Van der Waals Complexes of 2-Chloro-, 2-Methyl-, and 1,3-Dimethylazulene with Rare Gases:  Microscopic Solvent Shifts, Structures, and Binding Energies

Abstract

The S2−S0(1La) fluorescence excitation and emission spectra of the van der Waals complexes of three azulene (Az) derivatives, 2-chloroazulene (ClAz), 2-methylazulene (MAz), and 1,3−dimethylazulene (DMAz), with the rare gases, Ar, Kr, and Xe, have been measured under jet-cooled conditions. The microscopic solvent shifts, δν̄, of the origin bands in the S0−S2 spectra associated with complexation of the chromophores with one and two rare gas atoms increase with increasing polarizability of the adatom(s), consistent with the dominance of dispersion in the binding. Although there are substantial variations in the relative values of δν̄ among the Az derivatives examined, all of the δν̄ values are relatively small and are similar to those of the 1Lb(S0−S1) transitions in the rare gas complexes of naphthalene and its methyl-substituted derivatives. The theory of microscopic solvent shifts of Jortner et al. has been used to analyze the solvent shift data. Comparisons of the sources of the oscillator strengths and van der Waals binding interactions in the azulene− and naphthalene−rare gas systems are revealing and suggest that the variations in δν̄ with substitution pattern are primarily electronic in their origin and arise from variations in excited state configuration interactions, the magnitude of which depend on the S2−S n energy spacings. These spacings can be varied by placing substituents either along the long axis (2-position) or parallel to the short axis( 1,3-positions) so that they selectively perturb, respectively, the long axis polarized and the short axis polarized transitions. The structures and binding energies of the complexes of these derivatives have also been modeled using Lennard-Jones type calculations and have been compared with those of Az itself. The observed progressions in the low-frequency intermolecular vibrations in each case are assigned to that excited state bending mode which is parallel to the long axis of the chromophore, in agreement with model calculations using one-dimensional Morse and Taylor's series potential functions.

Control of the Photophysical Properties of Polyatomic Molecules by Substitution and Solvation: The Second Excited Singlet State of Azulene

“…Fortunately, the S 1 −T 1 energy gaps in all the azulene derivatives examined here are expected to be small, and approximately the same as that of azulene itself, namely 460 ± 30 cm -1 , a number which is known with some precision . The reasons for the small S 1 −T 1 energy spacings are associated with the small LUMO−HOMO overlap, and the resulting small correlation energies for the two orbitally unpaired electrons in these states . Thus, reasonable values of Δ E (S 2 −T 1 ) can be obtained by adding 460 cm -1 to the value of Δ E (S 2 −S 1 ) for each compound even though the triplet energies are difficult to obtain directly.…”

“…This is due almost exclusively to changes in the energies of the S 1 states because the S 2 states have energies which are only weak functions of the nature and positions of the substituents, at least for the compounds examined here. The reasons for these trends are related to the quantum mechanical descriptions of the S 1 and S 2 states in azulene and other nonalternant aromatic molecules. − The S 1 −S 0 transition in azulene is usefully described as an electric−dipole allowed HOMO−LUMO one-electron transition which has a low oscillator strength because HOMO and LUMO have large amplitudes on different atoms and their overlap is small. Its transition dipole is perpendicular to the long axis of the azulene ring system.…”

“…The reasons for these trends are related to the quantum mechanical descriptions of the S 1 and S 2 states in azulene and other nonalternant aromatic molecules. [21][22][23][24] The S 1 -S 0 transition in azulene is usefully described as an electricdipole allowed HOMO-LUMO one-electron transition which has a low oscillator strength because HOMO and LUMO have large amplitudes on different atoms and their overlap is small. Its transition dipole is perpendicular to the long axis of the azulene ring system.…”

“…32 The reasons for the small S 1 -T 1 energy spacings are associated with the small LUMO-HOMO overlap, and the resulting small correlation energies for the two orbitally unpaired electrons in these states. 24 Thus, reasonable values of ∆E(S 2 -T 1 ) can be obtained by adding 460 cm -1 to the value of ∆E(S 2 -S 1 ) for each compound even though the triplet energies are difficult to obtain directly. If only structurally rigid molecules are compared, one can use the data for 1,3-dichloroazulene and 1,3-dibromoazulene and compare them with the data for structurally rigid azulenes containing only light atoms in order to determine the effects of heavy atom substitution.…”

Vibration-Assisted Intersystem Crossing in the Ultrafast Excited-State Relaxation Dynamics of Halocoumarins

“…Recent intense research on phosphorescence − could be traced to G. N. Lewis’ statement to his student in 1942 namely, “You know, those triplet states that Mulliken was talking about in ethylene are probably the phosphorescent state” . The experimental evidence for the existence of triplet states in phosphorescent materials by Kasha and Lewis in 1944 also revealed the importance of intersystem crossing (ISC) through spin–orbit coupling (SOC). − The observation of heavy atom effect on phosphorescence by McClure in 1949 and the quadratic dependence of the SOC with the atomic number (∝ Z 4 ) found handy to tune the phosphorescence property of a system. − Halogen substitution in organic molecules like naphthalene, azulene, phenanthrene, quinoline, quinoxaline, and anthracene is a preferred way to introduce heavy atom effect in the past. − A change in orbital type from ππ * → nπ * or nπ * → ππ * has also been noted to facilitate ISC through stronger SOC (El Sayed rule) . However, El Sayed rule only considers the electronic aspect of the transition and ignores the vibrational component .…”