perpendicular to the longest axis of the BD molecule (inset in Figure 4a). This means that the transition dipole moment of the CT emission is perpendicular to the transition dipole moment of fluorescence, initially yielding an anisotropy value of −0.2 by the equation r CT = r0fl(
3 cos2 θ − 1 ) 2
(4)
where ≈ 0.4 and θ is the angle between the transition dipole moments of the initial (S1BDH*) and final (CS) states (here θ = 90°). To further illustrate the relation between the anisotropy and the CT emission, we designed and synthesized a D−A molecule in which μ* of BD and the direction of ET are expected to be parallel to each other. (See inset of Figure 4c for the structure.) Here the molecule is named (||)-BDH−TPA (compound 20) to signify the orientation. The photophysical properties are summarized in Figure S13, and electrochemical properties are reported in Table S2. 20 exhibits a significantly diminished BD fluorescence and good CT emission in chloroform (Φfl = 0.004 and ΦCT = 0.11). Time-resolved anisotropy measurements show that both of the anisotropies rfl0 and rCT 0 are ∼0.4, confirming from eq 4 that θ ≈ 0° in this molecule (Figure 4d). In addition to adding further support for the origin of CT emission, the time-resolved emission anisotropy results provide spectroscopic evidence of the direction of ET reactions, which is usually inferred from the molecular structures alone. Using intramolecular CT emission, we can synthetically modulate emission polarization patterns without modifying individual chromophores. Such modulation may be of interest to applications like time-resolved fluorescence anisotropy microscopy.35 Having established the origin of emission, the longest distance (rDA = 24 Å) we observed for CT emission in this work is longer than those for previously reported molecular systems15,16,25 and approaching those observed in thin layers36 and solids.17−19 The observed solvent dependence of the experimental radiative rates (krad) of the CT emission (Figure S14) calls for the three-state model that includes the local excited state.21,37 To quantify intensity borrowing, we apply the formalism of intensity borrowing to our set of molecules to determine the excited electronic coupling between the S1BDH* and the CS states (V*). Following the treatment by BJV21 (see SI Section 2 for details), we obtained V* for 1−5 in the range of 700−2000 cm−1 (Table 1 and more in Table S5). Obtained rfl0
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ASSOCIATED CONTENT
* Supporting Information S
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpclett.9b01269.
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Experimental methods, characterizations of new compounds, explanation of intensity borrowing, estimation of Gibbs energy changes, Figures S1−S14, and Tables S1−S5 (PDF)
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Tomoyasu Mani: 0000-0002-4125-5195 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by start-up funds from the University of Connecticut and JST PRESTO grant number JPMJPR17GA, Japan (T.M.). A part of this work, including the use of the computer Cluster at the Center for Functional Nanomaterials, was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, 3084
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