Microsolvation in superfluid helium droplets studied by the electronic spectra of six porphyrin derivatives and one chlorine compound

Abstract

After almost two decades of high resolution molecular spectroscopy in superfluid helium droplets, the understanding of microsolvation is still the subject of intense experimental and theoretical research. According to the published spectroscopic work including microwave, infrared, and electronic spectroscopy, the latter appears to be particularly promising to study microsolvation because of the appearance of pure molecular transitions and spectrally separated phonon wings. Instead of studying the very details of the influence of the helium environment for one particular dopant molecule as previously done for phthalocyanine, the present study compares electronic spectra of a series of non-polar porphyrin derivatives when doped into helium droplets consisting of 10(4)-10(5) helium atoms. Thereby, we focus on the helium-induced fine structure, as revealed most clearly at the corresponding electronic origin. The interpretation and the assignment of particular features obtained in the fluorescence excitation spectra are based on additional investigations of dispersed emission spectra and of the saturation behavior. Besides many dopant-specific results, the experimental study provides strong evidence for a particular triple peak feature representing the characteristic signature of helium solvation for all seven related dopant species.

Microsolvation of molecules in superfluid helium nanodroplets revealed by means of electronic spectroscopy

“…Much information on helium induced line broadening was provided by systematic investigations of a series of related dopant species. For three molecular species namely Pyrromethene (Pentlehner et al, 2011; Stromeck-Faderl et al, 2011), Porphyrin (Pentlehner et al, 2011; Riechers et al, 2013), and Anthracene (Pentlehner et al, 2010, 2011; Pentlehner and Slenczka, 2012, 2013) several derivatives have been investigated which differ in the number and the species of substituents such as methyl, ethyl, propyl, phenyl, and cyano groups which substitute hydrogen atoms in the periphery of the molecular compound. The main conclusions concerning the influence of electronic and vibrational degrees of freedom will be outlined for each of the three molecular species.…”

“…The study of Porphyrin (Riechers et al, 2013) includes derivatives such as 5,15-diphenylporphyrin (DPP), 5,10,15,20-tetraphenylporphyrin (TPP), 5,10,15,20-tetramethylporphyrin (TMP), 5,10,15,20-tetrapropylporphyrin (TPrP), and 2,7,12,27-tetraethyl-3,8,13,18-tetramethylporphyrin (Etio). In addition 5,10,15,20-tetraphenylchlorine (TPC) was investigated, which came as an impurity of the TPP sample (Riechers et al, 2013).…”

“…In addition 5,10,15,20-tetraphenylchlorine (TPC) was investigated, which came as an impurity of the TPP sample (Riechers et al, 2013). Again, for all derivatives the molecular symmetry is conserved if one ignores the configurational variants of the substituents.…”

“…None of the Porphyrin derivatives shows signals which could be attributed to an envelope or fully resolved progression of low energy modes representing torsional or bending modes of the substituents. Obviously, the equilibrium configuration of the substitunets is maintained upon electronic excitation as is the nuclear configuration of the Porphyrin moiety (Riechers et al, 2013). In contrast to the spectra recorded by means of a pulsed dye laser (Lindinger et al, 2001; Lehnig et al, 2007), the low photon flux and single mode radiation of a cw-dye laser enables resolution of a triple peaked ZPL of Porphyrin as shown in the top panel of Figure 3.…”

“…Within the first 10 cm −1 the signal can be separated into three parts. The first part is the signal within the first 1 cm −1 showing what was identified as the triple peak feature characteristic for the ZPL of Porphyrins in helium droplets (Riechers et al, 2013). The leading intense peak exhibits a line width of only 0.05 cm −1 .…”