Excited-State Aromaticity Reversals in Naphthalene and Anthracene

Abstract

Aromaticity reversals between the electronic ground (S 0 ) and low-lying singlet (S 1 , S 2 ) and triplet (T 1 , T 2 , T 3 ) states of naphthalene and anthracene are investigated by calculating the respective off-nucleus isotropic magnetic shielding distributions using complete-active-space self-consistent field (CASSCF) wavefunctions involving gauge-including atomic orbitals (GIAOs). The shielding distributions around the aromatic S 0 , antiaromatic S 1 ( 1 L b ), and aromatic S 2 ( 1 L a ) states in naphthalene are found to resemble the outcomes of fusing together the respective S 0 , S 1 , and S 2 shielding distributions of two benzene rings. In anthracene, 1 L a is lower in energy than 1 L b , and as a result, the S 1 state becomes aromatic, and the S 2 state becomes antiaromatic; the corresponding shielding distributions are found to resemble extensions by one ring of those around the S 2 and S 1 states in naphthalene. The lowest antiaromatic singlet state of either molecule is found to be significantly more antiaromatic than the respective T 1 state, which shows that it would be incorrect to assume that the similarity between the (anti)aromaticities of the S 1 and T 1 states in benzene, cyclobutadiene, and cyclooctatetraene would be maintained in polycyclic aromatic hydrocarbons.

Aromaticity in the Electronic Ground and Lowest Triplet States of Molecules with Fused Thiophene Rings

“…The changes in aromaticity between the S 0 and T 1 states of thiophene, the thienothiophenes, dithienothiophenes and sulflowers studied in this paper are, as a rule, much less pronounced than those observed in benzene, naphthalene and anthracene [105,106,108] ; surprisingly, our results indicate that hexathio [6]circulene 8 which is relatively weakly aromatic in its electronic ground state becomes more aromatic in its lowest triplet state. The extents of the changes in aromaticity between the S 0 and T 1 states of the molecules we examined are reduced by the use of geometries optimised separately for the two states: It is well-known that antiaromatic molecules molecules try to 'escape' antiaromaticity through a symmetry reduction, for example, square cyclobutadiene distorts to a rectangular geometry with 'single' and 'double' carbon-carbon bonds (for a more detailed discussion and further examples, see refs.…”

“…Each of the series of compounds with one, two and three fused thiophene rings including thiophene 1, thieno compounds do not exhibit trends consistent with the set of disputed predictions made on the basis of the NICS values for benzene, naphthalene and anthracene [113,114] known as the 'anthracene problem', [106,115,116] according to which both rings in naphthalene, as well as the central ring in anthracene should be more aromatic than the benzene ring, and the central ring in anthracene should be more aromatic than the outer rings. Instead, our NICS results suggest that the thiophene ring 1 a is more aromatic than the thienothiophene rings 2 a and 3 a which show levels of local aromaticity close to those of the dithienothiophene outer rings 6 a and 7 a; the central dithienothiophene rings 6 b and 7 b are less aromatic than the respective outer rings.…”

“…This produced much higher NICS(0) and NICS(1) T 1 values of 28.4 and 24.7 ppm, respectively, which are closer to but still noticeably lower than those for the T 1 state of benzene. It has been observed on the examples of naphthalene and anthracene [106] that, when used to calculate magnetic properties, UB3LYP-GIAO overestimates the antiaromaticity of the T 1 state in comparison to CASSCF-GIAO and a switch to a CASSCF-GIAO description of this state in thiophene (and in the other thiophene-based compounds studied in this paper) is likely to detect a lower level of antiaromaticity. This is an indication that, whereas the levels of aromaticity of the S 0 states of benzene and thiophene are comparable, the T 1 state of benzene is more antiaromatic than the T 1 state of thiophene.…”

“…The high antiaromaticity of this T 1 state is emphasized by the NICS(0), NICS(+ 1) and NICS(À 1) values of 35.2, 30.2 and 30.3 ppm, respectively, for the symmetry-equivalent thiophene rings 2 a. Despite the molecular symmetry reduction on switching from S 0 to T 1 (C 2h to C 2 ), the associated changes in the central region of the geometry of the carbon-sulfur skeleton of thieno[2,3b]thiophene 2 are relatively minor; as a result, the off-nucleus shielding picture in T 1 is very similar to those observed for S 0 to T 1 vertical excitations in naphthalene and anthracene, [106] and shows a high level of antiaromaticity. It should be noted that despite the complete deshielding of the fusion bond in the T 1 state, its length in the optimised geometry of this state is not much longer than that in the S 0 optimised geometry, 1.405 against 1.392 Å.…”

Tubular Magnetic Shielding Scan (TMSS): A New Technique for Molecular Space Exploration. (i) The Case of Aromaticity of Benzene and [n]Paracyclophanes

“…Over the years, several variants of the NICS index have been developed, including NICS(0), , NICS(1), NICS zz (0), NICS zz (1), , and NICS-Scan. , In addition, two- and three-dimensional multipoint magnetic probes have been developed, which are useful for scanning and imaging the magnetic shielding in specified spaces surrounding various molecules, including both typical − and biochemical , ones. The ability to evaluate not only aromaticity but also other chemical aspects, such as bonding ,,, and electron excitation effects − on molecular properties, makes these methods valuable in various chemical fields.…”