Tuning Electron-Accepting Properties of Phthalocyanines for Charge Transfer Processes

Abstract

Phthalocyanines play fundamental roles as electron-acceptors in many different fields; thus, the study of structural features affecting electron-accepting properties of these macrocycles is highly desirable. A series of low-symmetry zinc(II) phthalocyanines, in which one, three, or four benzene rings were replaced for pyrazines, was prepared and decorated with electron-neutral (alkylsulfanyl) or strongly electron-withdrawing (alkylsulfonyl) groups to study the role of the macrocyclic core as well as the effect of peripheral substituents. Electrochemical studies revealed that the first reduction potential (E_red¹) is directly proportional to the number of pyrazine units in the macrocycle. Introduction of alkylsulfonyl groups had a very strong effect and resulted in a strongly electron-deficient macrocycle with E_red¹ = -0.48 V vs SCE (in THF). The efficiency of intramolecular-charge transfer (ICT) from the peripheral bis(2-methoxyethyl)amine group to the macrocycle was monitored as a decrease in the sum of Φ_Δ + Φ_F and correlated well with the determined E_red¹ values. The strongest quenching by ICT was observed for the most electron-deficient macrocycle. Importantly, an obvious threshold at -1.0 V vs SCE was observed over which no ICT occurs. Disclosed results may substantially help to improve the design of electron-donor systems based on phthalocyanines.

Chart 1. Target Macrocycles (Pc1–Pc5, Pc9, Pc10) and Controls (Pc6–Pc8) Involved in the Studya

Scheme 1. Synthesis of Precursors (a) and Target Macrocycles (b): (i) bis(2-methoxyethyl)amine, K₂CO₃, anh. DMSO, rt, 240 h, 9%; (ii) pentane-3-thiol, K₂CO₃, DMSO, 60 °C, 48 h, 87%; (iii) pentane-3-thiol (1.35 equiv), K₂CO₃, DMSO, 60 °C, overnight, 85%; (iv) bis(2-methoxyethyl)amine, K₂CO₃, DMSO, 110 °C, 24 h, 0%; (v) pentane-3-thiol (2.5 equiv), K₂CO₃, anh. DMSO, rt, 72 h, 77%; (vi) m-CPBA, DCM, rt, 18 h, 14-73%; (vii) H₂O₂, AcOH, reflux, 2 h, 52%; (viii) bis(2-methoxyethyl)amine, THF, −12°C, 2 h, 81%; (ix) pentane-3-thiol, NaOH, THF, rt, 2 h, 64%; (x) pentane-3-thiol, NaOH, THF, rt, 2 h, 72%; (xi) Mg(BuO)₂, BuOH, reflux, 18 h followed by TsOH, THF, rt, 2 h, 13–24%; (xii) Zn(OAc)₂, pyridine, reflux, 30 min, 18–84%; (xiii) Zn(OAc)₂, anh. pyridine, reflux, 18 h, 5%; (xiv) Zn(OAc)₂, o-dichlorobenzene/anh. DMF, 135 °C, 18 h, 84%

Figure 1

Assignment of isolated fractions of Pc9 using ¹H NMR spectra (500 MHz, CDCl₃/pyridine-d₅ 3:1) to positional isomers Pc9-C_4h and Pc9-C_2v. The mobile phase used for the TLC was a 10:1 toluene/pyridine. Green dotted lines indicate the symmetry of the skeleton of the molecule.

Figure 2

(a) Cyclic voltammogram of Pc1 as a model compound of the series (in THF, rt, potential step 5 mV, scan rate 100 mV/s, tetrabutylammonium hexafluorophosphate as supporting electrolyte, potential vs SCE was determined according to oxidation of ferrocene used as internal standard (E_(Fc/Fc+) = 0.56 V vs SCE)). (b) Dependance of E_red¹ values on the number of pyrazine units in the macrocycle. The lines represent linear regression of the data for compounds bearing one donor center (solid line) and four donor centers (dashed line). Empty square = no donor for ICT; full squares = one donor for ICT; full triangles = four donor centers for ICT.

Figure 3

(a) Normalized absorption (blue), fluorescence emission (red), and fluorescence excitation (green) spectra of Pc1 as a model compound of the series in THF (c = 1 μM). (b) Comparison of absorption spectra of Pc1–Pc5 in THF (c = 1 μM).

Figure 4

(a, b) Fluorescence intensity decay curves of compounds Pc6 and Pc9-C_2v in different solvents. IRF = instrument response function. (c) Fluorescence quantum yield of Pc1, Pc6, and Pc9-C_2v dependent on the orientation polarizability of the solvents (Δf).

Figure 5

Lippert–Mataga plots of the studied derivatives in the different solvents (solvents are assigned to numbers in box “a” for Pc6). (a) Compounds bearing only pyrazine rings in the macrocycle. (b) Compounds bearing only benzene rings in the macrocycle. (c) Compounds with mixed macrocycle and/or bearing strongly electron withdrawing alkylsulfonyls.

Figure 6

Relationships between ICT efficiency (monitored as a decrease of sum of Φ_F + Φ_Δ in THF (a) and DMF (b)) and the first reduction potential (E_red¹, in THF). Empty squares = controls without a donor for ICT; full squares = one donor for ICT; full triangles = four donor centers for ICT. The dotted line at −1.0 V vs SCE indicates the threshold where more negative values of E_red¹ lead to ICT inefficiency.