Control and Switching of Aromaticity in Various All-Aza-Expanded Porphyrins: Spectroscopic and Theoretical Analyses

Abstract

Modification of aromaticity is regarded as one of the most interesting and important research topics in the field of physical organic chemistry. Particularly, porphyrins and their analogues (porphyrinoids) are attractive molecules for exploring various types of aromaticity because most porphyrinoids exhibit circular conjugation pathways in their macrocyclic rings with various molecular structures. Aromaticity in porphyrinoids is significantly affected by structural modification, redox chemistry, NH tautomerization, and electronic states (singlet and triplet excited states). Conversely, aromaticity significantly affects the spectroscopic properties and chemical reactivities of porphyrinoids. In this context, considerable efforts have been devoted to understanding and controlling the aromaticity and antiaromaticity of porphyrinoids. Thus, a series of porphyrinoids are in the limelight, being expected to shed light on this field because they have some advantages to demonstrate the switching of aromaticity; it is possible to control the aromaticity by lowering the temperature, adding and removing the protons of expanded porphyrins, changing the chemical environment, and switching the electronic states (triplet and singlet excited states) by photoexcitation. In this regard, this Review describes the control of aromaticity in various expanded porphyrins from the spectroscopic point of view with assistance from theoretical calculations.

Optical and magnetic properties of antiaromatic porphyrinoids

“…[43][44][45][46][47][48] Due to their high applicability in various fields, it is desirable to un-derstand the electronic structure of porphyrinoids in detail, which may open up routes to a tailored design of new porphyrinoid based molecules with desired properties. 49,50 Common spectroscopic techniques that have been used for investigating porphyrinoids are nuclear magnetic resonance (NMR), 51 ultraviolet (UV) absorption, 1 magnetic circular dichroism (MCD), 52,53 electronic circular dichroism (ECD), 54,55 photoelectron (PE), 56 and two-photon absorption (TPA) [57][58][59] spectroscopies to mention the most common ones.…”

Synthesis, Characterization, and Electrochemistry of Open‐Chain Pentapyrroles and Sapphyrins with Highly Electron‐Withdrawing meso‐Tetraaryl Substituents

“…[1] This macrocycle is an early member of the group of "expanded porphyrins" [2][3][4][5][6][7][8][9][10][11] and was first discovered in the early 1960s as ab yproduct in the synthesis of corroles. [12] Since then, sapphyrins have remained at opic of high interest, [13][14][15][16] in part because of their potential applicationsi nt he area of photodynamic therapy and their structurals imilarity to the well-known porphyrin and corrole macrocycles, [10,11,[17][18][19][20][21] and in part because of their unique properties as ar eceptor of neutraland anionic substrates. [22][23][24][25][26][27][28][29] Al arge number of studies have characterizedd ifferent sapphyrind erivatives, including b-alkylateds apphyrins, [30][31][32][33] coremodified sapphyrins with heteroatoms (O, S, or Se), [34][35][36][37][38][39][40][41][42][43]…”

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

“…Such observations are typical of what is seen in the case of aromatic expanded porphyrins. [25] As expected given its non-fluorescent nature, the excited-state lifetime of 5 was estimated to be 1.4 ps in toluene,av alue that is shortened to 1.0 ps in the case of 8. Presumably,t his reduced lifetime is the result of efficient intersystem crossing, which is due to the heavy atom effect of the coordinated rhodium metal cations.T he observation of long residual TA signals (up to 3ns) is consistent with the population of an excited-state triplet.…”