Electronic excited states of protonated aromatic molecules: Protonated Fluorene
Graphical abstract
Highlights
► We report the vibrationally resolved electronic spectrum of protonated fluorene. ► The ground and excited states of the x possible isomers have been calculated ab initio. ► Among these isomers, only two may contribute to the two band systems observed. ► Franck Condon simulations are used to assign the active vibrations.
Introduction
The excited states of protonated aromatic molecules, in particular those of polycyclic aromatic hydrocarbons (PAHs) have been extensively studied in the condensed phase [1], [2], [3], [4], [5], [6], [7]. However, the optical properties of protonated aromatic molecules isolated in the gas phase have been less studied experimentally, because of difficulties encountered in producing sufficient amount of these reactive species [8], [9], [10]. From another point of view, recent developments in quantum calculation methods have improved the description of the excited states of such species [11], [12], [13], [14], [15] and comparison of these calculations with gas phase spectroscopic data is now possible.
The photophysics of protonated polycyclic aromatic hydrocarbons became attractive since it was suggested that these ions might be present in the interstellar medium [14], [16], [17] being potential carriers for the diffuse interstellar bands (DIBs) that appear in astronomical observations.
We have recently studied the electronic spectra of linear protonated PAHs for which the S1–S0 transitions are all in the visible region and they do not show a monotonic red shift as a function of the molecular size, in contrast to their neutral analogues [18]. This behavior was interpreted using the results of ab initio calculations as due to the charge transfer character found in the excited states of PAHs, especially those with an even number of aromatic rings.
Previous work was focussed in protonated PAHs composed by different number of condensed six-membered rings. In the present paper, we will investigate the behavior of protonated fluorene, a PAH analogue having a five-membered ring in the structure, with particular interest in the effect of this moiety on the protonation site and on the charge transfer character of the excited state.
Protonation of fluorene has been used to change the optical properties of conjugated polymers [19]. This allows the design and development of new organic emitter and sensor materials tailored toward particular needs. The effect of protonation on the monomer might be useful to understand the optical properties of larger systems.
Finally, many efforts have been done recently to characterize the stability of the different conformers of protonated molecules through their infrared vibrational spectroscopy [20], [21], [22], [23], [24]. We will show in the present paper that the recent progress achieved by ab initio methods to describe an excited state is such that the electronic spectroscopy nowadays represents a quite accurate alternative to assign conformers, as long as the excited states are not too short lived, i.e. some vibrational analysis can be performed.
Section snippets
Experimental details
Protonated fluorene is produced in a pulsed high voltage electrical discharge source coupled to a pulsed nozzle (General valve) of 300 μm in diameter. The discharge is produced between two electrodes positioned 3 mm downstream from the nozzle. The gas mixture consists of 50% of Helium and 50% of H2 seeded with fluorene vapor, obtained by heating the solid at about 120 °C. The protonated species are obtained only in presence of H2. The typical backing pressure in the source is about 4 bars, while
Experimental
The experimental spectrum is presented in Fig. 1: the laser has been scanned to the red and no other bands were detected. Clearly, the spectrum shows two vibrational progressions: a band system A, starting at 479.8 nm (2.58 eV), and a more intense band system B, starting at 447.6 nm (2.77 eV). The vibrational progression found in system B appears well structured, whereas the progression in system A hardly gets out of the experimental noise, although it can be noted a clear onset with a doublet
Discussion
In all the spectra recorded with our set-up, the structured vibrational progressions are superimposed on a continuous background with a comparable intensity. We ascribe this effect to the presence of protonated molecules formed under very different cooling conditions, the colder ions being responsible for the vibrational band structure while the hot ones contribute to the background. The wide spread in the internal energy of protonated molecules produced by the electric discharge in a
Conclusions
Protonation of fluorene leads to a strong red shift of the electronic transition as compared to the neutral molecule. Two vibrational band systems are observed and assigned by comparison with ab initio calculations to the first excited state transitions of two different isomers. The agreement between the calculations (at the level of theory and basis set used in this paper) and the experiment is extremely good for the transition energies, as it has already been observed for linear protonated
Acknowledgments
The authors thank Prof. O. Dopfer for helpful discussion. This work has been supported by the Université Paris–Sud 11, by the ANR research Grant (ANR2010BLANC040501), the PROCOPE 17832NK program and the RTRA “triangle de la physique”. I.A. thanks the Atomic Energy Commission of Syria for financial support. The calculations have been performed on the GMPCS cluster of LUMAT. E.M. has been supported by the CONICET/CNRS exchange program.
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