Fluorescence of 4-(Diisopropylamino)benzonitrile (DIABN) Single

Article Cite This: J. Phys. Chem. A 2018, 122, 6985−6996

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Fluorescence of 4‑(Diisopropylamino)benzonitrile (DIABN) Single Crystals from 300 K down to 5 K. Intramolecular Charge Transfer Disappears below 60 K Klaas A. Zachariasse,*,† Sergey I. Druzhinin,*,†,§ Olaf Morawski,‡ and Boleslaw Kozankiewicz*,‡ †

Max-Planck-Institut für biophysikalische Chemie, Spektroskopie und Photochemische Kinetik, 37070 Göttingen, Germany Institute of Physics, Polish Academy of Sciences, Al. Lotników 32/46, 02-668 Warsaw, Poland

‡

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S Supporting Information *

ABSTRACT: Single crystals of 4-(diisopropylamino)benzonitrile (DIABN) undergo an intramolecular charge transfer (ICT) reaction in the excited singlet state. At 300 K, the fluorescence consists of emissions from the locally excited (LE) and from the ICT state. Upon cooling to 5 K, the ICT fluorescence intensity gradually decreases relative to that of the LE emission and is absent below 60 K. With crystalline 4-(dimethylamino)benzonitrile (DMABN), in contrast, only LE emission is found over the entire range from 300 to 5 K. The phosphorescence spectra of the DIABN and the DMABN crystals do not present any evidence for an additional ICT emission, showing that ICT does not occur in the triplet state. An activation energy Ea of ∼4 kJ/mol is determined for the LE → ICT reaction of DIABN crystals, from the temperature dependence of the fluorescence decay times τ2 and τ1. Ea is attributed to changes in the molecular conformation of DIABN other than a full rotation of the large diisopropylamino group with respect to the benzonitrile moiety. In a comparison with crystal and solution data, literature results from transient vibrational and absorption spectra are discussed and it is concluded that they cannot be employed to favor the TICT (perpendicular twist) over the PICT (planar) model for DIABN and DMABN.

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N,N-dimethylaniline (TCDMA) and derivatives.21 The absence of a rotation around the amino−phenyl bond in the LE → ICT reaction of DIABN also indicates that its activation energy Ea = 5.7 kJ/mol in n-hexane1 will involve changes in bond distances and angles. The ICT/LE fluorescence intensity ratio I′(ICT)/I(LE) of crystalline DIABN becomes smaller when the temperature is lowered, due to a decrease of the rate constant of the LE → ICT reaction.11 Similarly, the short decay time τ2 of the LE and ICT fluorescence decays becomes longer upon cooling, from 11 ps at +25 °C to 55 ps at −110 °C.11 Here, the fluorescence spectra and decays of DIABN single crystals are presented over an extended temperature range from 300 K down to 5 K.

INTRODUCTION 4-(Diisopropylamino)benzonitrile (DIABN) plays an exceptional role among the 4-(dialkylamino)benzonitriles, as it undergoes in the singlet excited state an intramolecular charge transfer (ICT) reaction in nonpolar solvents as n-hexane (ε25 = 1.88) and even in the gas phase.1,2 This is in contrast to the other (dialkylamino)benzonitriles such as 4-(dimethylamino)benzonitrile (DMABN), which only start to show ICT in media more polar than the alkanes: in toluene (ε25 = 2.37)3−6 and in dialkyl ethers: di(n-pentyl) ether (ε25 = 2.73), di(nbutyl) ether (ε25 = 3.05), di(n-propyl) ether (ε25 = 3.26), and diethyl ether (ε25 = 4.24), where ε25 is the dielectric constant at 25 °C.7−10 It was therefore no surprise that also the fluorescence spectrum of crystalline DIABN11 consists of a dual emission from a locally excited (LE) and an ICT state, observed from +80 °C down to −110 °C, different from DMABN crystals,12 for which only LE fluorescence is found, from +25 to −112 °C. This condition made it possible to determine the molecular structure of the ICT state of DIABN via picosecond X-ray diffraction.13 These investigations revealed that the torsional angle of the diisopropylamino group with respect to the plane of the phenyl ring of DIABN decreases from 14° in the electronic ground state S0 to 10° in the equilibrated ICT state, in full support of the planar ICT model PICT.5,9,14−23 In fact, primarily the energy gap between the two lowest singlet excited states ΔE(S1,S2) is the determining factor for the occurrence of an ICT reaction. A recent example of such a situation is found with 2,4,6-tricyano© 2018 American Chemical Society

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EXPERIMENTAL SECTION The synthesis and purification of DIABN1 and DMABN12 were described previously. Single crystals were grown from cyclopentane or from a n-hexane/diethyl ether mixture (10:1). For the fluorescence experiments in Figures 1 and 3−5 with an excitation wavelength λexc = 308 nm, the crystals were connected to a sample holder and inserted in a homemade optical cryostat allowing a temperature variation from 5 to 300 K.24 The experimental details of the absorption and fluorescence spectra of solutions in Figures 2 and 6 were Received: July 3, 2018 Revised: August 9, 2018 Published: August 9, 2018 6985

DOI: 10.1021/acs.jpca.8b06349 J. Phys. Chem. A 2018, 122, 6985−6996

Article

The Journal of Physical Chemistry A likewise reported earlier.25 The crystal fluorescence decays (Figures 7−9 and 13, Table 1) were obtained by single photon counting (SPC), setup SPC1, excited by 301 nm light pulses (optical half-width ∼5 ps, instrument response function halfwidth of ∼180 ps, 3.8 MHz repetition rate).24 The time resolution is ∼50 ps. The second set of fluorescence decays (Figures 10−12, Table 2) were determined with a different picosecond laser system, setup SPC2 (λexc 272 nm, optical halfwidth ∼0.2 ps, repetition rate 3.8 MHz),26,27 two ranges being routinely employed simultaneously (1.0 and 10 ps/channel in up to 1800 effective channels). The instrument response function has a half-width of ∼18 ps. The time resolution is ∼2 ps with an estimated reproducibility better than 10% for the picosecond decay times. The phosphorescence spectra (Figures 14 and 15) were measured with the same setup as that used for the fluorescence,24 but with a disc-chopper at the detection path. Its operation was synchronized with the excitation pulse emitted by an excimer laser in order to block an intense prompt fluorescence. In this case, the photomultiplier (Philips PM2254) is operated in the photon counting mode. The transients, observed with a delay of 80 μs after the laser pulses, were accumulated with a Stanford Research SR430 multichannel scaler.

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RESULTS AND DISCUSSION Fluorescence of DIABN Single Crystals at Room Temperature. The fluorescence spectrum of DIABN single crystals at 27 °C (Figure 1) consists of two components: a

Figure 2. Absorption (ABS) and fluorescence spectra of (a) the locally excited (LE) and intramolecular charge transfer (ICT) states of 4-(diisopropylamino)benzonitrile (DIABN) and (b) of the LE state of 4-(dimethylamino)benzonitrile (DMABN) in n-hexane at 25 °C.

is derived and similarly an ε ∼ 7 for a crystal melt at around 90 °C; see Supporting Information. Fluorescence Spectra of DIABN Single Crystals as a Function of Temperature down to 5 K. The fluorescence spectra of crystalline DIABN at a series of temperatures from 300 K (27 °C) to 200 K (−73 °C) and from 200 K (−73 °C) to 5 K (−268 °C) are shown in Figures 3 and 4. It is seen that the ICT/LE fluorescence intensity ratio I′(ICT)/I(LE) gradually decreases upon cooling,11 becoming very small below 80 K and reaching a value of practically zero at 5 K. These results mean that for DIABN single crystals the LE → ICT reaction can be stopped by lowering the temperature. LE ⇄ ICT Kinetics. For an LE ⇄ ICT equilibrium such as operating in the case of crystalline DIABN, Scheme 1 and eq 1 are applicable, where ka and kd are the rate constants of the forward and backward ICT reaction, τ0(LE) and τ0′(ICT) are the fluorescence lifetimes in the absence of a LE → ICT or an ICT → LE reaction, respectively, whereas kf(LE) and k′f (ICT) are the radiative rate constants.7,19

Figure 1. Fluorescence spectrum of 4-(diisopropylamino)benzonitrile (DIABN) single crystals at 27 °C, consisting of a major ICT band with a maximum at 23 200 cm−1 and a much weaker LE emission below 29 500 cm−1, reduced by self-absorption (see text). The halfwidth of the ICT fluorescence band is 3740 cm−1. Excitation wavelength 308 nm (32 500 cm−1).

major broad ICT emission band with a maximum ν̃max(ICT) at 23 200 cm−1 and a much weaker LE emission, starting below 29 500 cm−1, with a maximum at around 27 000 cm−1. For a dilute (4 × 10−5 M) solution of DIABN in n-hexane at 25 °C, absorption starts at 30 200 cm−1, whereas the LE fluorescence extends to 32 000 cm−1 (Figure 2a).1 This difference is caused by the strong self-absorption due to the high DIABN concentration of 5.4 M in the crystals, as will be further discussed below. For this reason, the ICT/LE fluorescence quantum yield ratio Φ′(ICT)/Φ(LE) cannot be determined precisely and the apparent value of 22 from the spectrum in Figure 1 is an upper limit. From a comparison of ν̃max(ICT) for crystalline DIABN at room temperature with data in a series of solvents over a range of polarities, an apparent polarity of ε ∼ 6

ka k ′(ICT) Φ′(ICT) = f Φ(LE) k f (LE) kd + 1/τ0′(ICT)

(1)

The continuous decrease of Φ′(ICT)/Φ(LE) upon lowering the temperature shown in Figures 3 and 4 means that the condition kd < 1/τ′0(ICT) holds below 300 K, the lowtemperature (LTL) limit.25 One then obtains eq 2. Φ′(ICT)/Φ(LE) = k f′(ICT)/k f (LE){kaτ0′(ICT)}

(2)

The observation that kd < 1/τ0′(ICT) indicates that the ICT reaction enthalpy will be substantial, such as, e.g., in the case of DMABN in acetonitrile (MeCN), for which −ΔH = 27.0 kJ/ mol and kd < 1/τ0′(ICT) below −10 °C25 An analysis of Φ′(ICT)/Φ(LE), to determine the activation energy Ea of the 6986


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The Journal of Physical Chemistry A Scheme 1

Fluorescence Spectra of DMABN Crystals. No ICT Down to 5 K. The fluorescence spectra of DMABN crystals from 300 to 5 K are presented in Figure 5. The observation

Figure 3. Fluorescence spectra of 4-(diisopropylamino)benzonitrile (DIABN) single crystals at various temperatures from 300 to 200 K. The spectra consist of two emissions ICT and LE, with a decreasing ICT/LE fluorescence intensity ratio as the temperature becomes lower. Excitation wavelength 308 nm (32 500 cm−1).

Figure 5. Fluorescence spectra of 4-(dimethylamino)benzonitrile (DMABN) crystals from 300 to 5 K. Only LE emission is observed. Excitation wavelength: 308 nm (32 500 cm−1).

that the fluorescence intensity practically does not depend on temperature is in accord with the absence of an LE → ICT reaction, as documented previously,12 clearly different from DIABN. Self-Absorption and Vibrational Structure. For DMABN crystals at 300 K in Figure 5, the fluorescence starts from 29 200 cm−1. A comparison with the absorption and fluorescence spectra of DMABN in n-hexane at 25 °C (Figure 2b) shows that the blue part of the spectrum has disappeared because of self-absorption, similar to what was observed for crystalline DIABN (Figures 3 and 4). Upon cooling, the extent of this self-absorption is seen to become smaller: at 5 K (Figure 5), the fluorescence starts at 30400 cm−1. The appearance of vibrational structure in the LE fluorescence spectrum of DIABN and DMABN below 100 K (see Figures 4 and 5) is likewise found for DMABN in dilute solution. The LE fluorescence spectrum of DMABN in alkane solvents starts to develop vibrational structure upon cooling, as seen from that in 2-methylpentane at −155 °C in Figure 6. Fluorescence Decays of DIABN Single Crystals from 5 to 314 K. Double Exponential LE and ICT Fluorescence

Figure 4. Fluorescence spectra of 4-(diisopropylamino)benzonitrile (DIABN) single crystals at various temperatures from 200 to 5 K. At 200 K down to 80 K, the spectra consist of two emissions ICT and LE, with a decreasing ICT/LE fluorescence intensity ratio. At the two lowest temperatures, only LE fluorescence is observed (see text). Excitation wavelength 308 nm (32 500 cm−1).

ICT reaction, is problematic in the case of the DIABN crystals because of the strong reabsorption, mentioned above. Instead, an analysis of the temperature dependence of the fluorescence decay times will be carried out in a later section. 6987


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The Journal of Physical Chemistry A

Figure 6. Absorption (ABS) and fluorescence (FLU) spectra of 4(dimethylamino)benzonitrile (DMABN) in 2-methylpentane at +25 and −155 °C.

Decays above 60 K. The fluorescence decays of DIABN single crystals were measured from 5 to 150 K (−268 to −123 °C, Table 1) (SPC1) and also at higher temperatures, from 135 to 287 K (−135 to +41 °C, Tables 2 and 3), on a different laser system (SPC2) with a better time resolution; see the Experimental Section. For double exponential LE and ICT fluorescence decays the following expressions hold (eqs 3−5). if (LE) = A11 exp(−t /τ1) + A12 exp(−t /τ2)

(3)

if (ICT) = A 21 exp( −t /τ1) + A 22 exp( −t /τ2)

(4)

A = A12 /A11

(5)

The expressions for τ1, τ2, and A appearing in eqs 6−8 are4 1 1/τ1,2 = {(X + Y ) ∓ (X − Y )2 + 4kakd } (6) 2

X = ka + 1/τ0(LE)

(8)

Y = kd + 1/τ0′(ICT)

(9)

Figure 7. Fluorescence decays of DIABN single crystals at three temperatures. (a) 6 K. λem: 352 nm (LE). (b) 100 K. λem: 352 nm (LE). (c) 140 K. λem: 352 nm (LE) and 450 nm (ICT). Experimental excitation pulse and LE and ICT decays are shown as gray lines, light blue lines, and magenta points, respectively. Calculated LE decays, weighted residuals, and autocorrelation functions are presented in blue for LE and in red for ICT. The decay times and their amplitudes are indicated together with the fitting quality parameters χ2.

Low-Temperature Fluorescence Decays (5−150 K, Table 1). The fluorescence decays of DIABN single crystals are measured with setup SPC1 from 5 to 150 K at 352 nm (28 400 cm−1) around the maximum of the LE fluorescence and for two temperatures (140 and 150 K) also at 440 or 450 nm (22 200 cm−1), near the ICT emission maximum (Figure 4). Three decay curves, at 6, 100, and 140 K, are presented in Figure 7. At 6 K, the LE decay is effectively single exponential with a time τ1 = 3.63 ns, showing a fit that is fully acceptable and typical for a crystal fluorescence decay (Figure 7a).11 The interpretation that the decay is single exponential agrees with our conclusion from Figure 4 that the fluorescence at 5 K exclusively consists of an LE emission. Likewise, the decays at 40, 50, and 60 K can be adequately fitted as a single exponential, with τ1 equal to 3.59, 3.54, and 3.36 ns (Table 1). This finding is in accord with our observation (Figure 4) that in the fluorescence spectrum of crystalline DIABN the additional ICT emission is not clearly visible below 80 K

(Figure 4). The shortening of the decay time τ1 above 60 K is an indication of the onset of the LE → ICT reaction. At higher temperatures, from 80 to 150 K (Table 1), the DIABN crystal fluorescence decays were found to be effectively double exponential (eqs 3−5), additional minor decay components (such as the 4370 ps in Figure 7c) being attributed to specific crystal defects and packing disorders, as was done in a previous publication11 on fluorescence decays of crystalline DIABN. This is shown in Figure 7b for the LE decay at 100 K and in Figure 7c for the global analysis of the LE and ICT decays at 140 K. At 100 K (Figure 7b), the decay is double exponential (eqs 3−5), having times τ2 = 0.82 ns and τ1 = 2.83 ns, with an amplitude ratio A = A12/A11 = 45 (Table 1). The double exponential character of the decay is in line with the presence of dual ICT + LE emission in the fluorescence spectrum (Figure 4). In Figure 7c, a global analysis of an LE and an ICT fluorescence decay at 140 K is

A=

kakdτ0(LE)2 X − 1/τ1 = 1/τ2 − X (1 + kaτ0(LE) − τ0(LE)/τ1)2

(7)

where

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Table 1. Data for Fluorescence Decay Parameters of DIABN Single Crystals as a Function of Temperature from 5 to 150 K, LE at 352 nm and ICT at 440 nm (140 K) or 450 nm (150 K) (Eqs 3−9)a T

T

λem

τ2

1/τ2

τ1

1/τ1

[K]

[°C]

[nm]

[ns]

[1010 s−1]

[ns]

[1010 s−1]

A = A12/A11

5 6 40 50 60 80 90 100 110 120 130 140 140 150 150

−268 −267 −233 −223 −213 −193 −183 −173 −163 −153 −143 −133 −133 −123 −123

352 352 352 352 352 352 352 352 352 352 352 352 440 352 450

0.050 0.073 0.122 0.238 0.353 0.485 0.516 0.515 0.671 0.671

3.63 3.59 3.59 3.54 3.36 2.99 2.79 2.83 2.88 3.10 2.50 2.48 2.48 2.07 2.07

0.028 0.028 0.028 0.028 0.030 0.033 0.036 0.035 0.035 0.032 0.040 0.040 0.040 0.048 0.048

1.60 10.7 45.5 132 299 273 494 (−0.74)b 647 (−2.31)b

1.99 1.37 0.820 0.420 0.283 0.206 0.190 0.190 0.149 0.149

ka

kd

τ0′(ICT)

1/τ0′(ICT)

[1010 s−1]

[106 s−1]

ka/kd

[ns]

[106 s−1]

0.002 0.016 0.023 0.093 0.209 0.325 0.456 0.487

41.2 10.4 17.1 14.8 10.6 15.8 9.4

3.9 22 54 141 307 289 520

2.79 2.96 2.82 2.88 3.10 2.50 2.49

358 338 355 348 323 401 402

0.643

9.3

690

2.07

483

τ0 = 3.63 ns, the decay time at 5 K (no ICT). 1/τ0 = 0.028 × 1010 s−1. bRatio A22/A21.

a

presented, fitted with two exponentials τ2 = 190 ps and τ1 = 2480 ps, with an amplitude ratio A12/A11 (eq 5) of 494 (Table 1). The short decay time τ2 of the DIABN fluorescence decreases from 1.99 to 0.149 ns, whereas τ1 ranges between 3.10 and 2.07 ns (Figure 8), a pattern to be expected for a two-

Figure 9. Arrhenius plot of the reciprocal decay time difference (1/τ2 − 1/τ0) for DIABN single crystals between 5 and 150 K (−268 to −123 °C). See eq 11 and Table 1.

Note that the results obtained for kd become inaccurate when A (∼ka/kd; see Table 1) reaches large values. Moreover, fluorescence decays of crystals are found to be considerably more complex than those in dilute solution, as can be seen from crystal decays in the literature11 as compared with those in solution.11,25 Therefore, an Arrhenius plot is only prepared for ka (equal to 1/τ2 − 1/τ0; see below) in Figure 9. Low-Temperature Limit (LTL): kd ≪ 1/τ′0(ICT) and A < ka/ kd. From the data in Table 1 and Table 2 (below) it is seen that above 90 K 1/τ2 ≫ 1/τ1 and ka ≫ kd. Likewise, ka is much larger than 1/τ0(LE) as well as 1/τ0′(ICT). Hence, with ka ≫ kd, ka ≫ 1/τ′0(ICT), and ka ≫ 1/τ0(LE), it follows from eq 10 (eqs 6, 8, and 9) that 1/τ2 ≅ ka; see eq 11. This means that the Arrhenius plot of (1/τ2 − 1/τ0) in Figures 9 and 12 (below) is equivalent to a plot of ln ka versus 1/T (K). From this plot, an activation energy Ea = 4.8 kJ/mol is calculated with a preexponential factor k0a = 0.33 × 1012 s−1. These results are similar to the Ea = 5.7 kJ/mol and k0a = 3.5 × 1012 s−1 measured for the ICT reaction of DIABN in n-hexane,1 an indication that in both media the solvent viscosity is not of primary importance, as to be expected for the formation of a planar ICT state, in which the rotational angle around the aminophenyl bond does not change appreciably.13

Figure 8. Fluorescence decay times τ1 and τ2 of DIABN single crystals as a function of temperature, from 5 to 150 K (−268 to −123 °C).

state LE ⇄ ICT equilibrium.25 The amplitude ratio A12/A11 (eq 5) increases from zero at 5 K (no ICT reaction), via A = 1.60 at 80 K, to A = 647 at 150 K (Table 1). This increase of A when going to higher temperatures indicates the growing importance of the LE → ICT reaction, A approaching the ratio ka/kd for large A values at high temperatures,25 as will be detailed in a following section. It further appears from Table 1 that kd ≪ 1/τ′0(ICT) for all temperatures between 80 and 150 K (low-temperature conditions (LTL), confirming the conclusion on the low-temperature limit for the ICT reaction in crystalline DIABN, derived above from the temperature dependence of Φ′(ICT)/Φ(LE). ICT Reaction Rate Constants and Arrhenius Plot from Low-Temperature Decays. From the two decay times τ2 and τ1, and the LE amplitude ratio A12/A11, together with the lifetime τ0(LE), the rate constants ka and kd and also the ICT lifetime τ′0(ICT), eqs 3−9 and Scheme 1, can be calculated.25 For τ0, the decay time of crystalline DIABN at 5 K (3.63 ns, Table 1) is adopted. Data from Table 1 are plotted in Figure 9.

1/τ2 + 1/τ1 = ka + kd + 1/τ0′(ICT) + 1/τ0(LE) 6989

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The Journal of Physical Chemistry A 1/τ2 − 1/τ0(LE) = ka + kd ≅ ka

The short decay time τ2 increases from 14.7 ps at 25 °C to 100 ps at −150 °C, whereas the long time τ1 remains fairly constant, ranging between 3.01 and 3.51 ns (Figure 10, Tables 2 and 3). The LE amplitude ratio A (eq 5) does not show a clear temperature dependence, with values of 860 at 25 °C and 2670 at −150 °C. Because of this large A, ka/kd (∼A; see above) is also large (Table 2), which means that practically all the data inaccuracy will be in kd. Therefore, according to eq 11, the Arrhenius plot in Figure 12 gives the activation energy Ea and the pre-exponential factor k0a of ka, similar to Figure 9. ICT Reaction Rate Constants and Arrhenius Plot for DIABN Single Crystals from Higher-Temperature Decays between 314 and 138 K (+41 to −150 °C) (Tables 2 and 3). From the data for τ1, τ2 and A12/A11 of the double exponential fluorescence decays of DIABN single crystals between 314 and 138 K (+41 to −150 °C) in Tables 2 and 3, together with the lifetime τ0(LE) = 3.6 ns (Table 1, at 5 and 6 K), the ICT rate constants ka and kd and also the ICT lifetime τ′0(ICT) can again be calculated; see eqs 3−9. As 1/τ2 ≫ 1/τ1, ka ≫ kd and ka is also much larger than 1/τ0(LE), as well as 1/ τ0′(ICT); see Tables 2 and 3, it follows again (eq 11) that 1/τ2 = ka. The Arrhenius plot of 1/τ2 − 1/τ0(LE) in Figure 12 is therefore equivalent to a plot of ln(1/τ2) versus 1/T, as in Figure 9. From this plot Ea = 3.4 kJ/mol and k0a = 0.23 × 1012 s−1 are obtained, which values are similar to the data (Ea = 4.8 kJ/mol and k0a = 0.33 × 1012 s−1) determined from the lowtemperature decays in a previous section, measured with setup SPC1 (Figure 9). For both sets of fluorescence decays described here, the temperature dependence of the decay times τ1 and τ2 and Arrhenius plots of 1/τ2 − 1/τ0 are presented together in the Supporting Information (Figures S1 and S2). Single Exponential Fluorescence Decays of DMABN Crystals. The fluorescence decays of DMABN crystals are single exponential over the temperature range from 27 °C (300 K)12 down to −268 °C (5 K), with times τ1 around 4 ns (Figure 13 and Table 4). This clearly indicates that crystalline DMABN does not undergo a LE → ICT reaction over this entire temperature range, supporting the conclusion reached above that the fluorescence spectra (Figure 5) consist of a single LE emission. The DMABN used in these experiments was synthesized, as it was shown previously12 that commercial DMABN contains an aldehyde impurity that could not be completely removed by our usual procedures, with HPLC as the last purification step. This impurity problem causes complex fluorescence decays and is also encountered in a recent publication on DMABN crystals.28 DIABN and DMABN Phosphorescence Spectra at 5 K. The phosphorescence spectra at 5 K of DIABN as (a) crystals, (b) n-hexane matrix, and (c) EPA (5:5:2 diethyl ether/ isopentane/ethanol, by volume) glass are shown in Figure 14. In EPA (Figure 14c) and n-hexane (Figure 14b) the highestenergy peak appears around 23 900 cm−1. With the crystals of DIABN this peak is shifted to lower energy due to the crystal field. Similar phosphorescence spectra are obtained with DMABN in the three media a 5 K (Figure 15). This means, as expected, that ICT does not occur in the triplet state. No Conclusive Structural Information on the Amino Group Configuration in the ICT State of 4-Aminobenzonitriles from Transient Vibrational and Absorption Experiments. PICT vs TICT. In discussions on the configuration of the amino group in the ICT state of 4aminobenzonitriles and in particular the amino twist angle θ

(11)

Under these LTL conditions, the ICT reaction equilibrium (Scheme 1) is shifted to the right and the longer decay time τ1 closely equals τ′0(ICT); see Table 1. Higher-Temperature Fluorescence Decays (135−287 K, Tables 2 and 3). Over the temperature range from 135 to 314 K (−138 to +41 °C) the fluorescence decays of DIABN single crystals were also measured with a laser system (setup SPC2) different from that used for the data in Table 1 (Figures 10 and

Figure 10. Fluorescence decay times τ1 and τ2 of DIABN single crystals as a function of temperature, from 135 to 314 K (−138 to +41 °C).

Figure 11. Global analysis of the LE (blue) and ICT (red) fluorescence decay curves of DIABN single crystals at 173 K (−100 °C). The ICT experimental points are indicated in magenta; the calculated residuals, in red. The excitation pulse profile is presented in gray. See the caption of Figure 7. The middle decay time of 344 ps in parentheses is attributed to crystal defects and other packing disorders (see text). Emission wavelengths: λem(LE) = 345 nm; λem(ICT) = 476 nm (see Figure 4). LE amplitude ratio A = A12/A11 = 2670 (eqs 3 and 5). ICT amplitude ratio A22/A21 = −0.93 (eq 4).

11). At all temperatures, the decays were determined at two wavelengths. One (at 340−380 nm) around the LE emission maximum and the other (at 430−480 nm) near the peak of the ICT fluorescence (Figures 3 and 4). These LE and ICT decays were fitted simultaneously (global analysis),25 with two exponentials (Figure 11), discarding other minor time components, which were attributed to crystal defects and other packing disorders, as done previously.11 Such an analysis at −100 °C (173 K) is presented in Figure 11. 6990


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Table 2. Data for LE Fluorescence Decay Parameters of DIABN Single Crystals as a Function of Temperature from +41 to −135 °C (314 to 138 K) (Eqs 3−9) T [K]

T [°C]

λ [nm]a

τ2 [ps]

1/τ2 [1010 s−1]

τ1 [ps]

A = A12/A11

ka + kd [1010 s−1]

ka [1010 s−1]

kd [106 s−1]

ka/kd

τ́0(ICT) [ns]

314 298 223 203 183 183 173 173 163 153 138 123 298b 163b

41 25 −50 −70 −90 −90 −100 −100 −110 −120 −135 −150 25 −110

370 340 380 380 380 380 340 345 385 370 360 340 373 378

18.3 14.7 32.2 37.5 44.1 48.4 35.4 35.4 56.4 76.1 72.3 100.1 11 55

5.47 6.80 3.11 2.67 2.27 2.07 2.83 2.83 1.77 1.31 1.38 1.0 9.09 1.82

3260 3250 3010 3050 2940 3100 3430 3430 3030 3510 3110 3370 2920 2790

1620 860 2160 4060 1860 1220 1900 2680 1080 1550 4520 2190 470 1970

5.45 6.77 3.08 2.64 2.24 2.04 2.80 2.80 1.75 1.29 1.36 0.97 9.06 1.79

5.43 6.77 3.08 2.64 2.24 2.04 2.80 2.80 1.74 1.29 1.36 0.97 9.04 1.79

34 79 14 6.5 12 17 15 11 16 8 3 4 190 9

1620 860 2170 4070 1870 1220 1900 2670 1080 1550 4540 2190 470 1990

3.26 3.25 3.01 3.05 2.94 3.10 3.43 3.43 3.03 3.51 3.11 3.37 2.92 2.78

LE emission wavelength. τ0 = 3.6 ns (Table 1: τ0 = 3.63 ns (5 K), τ0 = 3.59 ns (6 K)). bSimilar data from ref 11 are presented for comparison.

a

Table 3. Data for ICT Fluorescence Decay Parameters of DIABN Single Crystals as a Function of Temperature from +41 to −135 °C (314 to 138 K) (Eqs 3−9) T [K]

T [°C]

λ [nm]a

τ2 [ps]

1/τ2 [1010 s−1]

τ1 [ps]

A22/A21

314 298 223 203 183 183 173 163 153 138 298b 163b

41 25 −50 −70 −90 −90 −100 −110 −120 −135 25 −110

430 476 440 440 440 440 476 480 430 440 473 473

18.3 14.7 32.2 37.5 44.1 48.4 35.4 56.4 76.1 72.3 11 55

5.47 6.80 3.11 2.67 2.27 2.07 2.83 1.77 1.31 1.38 9.10 1.80

3260 3250 3010 3050 2940 3100 3430 3030 3510 3110 2920 2790

0.27 −0.44 0.09 −0.14 2.12 −0.18 −0.93 −0.80 −0.31 −0.35 1.32 −1.04

ICT emission wavelength. τ0 = 3.6 ns (Table 1: τ0 = 3.63 ns (5 K), τ0 = 3.59 ns (6 K)). bSimilar data from ref 11 are presented for comparison. a

Figure 13. Single exponential fits of fluorescence decays of DMABN crystals at (a) 5 K and (b) at 300 K. Excitation wavelength λexc 304 nm. Emission wavelength λem 350 nm at 5 K and 364 nm at 300 K.

differentiate between the TICT (perpendicular twist) and PICT (planar) models. CN Stretch Frequencies. In the Raman and IR spectra of DIABN single crystals in the electronic ground state S0, a CN stretch vibrational band centered at 2210 cm−1 is observed.38 During the LE → ICT reaction, this band undergoes a substantial downshift to 2104 cm−1, with a rise time of 10 ps.38 The decrease in frequency is caused by a change in bond character of the cyano group from triple toward double, resulting from the increase in dipole moment from ∼7 D (S0) to 18 D (ICT).1 For the ICT state of DIABN in dilute MeCN solution, an identical CN stretch vibrational frequency at ∼2100 cm−1 is found.35 Such a CN stretch frequency of ∼2100 cm−1 is likewise observed for the ICT state of DMABN,29,30,35,37 4-(diethylamino)benzonitrile

Figure 12. Arrhenius plot of the reciprocal decay time difference (1/ τ2 − 1/τ0), eq 11, for DIABN single crystals between 314 and 138 K (+41 to −135 °C), Tables 2 and 3.

between the planes of the amino and benzonitrile moieties, three sets of experimental data have appeared: (a) CN stretch frequencies, (b) transient absorption (TA) spectra, and (c) N-phenyl stretch frequencies.29−41 It will be outlined here that these data do not contain conclusive information on the ICT molecular structure and can therefore not be used to 6991


Article

The Journal of Physical Chemistry A Table 4. Fluorescence Decay Times of DMABN Crystals as a Function of Temperaturea T [K]

T [°C]

τ1 [ns]

5 50 100 !50 200 210 250 298 300

−268 −223 −173 −123 −73 −63 −23 25 27

4.05 4.0 4.25 4.2 4.15 4.40b 4.1 4.25b 4.0

Excitation wavelength λexc = 304 nm. Emission wavelengths: λem = 350 nm (5−100 K), 351 nm (150 K), 356 nm (200 K), 361.6 nm (250 K), 364 nm (300 K). Measured with the setup SPC1. bObtained with the setup SPC2, see Experimental Section.

a

Figure 15. Phosphorescence spectra at 5 K of DMABN as (a) crystals, (b) n-hexane matrix, and (c) EPA (5:5:2 diethyl ether/isopentane/ ethanol, by volume) glass.

between 0.0° for DMABN and 88.7° for CBQ and a substantial ICT charge transfer character (ICT dipole moment μe(ICT) around 17 D) (Table 5). This then means that structural information on the magnitude of θ cannot de derived for the 4aminobenzonitriles from an analysis of the ICT CN frequencies in the transient Raman or IR spectra, contrary to what has been reported31−37 in the literature. As a further complicating factor, it has been shown for DMABN in the triplet state that ν̃(CN) is not a reliable indicator of the extent of charge transfer as compared with the other states S0, LE, and ICT.43 For the DMABN triplet, the cyano frequency of 2035 cm−1 is much more downshifted relative to S0 at 2210 cm−1 (Δ = −175 cm−1) than that of the LE (2180 cm−1, Δ = −30 cm−1) and ICT (2095 cm−1, Δ = 115 cm−1) states. The shift is clearly not related to the extent of charge transfer, as seen from the DMABN dipole moments: 6.6 D (S0), 9 D (triplet), 9.9 D (LE), and 17 D (ICT).3,43 It hence follows that a discussion in terms of classical canonical bond structures and consequently bond lengths is problematic in a comparison of the electronic states of DMABN and frequencies in vibrational spectroscopy. ICT Transient Absorption (TA) Spectra. The ICT transient absorption (TA) spectra of DMABN,25,31,35,36,40 DEABN,35,36 DIABN, 35 MMD,6,35,36 2,4-dimethyl-3-(dimethylamino)benzonitrile (mMMD),6 and CBQ35 (all with a band maximum at ∼420 nm) do not depend on the nature or configuration of the amino group and they all resemble the spectrum,48,49 of the benzonitrile radical anion Bz(−). It can, therefore, not be concluded7,35,36,40,50 from this spectral similarity that the amino subgroup in the 4-aminobenzonitriles is electronically decoupled from the rest of the molecule, as also the absorption spectra of the radical anions of a series of p-

Figure 14. Phosphorescence spectra at 5 K of DIABN as (a) crystals, (b) n-hexane matrix, and (c) EPA (5:5:2 diethyl ether/isopentane/ ethanol, by volume) glass.

(DEABN),36 3,5-dimethyl-4-(dimethylamino)benzonitrile (MMD),36,37 and 6-cyanobenzoquinuclidine (CBQ),35 all practically the same as that of the ground state benzonitrile radical anion Bz(−)39 (2093 cm−1). The ICT state of crystalline DIABN has been shown to be effectively planar, with a twist angle θ of 10°, as compared to 14° in S0 (Table 5).13 It can therefore not be concluded from the similarity of the ICT stretch frequencies of DIABN, DMABN, DEABN, MMD, and CBQ with the ν̃(CN) of Bz(−) that the amino group of the 4-aminobenzonitriles is vibrationally decoupled from the benzonitrile moiety, caused by a perpendicular relative configuration. It merely indicates that ν̃(CN) is not sensitive to the nature of the amino substituent in these 4aminobenzonitriles, with a range of twist angles θ in S0 6992


Article


Table 5. Vibrational Frequencies ν̃(CN) and ν̃(CCN) (cm−1) in S0, Ground State Bond Lengths, Amino Twist Angles θ for LE and ICT, and Dipole Moments μe(ICT) of Aminobenzonitriles molecule

medium

DIABN DIABN DMABN DMABN DMABN DMABN DEABN MMD CBQ Bz(−)p,q Bzr,s

crystal MeCN MeOH MeCN MeCN calc THF THF MeCN THF liquid

S0 CN

ICT CN

2210a

2104a ∼2100e 2095f 2104i 2112i 2242k 2112l 2108j ∼2100e 2093

2210f 2213i 2215i 2352k 2213l 2227j 2230 2229

S0 Δ(CN) CCN

ICT CCN

Δ(CCN)

−106 −115 −109 −103 −110 −101 −119

1227f

−6

1221f

1231l

CN [pm]

CCN [pm]

NPh [pm]

θ(S0) [deg]

θ(ICT) [deg]

114.9b 114.9b 114.5g 114.5g 114.5g 113.7k 114.4m 114.4h 114.9n

143.7b 143.7b 142.7g 142.7g 142.7g 144.4k 143.6m 142.3h 144.6n

137.6b 137.6b 136.5g 136.5g 136.5g 138.6k 136.0n 141.4i 144.6n

14.1b 14.1b 0.0g 0.0g 0.0g 0.0k 5.4m 57.4h 88.7n

10c

μe(ICT) [D] 18d 18d 17h 17h 17i 17.5h 16.4h 13−15o

−137

a

Reference 38. bReference 44. cReference 13. dReference 1, in solution. eReference 35. fReference 33. Data used in ref 43. gReference 45. Reference 3. iReference 37. jReference 30. kReference 42. lReference 36. mReference 46. nReference 6. oReference 47. pBenzonitrile radical anion. q Reference 39. rBenzonitrile. sReference 48. h

such as ICT, the vibrational coupling between the N-phenyl and other bonds is not expected to be absent. Therefore, it is not established without doubt that the N-phenyl bond length can be deduced from the 1281 cm−1 line in the transient Raman or IR spectra of DMABN. In a review,34 Phillips agreed that the weak mode corresponding to the 1281 cm−1 band in the ICT spectrum has a complex nature, with contributions from numerous modes other than the N-phenyl vibration, as pointed out by Okamoto.41 It was then concluded that the proof of either the TICT or PICT model remained elusive.34 Experimental Basis for PICT. For comparison and completeness, the experimental basis for the PICT model is presented here: (a) no ICT with 4-(methylamino)benzonitrile (MABN);7,9,14 (b) absence for aminobenzonitriles of a linear correlation between the ICT emission maximum ν̃max (ICT) and the difference E(D/D+) − E(A−/A) of the redox potentials of the D and A subunits;9,14 (c) effectively planar ICT state of crystalline DIABN from picosecond X-ray diffraction experiments (a twist angle θ between the planes of the amino and bezonitrile moieties of 10° for the ICT state versus 14° in S0);13 (d) similar photophysics of the ICT reaction of a series of N-phenylpyrroles and their planarized fluorazene counterparts.52 (e) A planar ICT state exists for 1tert-butyl-6-cyano-1,2,3,4-tetrahydroquinoline (NTC6), a planarized DMABN derivative.15,16,23

substituted benzonitriles are largely independent of this substituent, all being the same as the absorption spectrum of Bz(−),48,49 as discussed6 previously. Similar to our interpretation for the C≡N stretch frequencies, the identical ICT TA spectra of the aminobenzonitriles do not contain information on the configuration of the 4-amino substituent and also do not imply a weak interaction for the D and A subgroups in the ICT state. It can hence likewise not be used to differentiate between the TICT and PICT reaction models. N-Phenyl Stretch Frequencies. The N-phenyl stretch vibration of the DMABN ICT state in methanol has been claimed to have a frequency of 1281 cm−1, being strongly (−89 cm−1) downshifted relative to the 1377 cm−1 in S0.32 It was then concluded that the N-phenyl bond length increases during the LE → ICT reaction, which was considered to be a proof of the TICT and contradicting the PICT model.32,33,35−37 The identification of the weak band at 1281 cm−1 was in part based on the transient vibrational spectra of the isotopically substituted DMABN-15N and DMABN-d6.32 The main evidence was, however, taken from CASSCF calculations of Dreyer and Kummrow,42 who found a 1286 cm−1 ν̃(N-phenyl) frequency for the TICT state and a 1356 cm−1 frequency for S0.32 The main problem in these computations is the calculated N-phenyl bond length of 138.6 pm, which is considerably longer than the 136.7 pm from X-ray crystal experiments.45 This will necessarily result in a smaller N-phenyl frequency than would be obtained for a correct bond length. A similar discrepancy is present for the strong CN stretch frequency of DMABN in S0.42 The calculated ν̃(CN), likewise after a scaling by 0.9, appears at 2352 cm−1, now upshifted as compared with the experimental frequency of 2215 cm−1, attributed to the shorter computed bond (113.7 pm) relative to the X-ray data45 (114.5 pm). There is hence no secure basis for the identification of the 1281 cm−1 line in the time-resolved resonance Raman (TR3) spectrum of the DMABN ICT state. A possible second problem in this identification of the ν̃(Nphenyl) is the substantial vibrational coupling that occurs for the N-phenyl stretch with a number of other internal coordinates. Already in S0, the experimental line around 1377 cm−1 only has a 23% contribution from the N-phenyl stretching vibration.51 This makes it difficult to extract exact information on bond lengths as N-phenyl. In an excited state

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CONCLUSIONS In the fluorescence spectra of DIABN single crystals measured from 300 to 5 K dual emission (LE and ICT) is observed down to 60 K. The Φ′(ICT)/Φ(LE) ratio gradually decreases upon cooling and below 60 K only LE fluorescence is found. The ICT emission maximum ν̃max(ICT) is similar to that of a solution with a polarity comparable to that of ethyl acetate. The structure of the ICT state of crystalline DIABN will therefore not be different from that in solution. From an analysis of the ν̃max(ICT) data in solution as a function of solvent polarity, it appears that the value for crystalline DIABN falls between those for ethyl acetate (ε25 = 5.99) of 22 260 cm−1 and n-propyl acetate (ε25 = 5.52) of 22 380 cm−1 (Table S1 in Supporting Information). The fluorescence decays of the DIABN crystals are effectively double exponential. The short decay time τ2 increases from 15 ps (298 K) to 1990 ps (80 K), whereas the longer time τ1 ranges from 3250 ps (298 K) to 6993


Article


(2) Daum, R.; Druzhinin, S. I.; Ernst, D.; Rupp, L.; Schroeder, J.; Zachariasse, K. A. Fluorescence Excitation Spectra of Jet-Cooled 4(Diisopropylamino)benzonitrile and Related Compounds. Chem. Phys. Lett. 2001, 341, 272−278. (3) Schuddeboom, W.; Jonker, S. A.; Warman, J. M.; Leinhos, U.; Kühnle, W.; Zachariasse, K. A. Excited-State Dipole Moments of Dual Fluorescent 4-(Dialkylamino)benzonitriles: Influence of Alkyl Chain Length and Effective Solvent Polarity. J. Phys. Chem. 1992, 96, 10809−10819. (4) Reichardt, Ch. Empirical Parameters of Solvent Polarity as Linear Free-Energy Relationships. Angew. Chem., Int. Ed. Engl. 1979, 18, 98−110. (5) Zachariasse, K. A.; Phuc, V. N.; Kozankiewicz, B. Investigation of Micelles, Microemulsions, and Phospholipid Bilayers with the Pyridinium N-Phenolbetaine ET(30), a Polarity Probe for Aqueous Interfaces. J. Phys. Chem. 1981, 85, 2676−2683. (6) Druzhinin, S. I.; Galievsky, V. A.; Demeter, A.; Kovalenko, S. A.; Senyushkina, T.; Dubbaka, S. R.; Knochel, P.; Mayer, P.; Grosse, C.; Stalke, D.; et al. Two-State Intramolecular Charge Transfer (ICT) with 3,5-Dimethyl-4-(Dimethylamino)benzonitrile (MMD) and its Meta Isomer mMMD. Ground-State Amino Twist Not Essential for ICT. J. Phys. Chem. A 2015, 119, 11820−11836. (7) Leinhos, U.; Kühnle, W.; Zachariasse, K. A. Intramolecular Charge Transfer and Thermal Exciplex Dissociation with p-Aminobenzonitriles in Toluene. J. Phys. Chem. 1991, 95, 2013−2021. (8) Grabowski, Z. R.; Rotkiewicz, K.; Rettig, W. Structural Changes Accompanying Intramolecular Electron Transfer: Focus on Twisted Intramolecular Charge-Transfer States and Structures. Chem. Rev. 2003, 103, 3899−4031. (9) Zachariasse, K. A.; Grobys, M.; von der Haar, Th.; Hebecker, A.; Il’ichev, Yu. V.; Jiang, Y.-B.; Morawski, O.; Kühnle, W. Intramolecular Charge Transfer in the Excited State. Kinetics and Configurational Changes. J. Photochem. Photobiol., A 1996, 102, 59−70. Zachariasse, K.; et al. Erratum to “Intramolecular Charge Transfer in the Excited State. Kinetics and Configurational Changes”. J. Photochem. Photobiol., A 1998, 115, 259. (10) Yoshihara, T.; Galievsky, V. A.; Druzhinin, S. I.; Saha, S.; Zachariasse, K. A. Singlet Excited State Dipole Moments of Dual Fluorescent N-Phenylpyrroles and 4-(Dimethylamino)benzonitrile from Solvatochromic and Thermochromic Spectral Shifts. Photochem. Photobiol. Sci. 2003, 2, 342−353. (11) Druzhinin, S. I.; Demeter, A.; Zachariasse, K. A. Dual Fluorescence and Intramolecular Charge Transfer with Crystalline 4-(Diisopropylamino)benzonitrile. Chem. Phys. Lett. 2001, 347, 421− 428. (12) Demeter, A.; Zachariasse, K. A. Fluorescence of Crystalline 4(Dimethylamino)benzonitrile. Absence of Dual Fluorescence and Observation of Single Exponential Fluorescence Decays. Chem. Phys. Lett. 2003, 380, 699−703. (13) Techert, S.; Zachariasse, K. A. Structure Determination of the Intramolecular Charge Transfer State in Crystalline 4(Diisopropylamino)benzonitrile from Picosecond X-Ray Diffraction. J. Am. Chem. Soc. 2004, 126, 5593−5600. (14) von der Haar, Th.; Hebecker, A.; Il’ichev, Yu. V.; Jiang, Y.-B.; Kühnle, W.; Zachariasse, K. A. Excited-State Intramolecular Charge Transfer in Donor/Acceptor-Substituted Aromatic Hydrocarbons and in Biaryls. The Significance of the Redox Potentials of the D/A Subsystems. Recl. Trav. Chim. Pays-Bas 1995, 114, 430−442. (15) Zachariasse, K. A.; Druzhinin, S. I.; Bosch, W.; Machinek, R. Intramolecular Charge Transfer with the Planarized 4-Aminobenzonitrile 1-tert-Butyl-6-cyano-1,2,3,4-tetrahydroquinoline (NTC6). J. Am. Chem. Soc. 2004, 126, 1705−1715. (16) Druzhinin, S. I.; Kovalenko, S. A.; Senyushkina, T.; Zachariasse, K. A. Dynamics of Ultrafast Intramolecular Charge Transfer with 1tert-Butyl-6-cyano-1,2,3,4-tetrahydroquinoline (NTC6) in n-Hexane and Acetonitrile. J. Phys. Chem. A 2007, 111, 12878−12890. (17) Zachariasse, K. A.; Grobys, M.; von der Haar, Th.; Hebecker, A.; Il’ichev, Yu. V.; Morawski, O.; Rü ckert, I.; Kü hnle, W. Photoinduced Intramolecular Charge Transfer and Internal Con-

3630 ps (5 K). The much smaller temperature dependence for τ1 is generally observed for a reversible LE ⇄ ICT equilibrium reaction. An activation energy Ea for the forward LE → ICT reaction of around 4 kJ/mol is determined for crystalline DIABN, as compared with 6 kJ/mol for DIABN in dilute nhexane solution. With crystalline DMABN, in contrast, an ICT reaction does not take place, as only LE fluorescence is found, over the entire temperature range from 300 to 5 K. The phosphorescence spectra of DIABN and DMABN crystals at 5 K only show a single emission band, which indicates, as expected, that an ICT reaction does not occur in the triplet state. For crystalline DIABN in the literature, the CN stretch frequency at 2210 cm−1 undergoes a substantial downshift to 2104 cm−1 during the S0 → ICT reaction. As this ICT state is effectively planar, this is not caused by a perpendicular shift of the amino group, but by a change in bond character of the cyano group from triple toward double due to the increase in dipole moment from ∼7 D (S0) to 18 D (ICT). Literature data sets on CN and N-phenyl stretch frequencies from transient vibrational (Raman and IR) spectroscopy and also transient absorption spectra are discussed. It is concluded that a proof of either the PICT or the TICT model for the LE ⇄ ICT reaction of aminobenzonitriles can not be made from these data.

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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.8b06349.

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Effective polarity of DIABN crystals, fluorescence decay times and Arrhenius plots for DIABN single crystals as a function of temperature, complete list of the authors of refs 6 and 27 (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (K.A.Z.). *E-mail: [email protected] (S.I.D.). *E-mail: [email protected] (B.K.). ORCID

Klaas A. Zachariasse: 0000-0003-3390-0956 Sergey I. Druzhinin: 0000-0002-6545-8819 Boleslaw Kozankiewicz: 0000-0002-2507-2356 Present Address §

Physical Chemistry I, Department of Chemistry and Biology, University of Siegen, D-57076 Siegen, Germany. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Many thanks are due to Mr Claus-Peter Adam, MedienService, Max-Planck-Institut für biophysikalische Chemie Göttingen for preparing the Figures.

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REFERENCES

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