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Multiple Intermolecular Exciplexes in Highly Polar Solvents Joseph P. Dinnocenzo,* Adam M. Feinberg, and Samir Farid* Department of Chemistry, University of Rochester, Rochester, New York 14627, United States S Supporting Information *

ABSTRACT: Exciplexes of 2,6,9,10-tetracyanoanthracene (TCA) with alkylbenzenes were investigated in solvents ranging from cyclohexane to acetonitrile. Plots of the reduced emission maxima or the average emission frequency (hνav) versus redox potential differences (Eredox) were linear with a slope of ∼1 in all solvents, which is consistent with the highly ionic character of the exciplexes. The exciplex spectra were analyzed in terms of the energy gap between the exciplex minimum and the AD pair (ΔG), the energy difference between ΔG and Eredox (δEx), and the total reorganization energy (Σλ). A plot of (Eredox − hνav), equivalent to (Σλ − δEx), versus a solvent polarity function showed a linear dependency for the low-to-moderate polarity solvents, whereas highly polar solvents deviated significantly. δEx showed a smooth linear dependency for all solvents. Thus, the deviation of the polar solvents is due to a larger-than-expected Σλ. Additionally, the full width at half-maximum (fwhm) of the emission spectra in polar solvents deviates significantly from the extrapolated trend in less-polar solvents. The deviations of Σλ and fwhm in highly polar solvents can plausibly be explained by composite emissions from two exciplex structures, with the donor overlapping with the inner or outer ring of TCA.

1. INTRODUCTION Exciplexes, mixed locally excited (LE) and ion pair (IP) states,1,2 have been the subject of numerous investigations. Exciplex deactivation occurs via radiative decay (fluorescence), nonradiative decay (internal conversion to the ground state), intersystem crossing, solvent separation in polar media to produce free radical ions, or in certain cases chemical reactions (product formation).3−7 Of these processes, exciplex fluorescence has attracted the most attention because of its potential utility in a wide variety of applications.8−14 A wide range of acceptors and donors was initially instrumental in showing that the emission maxima correlate with redox potentials15 and solvent polarity.2 For a critical assessment of such correlations, however, very accurate oxidation and reduction potentials are required, which were not usually available in earlier studies. The purpose of the present work is to reassess the correlation of exciplex emission maxima with redox potentials and solvent parameters using a single acceptor (2,6,9,10-tetracyanoanthracene, TCA) and a homologous series of donors (D) for which highly accurate oxidation potentials have been established16−19 in order to minimize scatter in the data that would otherwise preclude a critical evaluation. In addition, these exciplexes are largely ionic (charge-transfer character > 85%),20,21 thus minimizing potential deviations due to wide degrees of mixing with the LE state. We have investigated the effect of solvent on TCA/D exciplexes using a wide range of polarities, from cyclohexane to acetonitrile. The data presented here reveal an anomalous behavior of these exciplexes in highly polar solvents, such as acetonitrile and nitromethane, i.e., large deviations of the © XXXX American Chemical Society

emission spectra relative to the trend seen in less-polar media. A possible explanation for this anomaly is discussed.

2. EXPERIMENTAL SECTION 2.1. Materials. Most of the solvents were spectroscopicgrade or HPLC-grade. Nitromethane (Acros, 99+%) was fractionally distilled at reduced pressure (100 mmHg). Valeronitrile (Aldrich, 99.5%) was passed through Brockman I activated acidic alumina and then fractionally distilled. Acetonitrile (99.93+%, Baker, HPLC-grade, 2.4 eV) are below that.

f (solv) = [(ε − 1)/(2ε + 1)] − [(n2 − 1)/2(2n2 + 1)] (4)

The energetic parameters controlling exciplex emissions are discussed in Section 3.2. 3.2. Relationship between Exciplex Energetics and Redox Potentials. Exciplex energetics and their relationship to Eredox, the free energy of the corresponding radical ions in AN, can be defined by four parameters (Scheme 1): (1) ΔG, the energy gap between the exciplex minimum and the A,D pair in the ground state; (2) δEx, the energy difference between ΔG and Eredox; (3) hνav, the average exciplex emission frequency; and (4) Σλ, the total reorganization energy. The relationships between these parameters are given by eqs 1 and 2, from which

As shown in Figure 3, there is a simple linear correlation of (Eredox − hνav) versus f(solv) for the nonpolar and mediumpolarity solvents, but a major deviation (0.18 eV) for AN, indicating a larger-than-expected (Σλ − δEx) for that solvent. To test whether the deviation of the AN data is unique for this solvent or is a feature of other highly polar solvents, we compared the exciplex spectrum of TCA/p-Xy in AN to those in two other polar solvents: nitromethane (NM) and valeronitrile (VN), Figure 4. For analogous spectra of TCA/ m-Xy exciplexes in AN and NM see Supporting Information. C

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Figure 3. Difference between the redox potential, Eredox, and the average emission frequency of TCA exciplexes, hνav, as a function of solvent polarity. It is noted that (Eredox − hνav) = (Σλ − δEx), eq 3. The data of AN and the other highly polar solvents (see text) are not included in the fitting.

Figure 5. Normalized reduced absorption [ε(λ)ν] of the TCA/HMB CT complex, black curve, and exciplex emission [I(λ)/ν3], blue curve, versus wavenumber in (a) CHX and (b) DCM. The red curve is the mirror image of the emission spectrum, displaced horizontally to match the CT absorption spectrum. The crossing point corresponds to ΔG.

shifted and too weak to measure accurately. In addition, as discussed below, the emission spectra with the other donors in AN are exceptional and thus not expected to provide a proper image relationship with the absorption spectra. Several pieces of independent data, however, point to a value of ∼0.06 eV for δEx in AN. First, the free energy differences determined between contact and free radical ion pairs in AN are estimated to be ∼0.06 eV.37 A similar estimate was determined from the fitting of electron-transfer rate constants to driving force data in AN.37,38 Interestingly, the free energy difference between contact and solvent-separated radical ion pairs in AN is also estimated to be ∼0.06 eV.37,39 Consistent with using a value of 0.06 eV for δEx in AN, a plot of δEx versus f(solv), Figure 6, shows a reasonable linear dependency, including the data point

Figure 4. Reduced [I(λ)/ν3] emission spectra of TCA/p-xylene (pXy) exciplexes in acetonitrile (AN), nitromethane (NM), and valeronitrile (VN).

The deviation of these two solvents from the trend of the lower-polarity solvents is similar to that of AN (Figure 3). As described below, to determine the contributions of the two parameters Σλ and δEx to the deviation in polar solvents, we used CT absorption and emission spectra to determine δEx. 3.4. Energy Gap between ΔG and Eredox, δEx. In cases where charge-transfer absorption is adequately separated from the LE absorption of the acceptor, mostly with hexamethylbenzene (HMB) as donor, the exciplex free energy (ΔG) can be determined from the midpoint of the reduced absorption and emission spectra. Two examples are shown in Figure 5a,b; the corresponding spectra in the other solvents are shown in the Supporting Information. In cases like these, δEx can be evaluated directly as ΔG − Eredox, eq 2. For the other donors in a given solvent, δEx can reasonably be assumed to be the same as that for HMB because plots of hνmax versus Eredox are linear with a slope of ∼1, Figure 2, and the TCA exciplexes included in that set are largely ionic in character. For the less-polar solvents, the higher-Eox donors (unfilled circles in Figure 2) might actually have slightly smaller δEx values because of mixing with the LE state, and a smaller Σλ,36 and therefore, the values of δEx and Σλ in the least polar solvents (CHX and CTC) probably apply only to the low-Eox donors. The value of δEx in AN had to be derived differently because, in this case, the TCA/HMB exciplex emission was strongly red-

Figure 6. Energy difference between ΔG of TCA exciplexes and Eredox, δEx, as a function of solvent polarity, eq 4. D

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The Journal of Physical Chemistry A Table 1. Energetic Parameters (in eV) for TCA Exciplexes in Different Solvents solva

na

εa

f(solv)b

δExc

Σλ − δExd

Σλe

fwhmf

CHX CTC TCE FB DCM AN NM VN

1.426 1.459 1.476 1.465 1.424 1.344 1.382 1.397

2.02 2.24 3.42 5.42 8.9 37 35.9 17.7

0.100 0.119 0.199 0.265 0.319 0.393 0.385 0.362

0.312 0.249 0.208 0.162 0.130 0.06

−0.009 0.071 0.162 0.289 0.365 0.665 0.651 0.612

0.303 0.320 0.370 0.451 0.495 0.725

3.45 3.50 3.61 3.83 3.96 ∼5g ∼5g ∼5g

Solvents: cyclohexane (CHX), carbon tetrachloride (CTC), trichloroethylene (TCE), fluorobenzene (FB), dichloromethane (DCM), acetonitrile (AN), nitromethane (NM), and valeronitrile (VN); n is the refractive index and ε the dielectric constant. bSolvent polarity function, given by eq 4. c δEx is the energy difference between ΔG and Eredox, Scheme 1; no data for NM and VN are available. dFrom Eredox − hνav, eq 3. eThe total reorganization energy, Σλ, is obtained from the sum of the two preceding entries. fFull width at half-maximum of the reduced spectra, in 103 cm−1. g Values for the TCA exciplexes with m- and p-Xy; the spectra of the lower Eox donors are too red-shifted to accurately determine the fwhm. a

point to acetonitrile [f(solv) = 0.393], the decrease of hνmax with increasing solvent polarity is also linear, but considerably steeper. The ratio of the slopes of the two lines is ∼2.8, which corresponds to the ratio of the dipole moments of the loose to the tight structures.41 The deviation of highly polar solvents for the TCA/alkylbenzene exciplexes differs from those for naphthalene/N,N-diethylaniline in two ways. There is no discontinuity in solvent dependency from nonpolar to moderately polar solvents such as DCM [f(solv) = 0.319], and there is no indication of two domains with different linear slopes. Instead, the change of hνav versus f(solv) for the TCA exciplexes in the limited range of highly polar solvents studied here has a similar slope to that of the other solvents, Figure 3. A likely cause of the different response of the two sets of exciplexes is the nature and charge distribution of the radical cation of the aromatic donors, alkyl versus amino substituents. 3.6. Spectral Bandwidth. In addition to their emission maxima, the broad, structureless emissions of predominantly ionic exciplexes are commonly characterized by their bandwidths, typically the full width at half-maximum (fwhm). The fwhm values of the TCA exciplexes in the six principal solvents studied are shown in Figure 8. The data for the

for AN. Finally, as noted above, although the crossing of absorption and emission spectra is not the preferred way to determine δEx for the TCA exciplexes in AN, the value of 0.06 eV is in agreement with the absorption spectrum of the TCA/ HMB CT complex and the estimated exciplex spectrum for TCA/HMB in AN derived from the exciplex spectra from other donors (see Supporting Information). 3.5. Reorganization Energy, Σλ. As discussed in Section 3.2, (Eredox − hνav) equals (Σλ − δEx), eq 3. Thus, Σλ can be determined from (Eredox − hνav) − δEx. The data are summarized in Table 1. A plot of Σλ versus the solvent function f(solv) is shown in Figure 7. These data show that the deviation of the AN data in

Figure 7. Sum of reorganization energies Σλ as a function of solvent polarity. Data for acetonitrile, AN, are not included in the fitted line.

Figure 3 can be attributed to an unusually large Σλ for AN compared to those for the less-polar solvents. It is noteworthy that, within the estimated experimental error (±0.02 eV), the Σλ deviation for AN in Figure 7 accounts for all of the deviation observed for AN in Figure 3. Solvent-triggered changes in exciplex structure have been reported for linked (intramolecular) exciplexes.40,41 In lowpolarity solvents strong Coulombic attraction favors tight structures of the A•−D•+ pair, whereas in polar solvents increased solvent stabilization favors conformers with stretched (looser) structures, which additionally increase the solvent reorganization energy. Examples that exhibit a similar trend for unlinked (bimolecular) exciplexes have been reported by Verhoeven.41 The emission maxima (hνmax) of naphthalene/ N,N-diethylaniline exciplexes decrease linearly with increasing f(solv) from 0.09 (hexane) to 0.25 (diethyl ether). From that

Figure 8. Full width at half-maximum, fwhm, of reduced spectra of TCA exciplexes as a function of solvent polarity. Data for AN are not included in the fitted line.

solvents of low-to-medium polarity (CHX to DCM) show a linear increase of ∼500 cm−1 with increasing solvent polarity attributable to the increase in solvent reorganization energy, λs. The AN data point (derived from the data for m-Xy and p-Xy) deviates from the trend line for the other solvents by ∼880 cm−1. E

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overlapping with the inner ring of TCA•− would likely outweigh the gain from increased solvation from outer-ring overlap. Thus, an inner-ring-overlapping structure should be dominant in the less-polar solvents, leading to emission from a single species. Possible combinations are shown in Figure 10 of two spectra of exciplexes with different λs values that reproduce the

A plot of fwhm versus Σλ (Figure 9) shows that the additional increase in Σλ over the trend in the less-polar

Figure 9. Full width at half-maximum, fwhm, of reduced spectra of TCA exciplexes versus the total reorganization energy, Σλ. Data for AN are not included in the fitted line.

solvents by ∼0.17 eV (Figure 7) can account for an increase in fwhm of only ∼440 cm−1, that is, only half the increase by 880 cm−1 shown in Figure 8. Thus, the unusually large fwhm for the AN emission spectra must have another origin. 3.7. Source of Anomalous Exciplexes in Highly Polar Solvents. In summary, the TCA/D exciplexes in AN and other highly polar solvents deviate from the trends in low-to-mediumpolarity solvents in two aspects: (i) the total reorganization energy, Σλ, is ∼0.17 eV larger than expected, and (ii) the observed deviation in fwhm of the exciplexes (at least those of m-Xy and p-Xy) in acetonitrile is more than twice that required to explain the increase in solvent reorganization energy, λs, beyond the expected trend from the other solvents. Thus, the dramatic deviation of the bandwidth of the exciplexes in highly polar solvents cannot be explained just in terms of a single exciplex with exceptionally large λs. The internal, skeletal reorganization energy (associated with the vibrational relaxation of A•−D•+ to AD), λv, which affects the fwhm much more than λs does, also contributes to Σλ. However, λv is expected to be largely solvent-independent. Thus, it is evident that the exciplexes in the highly polar solvents somehow differ in some other fundamental way from those in the less-polar media. The absence of dependency of the exciplex emissions on donor concentration (from 0.08 to 0.4 M) rules out that the anomaly is due to fluorescence from 1:1 and 1:2 exciplexes. A plausible hypothesis to explain the spectral broadening of the TCA/D exciplexes in polar solvents is that these exciplexes consist of composite emissions from more than one exciplex structure that have different λs values. One possibility would be to have exciplexes in which the donor is situated over the inner or the outer ring of TCA. An exciplex with the latter structure might be expected to have a significantly larger λs because of the greater solvent exposure of the negatively charged TCA moiety. A quantum chemical calculation using the B3LYP density functional42,43 with a 6-31+G** basis set and optimized with the SM8 solvation model44 for AN predicts the electrostatic potential (ESP) group charges for the central and outer rings in TCA•− to be significantly different: −0.70 and −0.15, respectively. The calculated natural group charges are comparable: −0.74 and −0.13, respectively. Thus, an exciplex with the more highly charged central ring exposed to solvent would be expected to interact significantly more strongly with a polar solvent, resulting in a larger λs. In the less-polar solvents the gain from the donor radical cation

Figure 10. Black noisy curves: measured reduced emission spectra in acetonitrile of TCA exciplexes with (a) m-xylene, (b) p-xylene, and (c) 1,2,4-trimethylbenzene. Blue and green curves: simulated exciplex spectra (see text for details), appropriately scaled from the normalized peak intensity such that their sum (the red curves) reproduced the measured spectra. The scaling factors are given in the figure.

measured spectra of TCA in AN with m-Xy, p-Xy, and 1,2,4trimethylbenzene (124-TMB) as donors. The predicted spectra were generated using a previously described simulation program27 based on four parameters: ΔG (Scheme 1), a lowfrequency (mostly solvent) reorganization energy (λs), and a vibration reorganization energy (λv) associated with a single averaged frequency (νv). For the determination of an appropriate value for λv, the exciplex spectra of TCA in DCM were fitted using ΔG = Eredox + δEx (eq 2, with δEx = 0.13, Table 1) and a typical18 νv of 0.17 eV as fixed parameters. Fitting the spectra of the different donors in DCM yielded a λs value of 0.305 eV and a λv value of 0.19 eV (within ±0.002 eV). As mentioned above, the skeletal vibration parameters λv and νv F

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in highly polar solvents cannot yet be precisely determined. The combined data presented here strongly indicate, however, the presence of multiple exciplexes from TCA−donor pairs in highly polar solvents. Well-supported examples of multiple exciplex emissions with organic acceptors and donors in a 1:1 ratio in homogeneous solution are rare. On the basis of magnetic field effects, exciplex emissions from pyrene/N,N-dimethylaniline in binary solvent mixtures with dielectric constants >15 and in acetone were attributed by Nath et al. to two species: a relaxed exciplex and a very short-lived, “unrelaxed” exciplex.45 Van der Auweraer, De Schryver, and co-workers reported that pyrene linked through a propyl group to N-methylindole at the C-3 position showed wavelength-dependent fluorescence consistent with an emission from two species with different lifetimes.46 In this case, the two exciplexes with tethered acceptor and donor were present in both nonpolar (iso-octane) and polar solvent (AN), however, and were attributed to conformational isomers. Consistent with that explanation, two isomeric compounds linked through the C-2 position or the nitrogen atom did not show evidence for multiple exciplexes.46 It will be of interest to explore the generality of the anomalous properties of exciplexes in highly polar solvents presented here and what factors affect these properties, e.g., the ring sizes of the acceptor or donor, the substitution pattern, and the degrees of charge localization of their radical ions.

are expected to be solvent-independent and should apply to the data in AN. We assume that one of the exciplex structures in AN has the Σλ value predicted from the trend of the other solvents (equation in Figure 7) of 0.558 eV (λv value of 0.19 plus λs value of 0.368). The predicted emission spectra for these TCA exciplexes are shown by the blue curves in Figure 10. These emissions can be ascribed to exciplex structures where the donor radical cation overlaps the inner ring of TCA•−. For mXy and p-Xy, a second exciplex (with the same λv) that is needed to reproduce the measured spectra required a 0.22 eV larger λs; these spectra are shown in the green curves in panels a and b in Figure 10. For the reproduction of the measured spectrum of 124-TMB, the second exciplex required a 0.17 eV larger λs, panel c in Figure 10. The much larger λs value of the second component in the three cases might be expected from the increased solvation of an exciplex structure with outer-ring overlap. The red curve in Figure 10a shows the sum of the two simulated spectra in a ∼2:1 ratio, which matches the measured spectrum well. Similar fittings of the exciplexes with p-Xy and 124-TMB are shown in panels b and c in Figure 10. It is noted that the ratios of the larger-to-smaller λs components increase with the decreasing Eox of the donors: ∼2:1 for m-Xy, ∼3:1 for p-Xy, and ∼5:1 for 124-TMB. This trend can be rationalized as follows. As Eox decreases, the positive charges of the donor radical cations become increasingly delocalized. As a result, the importance of the Coulombic interactions between the donor radical cations and TCA•− decreases with the decreasing Eox of the donor. This allows the greater solvation of the outer-ring exciplex structures to become increasingly important. Previously determined lifetimes of the TCA exciplexes with the high-Eox donors (m-Xy to 124-TMB) in AN were in the range 84−89 ps, with the main deactivation path being the formation of the solvent-separated radical ion pair.27 Unlike intramolecular exciplexes, where diffusional processes are not required and can be carried out at low concentrations, the rate of exciplex formation of the TCA exciplexes exceeds that of their decay only at donor concentrations >0.6 M. That the data could be fit with a single decay constant indicates that the exciplexes either are in rapid equilibrium or have nearly the same lifetimes, which would not be surprising for the proposed structures. It is noteworthy that the break between the highly polar and less-polar solvents exhibited by the TCA exciplexes parallels a trend in the free energies of highly ionic exciplexes (predominantly contact radical ion pairs, A•−D•+, in character) relative to the corresponding solvent-separated radical ion pairs. In DCM and less-polar solvents the energy of the contact radical ion pairs is lower than that of the solvent-separated radical ion pairs.37,39 As the solvent polarity increases, the energy gap decreases because of both a decrease in Coulombic stabilization and an increase in solvation energy. Ultimately, the energetic situation is reversed in highly polar solvents such as valeronitrile and acetonitrile, where the free energy of a contact radical ion pair is higher than that of the corresponding solventseparated pair. Both of these related systems illustrate similar interplay between stabilization by solvation versus Coulombic interactions as the solvent polarity increases. Despite the good agreement between the simulated and observed emission spectra in Figure 10, fitting the spectra with two exciplexes should be considered plausible rather than unique. The number of emissive exciplexes and their structures

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpca.7b01461. Spectra of TCA/alkylbenzene exciplexes, hνmax − hνav, and TCA/m-xylene exciplex in highly polar solvents; spectra for the determination of ΔG and δEx in AN and the evaluation of δEx in AN; table of calculated properties for TCA•− in CH3CN (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Joseph P. Dinnocenzo: 0000-0003-0206-3497 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Ralph Young (University of Rochester) for providing the spectral simulation programs. The NSF (CHE-1464629) is gratefully acknowledged for financial support.

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REFERENCES

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DOI: 10.1021/acs.jpca.7b01461 J. Phys. Chem. A XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry A Solvents: Evidence of Multiple Exciplex Formation at Higher Bulk Dielectric Constant. Chem. Phys. Lett. 2009, 474, 297−301. (46) Helsen, N.; Viaene, L.; Van der Auweraer, M.; De Schryver, F. C. Influence of the Substitution on Intramolecular Exciplex Formation Between Pyrene and Indole Moieties. J. Phys. Chem. 1994, 98, 1532− 1543.

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DOI: 10.1021/acs.jpca.7b01461 J. Phys. Chem. A XXXX, XXX, XXX−XXX