Experimental and computational evaluation of the barrier to torsional rotation in a butadiyne-linked porphyrin dimer

Abstract

The barrier to torsional rotation in a butadiyne-linked porphyrin dimer has been determined in solution using variable temperature UV-vis-NIR spectroscopy: ΔH = 5.27 ± 0.03 kJ mol(-1), ΔS = 10.69 ± 0.14 J K(-1) mol(-1). The value of ΔH agrees well with theoretical predictions. Quantum chemical calculations (DFT) were used to predict the torsion angle dependence of the absorption spectrum, and to calculate the vibronic fine structure of the S0 → S1 absorption for the planar dimer, showing that the absorption band of the planar conformer has a vibronic component overlapping with the 〈0|0〉 absorption of the perpendicular conformer. The torsion barrier in the porphyrin dimer is higher than that of 1,4-diphenylbutadiyne (calculated ΔH = 1.1 kJ mol(-1)). Crystallographic bond lengths and IR vibrational frequencies confirm that there is a greater contribution of the cumulenic resonance form in butadiyne-linked porphyrin dimers than in 1,4-diphenylbutadiyne. The DFT frontier orbitals of the twisted conformer of the porphyrin dimer are helical, when calculated in the absence of symmetry. The helical character of these orbitals disappears when D2d symmetry is enforced in the 90° twisted conformer. Helical representations of the frontier orbitals can be generated by linear combinations of the more localised orbitals from a symmetry-constrained calculation but they do not indicate π-conjugation. This work provides insights into the relationship between electronic structure and conformation in alkyne-linked conjugated oligomers.

Helical orbitals and circular currents in linear carbon wires

“…Helical orbitals appear in electronic structure calculations on carbon chains whenever appropriate helicogenic symmetry is achieved, 1,2,9-14 for example through mechanical torsion along the molecular axis. [15][16][17][18] While axial torsion may be a promising avenue, only the even [n]cumulene series has the appropriate molecular symmetry in the ground-state structure. Though cumulenes are not as stable as polyynes, the even [n]cumulenes have been synthesized up to the [6] cumulene, [19][20][21][22][23][24][25] the odd [n]cumulenes up to [9]cumulene, 26 and metallacumulenes of similar lengths have been reported.…”

Coarctate and Möbius: The Helical Orbitals of Allene and Other Cumulenes

“…Helical MOs also appear in terminally functionalized polyynes from axial torsion of the terminal groups when the molecules achieve the appropriate symmetry. 8 , 91 Subjecting cumulenes to axial torsion is an interesting way to tune the electrohelicity effect in situ, which was also explored in separate studies by Imamura and Aoki, 6 and Yakobson and co-workers. 7 …”

Discrete π‐Stacks from Self‐Assembled Perylenediimide Analogues

“…Theq uantum yields (QYs) were 64 %a nd 37 %, respectively,s uggesting that rotation of the PDIs around the central linker induces ad ecay of the excited state. [16] In contrast, for 1a (0.25 mm), the spectra displayed astrong redshifted emission at 600 nm with alonger lifetime of 17.2 ns,as expected for PDI excimers. [17] TheQ Yo ft his sample is somewhat improved to 65 %, as the formation of discrete 1 aggregates potentially restricts the rotation of PDI units and thereby inhibits non-radiative energy transfer.…”