Absence of Intramolecular Charge Transfer with 4-Fluoro-N,N

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Absence of Intramolecular Charge Transfer with 4-Fluoro-N,N-Dimethylaniline (DMA4F), Contrary to an Experimental Report Supported by Computations Klaas A. Zachariasse, Attila Demeter, and Sergey I. Druzhinin J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.6b12142 • Publication Date (Web): 18 Jan 2017 Downloaded from http://pubs.acs.org on January 20, 2017

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The Journal of Physical Chemistry

Absence of Intramolecular Charge Transfer with 4-Fluoro-N,N-Dimethylaniline (DMA4F), Contrary to an Experimental Report Supported by Computations

Klaas A. ZACHARIASSE,*,a Attila DEMETER,b Sergey I. DRUZHININ,*,a,c

a

Max-Planck-Institut für biophysikalische Chemie, Spektroskopie und Photochemische Kinetik, 37070 Göttingen, Germany

b

Institute of Materials and Environmental Chemistry, Research Centre for Natural Sciences, Hungarian Academy of Sciences, 1519 Budapest, P.O.Box 286, Hungary

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ACS Paragon Plus Environment


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ABSTRACT With 4-fluoro-N,N-dimethylaniline (DMA4F) only a single fluorescence from a locally excited (LE) state is observed, irrespective of solvent polarity, temperature, and excitation wavelength. The relatively small excited state dipole moment µe = 7.3 D confirms the identification as LE. The single exponential fluorescence decays in the nonpolar n-hexane (2.04 ns) and in the strongly polar acetonitrile (5.73 ns) are a further support. Similar results are obtained with 4-chloro-N,N-dimethylaniline (DMA4Cl), having a chlorobenzene subgroup, a somewhat better electron acceptor than the fluorobenzene moiety in DMA4F. The absence of intramolecular charge transfer (ICT) with DMA4F is in accord with its large energy gap ∆E(S1,S2) of 8300 cm-1 in n-hexane between the two lowest singlet excited states, which is even larger than that (6300 cm-1) of N,N-dimethylaniline (DMA), for which an LE → ICT reaction likewise does not occur. The results with DMA4F are in contradiction with a publication by Fujiwara, T.; Reichardt, C.; Vogt, R. A.; Crespo-Hernández, C. E.; Zgierski, M. G.; Lim, E. C. Chem. Phys. Lett. 2013, 586, 70-75, in which the appearance of dual LE + ICT emission is reported for DMA4F in n-hexane and MeCN at room temperature. The ICT/LE fluorescence quantum yield ratio Φ´(ICT)/Φ(LE) reached a maximum value of ∼2, in n-hexane and surprisingly also in MeCN, as the excitation wavelength approaches the rededge of the absorption spectrum. These, in our opinion, erroneous observations were supported by Time-Dependent Density Functional Theory (TDDFT) calculations, which compute a perpendicularly twisted lowest ICT state (TICT) state. This is a further example of the general tendency of computations to find a TICT conformation for the lowest excited singlet state of electron donor/acceptor molecules such as p-substituted anilines.

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INTRODUCTION Among the N,N-dimethylanilines, 4-(dimethylamino)benzonitrile (DMABN) plays a central role in dual luminescence studies. DMABN undergoes intramolecular charge transfer (ICT) in solvents more polar than n-hexane, as shown by the appearance of new red-shifted emission from the ICT state, next to that from the locally excited (LE) state precursor: dual LE + ICT fluorescence.1-5 From the start of the investigations on DMABN and related electron donor(D)/acceptor(A) molecules, it was noticed that the possibility to undergo an ICT reaction was associated with a close proximity of the two lowest-energy singlet excited states S1 and S2.1,2 In our planar ICT (PICT) model, this energy gap ∆E(S1,S2) is of prominent importance, its magnitude governing whether a LE → ICT reaction with the appearance of an ICT emission next to that of LE is possible or not.6-18 For a particular molecule, ∆E(S1,S2) is a function of solvent polarity and somewhat decreases from n-hexane to MeCN.4-14 These conclusions are supported by computational studies.19,20 In a publication on 4-substituted halobenzenes, the fluorescence spectrum of 4-fluoro-N,Ndimethylaniline (DMA4F) was found to show a mirror-image relationship with the absorption spectrum, consisting of a single emission from a LE state in the nonpolar solvent isooctane as well as in the strongly polar acetonitrile (MeCN).21 This finding reveals that an ICT reaction resulting in dual LE + ICT fluorescence does not occur. The absence of ICT emission with DMA4F was confirmed in a later investigation.22 Recently,23 Fujiwara et al. reported on observing dual fluorescence for DMA4F in n-hexane and MeCN at room temperature. They concluded that in both solvents these two emissions originate from a ππ* state and a nearby ICT state with perpendicularly twisted dimethylamino and fluorobenzene moieties. On the basis of the practically identical fluorescence decay times of these states, the conclusion was made that these states were in fact in thermal equilibrium at room temperature. It was further noted that for DMA4F in n-hexane as well as in MeCN, the 3



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excitation spectra at the high-energy side of the emission spectra were somewhat different from the absorption spectra.23 Moreover, the overall fluorescence spectra of DMA4F in MeCN and n-hexane were found to be a function of the excitation wavelength λexc. Such a dependence has not previously been observed with 4-substituted anilines. The ICT/LE fluorescence quantum yield ratio Φ´(ICT)/Φ (LE) increased to a value of ∼2, when λexc goes from 270 to 308 nm in n-hexane (ε = 1.88), as well as from 270 to 312 nm in MeCN (ε = 36.7), i.e., Φ´(ICT)/Φ(LE) is found to be independent of solvent polarity. In all D/A-systems known to us, Φ´(ICT)/Φ(LE) becomes larger with solvent polarity, as for example with DMABN, due to the stabilization of the ICT state relative to LE and the consequent increase of ka/kd, the ratio of the forward and backward rate constants in the ICT reaction equilibrium LE⇄ICT.7,12,24,25 The minor impact of the introduction of p-F substituent in the phenyl ring of a D/A-molecule, is documented by the following examples. Even with 2,6-dimethyl-4-fluoro-N,Ndimethylaniline (MMF), ICT is not observed, neither in n-hexane nor in MeCN.26 For the comparable pair DMABN/2,6-dimethyl-4-cyano-N,N-dimethylaniline (MMD), the ICT reaction is much faster for the latter, with a ka of 1000 x 1010 s-1 for MMD27 against 24 x 1010 s-1 for DMABN4 in MeCN at 25 oC. Similarly, in the series N-phenylpyrrole (PP), 4-fluoro-Nphenylpyrrole (PP4F), and 4-cyano-N-phenylpyrrole (PP4CN) as well as for their rigidized counterparts fluorazene (FPP), and 4-fluorofluorazene (FPP4F), and, 4-cyanofluorazene (FPP4CN) in n-hexane at 25 oC, only LE emission is observed for PP, PP4F, FPP and FPP4F, whereas an additional ICT fluorescence is found for the cyano derivatives PP4C and FPP4CN.28-30 Likewise, an ICT reaction does not occur, in n-hexane nor in MeCN, with Nphenylcarbazole (NPC) and its 4-F derivative NPC4F, whereas ICT emission starts to appear with the 4-CF3-substituted NPC4CF3 and is strongly present with NPC4CN,31 a development similar to that in the series N,N-dimethylaniline (DMA), DMA4F, N,N-dimethyl-44


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(trifluoromethyl)aniline (DMACF3), and DMABN discussed here (Figures 1 and 2, below). A prudent and cautious procedure when confronted with excitation wavelength dependent fluorescence should be to carefully inspect the purity of the molecule employed. This was not done with the DMA4F (Oakwood Products, 98%) in ref 23, which was used as received. To clarify this controversial situation, we have investigated the fluorescence spectra and decays of DMA4F in n-hexane and MeCN. The results are presented here.

EXPERIMENTAL SECTION The compounds appearing in our experiments described here were purified by HPLC. The DMA4F was obtained from Fluorochem. Impurities could not be detected in the HPLC. The solvents were chromatographed over Al2O3 just prior to use. The solutions, with an optical density between 0.4 and 0.6 for the maximum of the first band in the absorption spectrum, were deaerated by bubbling with nitrogen for 15 minutes. The measurement and treatment of the fluorescence spectra, quantum yields, triplet yields, nanosecond and picosecond single photon counting decays has been described elsewhere.4,9,32 The decay time accuracy is estimated to be of 2%. The triplet experiments were made by a transient absorption setup,33 applying for excitation the fourth harmonic of the light pulse of a Surelight Nd-YAG laser (266 nm) at room temperature (25 oC), see Supporting information.

RESULTS AND DISCUSSION Fluorescence and Absorption Spectra of DMA, DMACF3 and DMABN. In N,Ndimethylanilines, the appearance or absence of dual fluorescence from a LE and an ICT state is determined by the nature of the p-substituent on the phenyl ring.1,2,15,16,30 This substituent governs the magnitude of the energy difference ∆E(S1,S2) = ν~ max (S2,abs) - ν~ max (S1,abs) between the two lowest excited singlet states.1,2,4-14,27-32 The influence of the electron affinity 5



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of the phenyl subgroup is of smaller importance.17 The fluorescence and absorption spectra of three N,N-dimethylanilines, DMA, DMACF3, and DMABN, are presented in Figure 1.

Chart 1

Absorption Spectra at 25 oC. ∆E(S1,S2). The energy difference ∆E(S1,S2) for the three molecules DMA, DMACF3, and DMABN decreases from 6300 cm-1, via 4770 cm-1, to around 2700 cm-1 in n-hexane. In MeCN, similar gaps are observed: 6110 cm-1 for DMA and 4650 cm-1 for DMACF3 (Table 1). For DMABN in MeCN, ∆E(S1,S2) cannot be determined because of the full overlap of the S1 absorption band by the dominant S2, but can be expected (Table 1) to be smaller than the ∼2700 cm-1 determined for n-hexane.4,5

Fluorescence Spectra at 25 oC. With DMA, the fluorescence spectrum consists of a single LE emission (Figure 1a), in n-hexane as well as in MeCN at 25 oC.30,34,35 In the case of DMACF3 in n-hexane,5 likewise only LE fluorescence is observed, whereas dual (LE + ICT) emission starts to appear in MeCN, with an ICT fluorescence maximum at 22120 cm-1 and a small ICT/LE fluorescence quantum yield ratio Φ´(ICT)/Φ (LE) = 0.06.18 With DMABN, ICT fluorescence is absent in n-hexane,1-4 but starts to appear in slightly more polar solvents as cyclohexane,36 toluene,25 and DEE,3,7 becomes by far the prominent emission in the strongly polar MeCN,4 with Φ´(ICT)/Φ (LE) = 39.5 at 25 oC. The appearance of ICT fluorescence for DMACF3 in MeCN and to a much larger extent with DMABN in this solvent, has been correlated with the decrease in ∆E(S1,S2) as compared with DMA.18 No doubt, the magnitude of this energy gap will not be the only factor governing the appearance of ICT + LE fluorescence. Other factors, such the electron acceptor character of

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the substituted phenyl moiety will also play a role.17

Figure 1. Fluorescence and absorption spectra in n-hexane and acetonitrile (MeCN) at 25 oC of (a) N,Ndimethylaniline (DMA), (b) N,N-dimethyl-4-(trifluoromethyl)aniline (DMACF3), and (c) 4(dimethylamino)benzonitrile (DMABN). The absorption spectrum of DMA in MeCN is in arbitrary units (a.u.). The fluorescence of DMA in n-hexane and MeCN and that of DMACF3 in n-hexane consists of a single LE emission, whereas dual emission from a locally excited (LE) and an intramolecular charge transfer state (ICT) is observed for DMACF3 and DMABN in MeCN. Excitation wavelength: 297 nm.

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TABLE 1. Spectroscopic Data for DMA, DMACF3, DMABN, and DMA4F in n-Hexane and Acetonitrile (MeCN) at 25 oC Φ´(ICT)/ ν~ max (S1) ν~ max (S2) ∆E(S1,S2)a ν~ max (LE) ν~ max (ICT) εmax(S1) εmax(S2) Φ(LE) solvent [cm-1] [cm-1] [cm-1] [cm-1] [cm-1] [M-1cm-1] [M-1cm-1] DMAb DMACF3c DMABNd DMA4Fh ∆E(S1,S2) = ν~

a

n-hexane MeCN n-hexane MeCN n-hexane MeCN n-hexane MeCN max

33610 33350 32150e 31530e 31650e,g -g 32350 32050

(S2,abs) - ν~

max

39910 39240 38020 37280 35620 34250 40650 40180

6300 6110 4770f 4650f 2720f

Absence of Intramolecular Charge Transfer with 4-Fluoro-N,N

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