[page 51↓]

5  Excited State Properties of Fluorinated Analogues of DMABN and PBN

Abstract

The excited state characteristics of fluorinated derivatives of N,N-dimethylaminobenzonitrile and N-pyrrolobenzonitrile have been characterized by both absorption and emission spectroscopy and the fluorinated have been compared with the corresponding nonfluorinated compounds. The low fluorescence quantum yield values observed in these compounds are not due to intersystem crossing (no phosphorescence observable) and have been rationalized in terms of the fluoro atom substitution which can enhance the benzene channel III nonradiative pathways. It is tentatively proposed that the non-radiative pathway in the excited state of these compounds can be either bending (Dewar path) or folding (prefulvene path) of the benzene ring.

5.1 Introduction

Electronically excited molecules are known to undergo a variety of decay processes. In addition to the radiative decay (fluorescence or phosphorescence), the non-radiative decay takes place either by internal conversion (IC) or by intersystem crossing (ISC). An ISC mechanism from S1 to T1 can be evidenced through phosphorescence. For the other non-radiative channel (IC to the ground state), formation of dark transient structures such as conical intersections and excited state chemical reactions such as fragmentation and isomerisation have been discussed in this type of molecules. In a simple molecule like benzene, very low fluorescent quantum yields have been observed and interpreted by the existence of a very efficient photochemical pathway (channel III photochemistry) [57, 58, 59, 60, 61, 62] to the electronic ground state.


[page 52↓]

Figure 5.1: Structures of the molecules

The motivation of this chapter arises from the fact that the fluoro analogues of DMABN, namely DMABN-F4 possess a benzonitrile group with strongly increased acceptor properties as compared to DMABN. By increasing the acceptor strength by introducing F-atoms into the donor-acceptor systems, the energy of the intramolecular charge transfer (ICT) state will be preferentially stabilized. This should therefore lead to enhanced charge transfer (CT) formation. But on the other hand, the literature indicates that this substitution will lead to an increased efficient non-radiative decay. Druzhinin [45] reported that F-substitution into the corresponding azetidinyl benzonitriles creates an efficient internal conversion channel. It is possible that the IC channels will be further increased by higher fluoro-substitution. In order to investigate this behaviour of fluorinated compounds, further aniline derivatives such as ABN-F4, A-F5 and PBN-F4 are spectroscopically investigated here by means of both absorption and fluorescence spectroscopy.

5.2 Experimental

Technical details about the measurement of absorption and fluorescence spectra and quantum yields are reported in section 3.3

5.2.1 Synthesis of the Compounds used in this Study

The compounds were a gift of Prof. A.I. Tolmachev, Institute for Organic Chemistry of the National Academy of Sciences of Ukraine, Kiew, Ukraine.


[page 53↓]

5.3  Results and Discussion

5.3.1 Absorption and Emission Spectroscopy

Fig. 5.2 shows the absorption and emission spectra of all the compounds investigated, and the photophysical data are compiled in Table 5.1. The absorption spectrum of ABN-F4 in polar solvents is slightly red shifted when compared to that in non-polar solvent. The absorption spectra of all the compounds in solvents of various polarity are independent of concentration effects. There is no emission observable for ABN-F4 in any of the solvents. Even though, the acceptor strength in ABN-F4 is considerably stronger than in the parent compound ABN, the absorption spectrum in hexane is slightly shifted to the blue as compared to ABN in hexane [63]. The latter phenomenon was also taking place in the case of PBN-F4 with respect to p-PBN. The absorption spectra of DMABN-F4 in all solvents are slightly red shifted with respect to DMABN. The difference between the behaviour of p-PBN and DMABN-F4 with respect to their tetrafluorinated derivative can be linked to the different influence of solvent polarity on the absorption spectra (large for DMABN-F4, negligible for p-PBN), which is a sign of the smaller contribution of the quinoidal resonance structure in p-PBN.

Figure 5.2: Normalised Absorption and fluorescence spectra of ABN-F4, A-F5 and PBN-F4 in various solvents of different polarity (Hex – n-hexane, EOE – diethylether, ACN - acetonitrile). Abs. spectra are superimposed in the case of PBN-F4.


[page 54↓]

Table 5.1: Photophysical parameters of fluorinated analogues of DMABN and aniline in various solvents at room temperature and comparison to nonfluorinated derivatives

For A-F5, the absorption spectra do not show much variation in their maximum in the solvents studied. In contrast to ABN-F4, this compound shows emission in n-hexane and diethyl ether, and the spectra are very broad and show a strong solvatochromic redshift indicating an emissive CT state. Moreover, they are blue shifted when compared to DMABN-F4, which indicates that the donor is probably the amino and dimethylamino group. The emission spectra of A-F5 in hexane and diethyl ether also show broad spectra and significant solvatochromic shifts indicating CT emission. The quantum yield values of A-F5 are drastically decreasing from n-hexane, and the compound eventually is non-fluorescent in acetonitrile. This could be a polarity effect energetically favouring a polarity-sensitive non-radiative channel. Apart from the investigations of fluorinated anilines, the fluorinated analogue of PBN, namely PBN-F4 is also investigated. The absorption spectra do not change [page 55↓]with the polarity of the solvent. Emission is only observable in n-hexane. The compound is non-emissive in medium and highly polar solvents. The fluorescence behaviour of ABN-F4, A-F5 and PBN-F4 has also been studied at 77 K in EOE. Neither fluorescence nor phosphorescence was observed in such conditions.

Regarding the nature of the CT state, it is well-known, that DMABN populates a Twisted Intramolecular Charge Transfer TICT state [9],and this has also been verified for DMABN-F4 [64] The strongly increased TICT formation tendency for DMABN-F4 as compared to DMABN can be explained on the basis of two facts: a) the increased acceptor strength b) a change of equilibrium conformation in the ground state. In the ground state, DMABN assumes a planar equilibrium conformation. The addition of fluorine atoms into the ring considerably increases the twisting of the acceptor moiety as evidenced by quantum chemical calculations [64]. Due to the fluoro substituents on the benzene moiety, PBN-F4 is also expected to be more strongly twisted in the ground state than the parent compound PBN.

Figure 5.3: Plot of log knr of DMABN against the number of fluorine atoms. The numbers in the brackets represent the position of the fluorine atoms with respect to the cyano group. The knr values for the monofluorinated DMABN-derivatives have been extrapolated for labels F1(2) and F1(3) from the compounds (ref.7) P4CF2 and P4CF3 in Fig. 5.1.

The low fluorescence quantum yield values can be explained in terms of the fluorine substitution operating as an additional photochemical pathway such as butterfly folding of the benzene ring. The non–fluorescent behaviour of ABN-F4 can be compared to 1,2,4,5-tetrafluorobenzene where laser-induced fluorescence spectra have evidenced a folding (butterfly motion) resulting in a double minimum excited-state potential [57]. An additional question one can ask is whether the increase of the non-radiative rate constant of DMABN shows a linear dependence on the number of fluorine atoms. This is conceptually represented in Fig. 5.3 by plotting log k nr DMABN versus the number of fluorine atoms. It can be seen [page 56↓]that already one fluoro atom can be as efficient as four fluorine atoms in enhancing k nr . The detailed nature of this fluoro effect necessitates further experimental and theoretical studies.

5.4 Conclusion

The excited statecharacteristics of fluorinated aniline and phenylpyrrole derivatives have been analysed by absorption and emission spectroscopy at room temperature. The high solvatochromic effect of the emission spectra of DMABN-F4 and A-F5 points to the formation of a CT state. The increase in non-radiative rate constant by the addition of fluorine atoms can be tentatively rationalised on the basis of the benzene channel III pathways [57, 58, 60]. The reaction pathway for non-radiative decay in the excited state of these compounds might be either bending (Dewar path) or folding (prefulvene path) of the benzene ring. Introduction of fluorine atoms into the acceptor part strongly lowers the energy of the CT state and enhances and enhances its population efficiency. But it also lowers the energy of possible conical intersections related with the channel III pathways and therefore induces a k nr pathway efficiently competing with CT formation.


© Die inhaltliche Zusammenstellung und Aufmachung dieser Publikation sowie die elektronische Verarbeitung sind urheberrechtlich geschützt. Jede Verwertung, die nicht ausdrücklich vom Urheberrechtsgesetz zugelassen ist, bedarf der vorherigen Zustimmung. Das gilt insbesondere für die Vervielfältigung, die Bearbeitung und Einspeicherung und Verarbeitung in elektronische Systeme.
DiML DTD Version 4.0Zertifizierter Dokumentenserver
der Humboldt-Universität zu Berlin
HTML generated:
07.07.2005