β‐Octamethoxy‐Substituted 22π and 26π Stretched Porphycenes: Synthesis, Characterization, Photodynamics, and Nonlinear Optical Studies

Abstract

Three meso-expanded tetrapyrrolic aromatic macrocycles, including 22π and 26π acetylene-cumulene bridged stretched octamethoxyporphycenes and octamethoxy[22]porphyrin-(2.2.2.2), are reported, for the first time, by modification of previously reported synthetic methods. This strategy led to an enhancement in the overall yield of their corresponding octaethyl analogues. The methoxy-substituted expanded porphycenes display slightly blueshifted absorption relative to their ethyl analogues, along with very weak fluorescence, probably due to efficient intramolecular charge transfer (ICT). Additionally, the two-photon absorption (TPA) cross sections of these macrocycles were evaluated; these are strongly related to core expansion of the porphyrin aromaticity through increased meso-bridging carbon atoms as well as conformational flexibility and substitution effects at the macrocyclic periphery. In particular, the octamethoxy stretched porphycenes display strong TPA compared with the octaethyl analogues due to the dominant ICT character of methoxy groups with a maximum TPA cross section of 830 GM at 1700 nm observed for 26π-octamethoxyacetylene-cumuleneporphycene.

Stable Expanded Porphycene‐Based Diradicaloid and Tetraradicaloid

“…[14] Therefore,T PA measurement of compound 1 was conducted by using Z-scan technique in the wavelength range of 1800 to 2400 nm, where the contribution from one-photon absorption is negligible.T he highest TPAc ross-section was measured to be 670 GM at 2000 nm in toluene (Figure 4a; Supporting Information, Figure S11). Thee nhanced TPA cross-section observed for 1,c ompared to the previously reported TPAc ross-section value for closed-shell expanded porphycene 9 (380 GM at 1500 nm), [7] can be explained by its relatively larger p-conjugation pathway and moderate diradical character.N evertheless,t he TPAc ross section value of 1 is not high compared with other polycyclichydrocarbon and porphyrinoid-based diradicaloids, [1] which is presumably due to its conformational flexibility. [15] Theexcited state dynamics of 1 was studied by femto-second transient absorption (TA) measurement in toluene.T he TA spectrum of 1 displayed ground-state bleaching signals around 550 and 670 nm and broad excited-state absorption signals in the range of 580-650 and 700-850 nm (Supporting Information, Figures S12, S13).…”

“…However,t he macrocyclic structure of expanded porphycene 1 exhibits as light out-ofplane distortion of bithiophene part (Supporting Information, Figure S3) owing to steric repulsion of opposite thiophene units.T here is also intermolecular bithiophene-bithiophene p-p stacking with ad istance of about 3.44 (Supporting Information, Figure S4). Theb ond lengths of pyrroles and cisoid positions show as maller bond length alternation compared to the structurally resembled expanded porphycene analogue,that is,octamethoxy[22]-porphyrin(2.2.2.2) (9, Figure 2a), [7] indicating more effective cyclic conjugation in 1. However,w ec ould not crystallize 2 despite our numerous efforts.…”

Stable Expanded Porphycene‐Based Diradicaloid and Tetraradicaloid

“…There is also intermolecular bithiophene–bithiophene π–π stacking with a distance of about 3.44 Å (Supporting Information, Figure S4). The bond lengths of pyrroles and cisoid positions show a smaller bond length alternation compared to the structurally resembled expanded porphycene analogue, that is, octamethoxy[22]‐porphyrin(2.2.2.2) ( 9 , Figure a), indicating more effective cyclic conjugation in 1 . However, we could not crystallize 2 despite our numerous efforts.…”

“…The highest TPA cross‐section was measured to be 670 GM at 2000 nm in toluene (Figure a; Supporting Information, Figure S11). The enhanced TPA cross‐section observed for 1 , compared to the previously reported TPA cross‐section value for closed‐shell expanded porphycene 9 (380 GM at 1500 nm), can be explained by its relatively larger π‐conjugation pathway and moderate diradical character. Nevertheless, the TPA cross section value of 1 is not high compared with other polycyclic hydrocarbon and porphyrinoid‐based diradicaloids, which is presumably due to its conformational flexibility .…”

An Expanded Porphycene with High NIR Absorptivity That Stabilizes Two Different Kinds of Metal Complexes

“…Examples include bronzaphyrin (2) [14] and an acetylenecumulene porphycene derivative (3). [15,16] Both 2 and 3 display red-shifted absorption features compared to 1,r eflecting an increase in the electronic pathway from 18 to 26 p-electrons. Thea nnulated meso-dibenzoporphycene derivative 4 also displays ar ed-shifted infrared absorption maximum (in the NIR).…”