Rotationally resolved ultrahigh-resolution laser spectroscopy of the S2 A11←S A11 transition of azulene

Abstract

We have observed rotationally resolved ultrahigh-resolution fluorescence excitation spectra of the 0(0)(0) (a-type) and 0(0)(0)+467 cm(-1) (b-type) bands of the S(2) (1)A(1)<--S(0) (1)A(1) transition of jet-cooled azulene. The observed linewidth is 0.0017 cm(-1), which corresponds to the lifetime of 3.1 ns in the S(2) state. Zeeman splitting of rotational lines is very small so that intersystem crossing to the triplet state is considered to be very slow. Inertial defect is very small and the molecule is considered to be planar in the S(0) and S(2) states (C(2v) symmetry). Rotational constants of the S(2) state are almost identical to those of the S(0) state, indicating that geometrical structure is similar in both electronic states. In this case, internal conversion (IC) by vibronic coupling is thought to be inactive. Therefore, the main radiationless transition process in the S(2) (1)A(1) state of azulene was identified to be IC to the S(1) (1)B(2) state. However, this S(2)-->S(1) IC is still slower than that of conventional polycyclic aromatic hydrocarbons. We consider it to be due to the shallower potential energy curve in the S(1) (1)B(2) state, which is also responsible for the extraordinarily fast S(1)-->S(0) IC in the isolated azulene molecule.

Infrared spectra of protonated polycyclic aromatic hydrocarbon molecules: Azulene

“…Table IV summarizes the salient parameters, namely, the C-C bond lengths ͑r i ͒, the C-H bond lengths ͑R i ͒, and the C-C-C bond angles ͑␣ i ͒. In agreement with previous high-level calculations [47][48][49] and spectroscopic evidence, [50][51][52][53][54] azulene is found to have a planar geometry with C 2v symmetry and quite regular five-and seven-membered aromatic rings in its 1 A 1 electronic ground state. All C-C bond lengths are similar ͑1.3895-1.4040 Å͒, with the notable exception of the significantly longer C-C bond fusing both rings ͑r 6 = 1.4988 Å͒.…”

“…Similarly, all C-H bonds of Azu are rather similar ͑1.0810-1.0879 Å͒, as are the C-C-C bond angles within the five-and also within the seven-membered ring ͑106.5°-109.9°, 127.3°-129.1°͒. The deviations of the equilibrium rotational constants ͑A e = 0.095 246, B e = 0.041 870, C e = 0.029 084 cm −1 ͒ from those measured 51,54 for the ground vibrational state ͑A 0 = 0.094 797, B 0 = 0.041 857, C 0 = 0.028 639 cm −1 ͒ are rather small ͑⌬A = 0.5%, ⌬B = 0.03%, ⌬C = 1.6%͒, confirming the suitability of the chosen theoretical level.…”

“…1͒, [47][48][49] in agreement with microwave, [50][51][52] vibrational Raman and IR, 53 and rotationally resolved electronic spectra. 54 In contrast with neutral Azu, theoretical and experimental information on AzuH + is extremely scarce. 8,[55][56][57][58] The only experimental information available comes from mass spectrometry, yielding values for the proton affinity in the range between 925 and 931 kJ/mol.…”

Towards an understanding of the singlet–triplet splittings in conjugated hydrocarbons: azulene investigated by anion photoelectron spectroscopy and theoretical calculations

“…3(a) and (b)), for T 1 and S 1 the calculated vibrational energies are too high in comparison to those found in the PE spectrum. 30 Our geometry optimizations of the higher excited-state structures revealed a conical intersection of the T 2 with the T 1 state. When we investigated this more in detail, we found that TDDFT provides a T 1 double minimum structure in which the twofold rotational symmetry of azulene is destroyed leading to bond alterations in the rings.…”

Electronic Spectroscopy of PAHs

“…Although the spectra were somehow all known, the unprecedented spectral resolution opened the possibility of a complete analysis, from spectral linewidths to rotational constants and detailed geometries. Azulene, naphthalene, pyrene and perylene, as well as benzene, have all been measured Yoshida et al 2009;Baba et al 2009b;Semba et al 2009;Kowaka et al 2010). In addition, application of a magnetic field in the lasermolecules interaction zone was performed in order to probe the Zeeman effect.…”