Synthesis of phosphorescent asymmetrically π-extended porphyrins for two-photon applications

Abstract

Significant effort has been directed in recent years toward porphyrins with enhanced two-photon absorption (2PA). However, the properties of their triplet states, which are central to many applications, have rarely been examined in parallel. Here we report the synthesis of asymmetrically π-extended platinum(II) and palladium(II) porphyrins, whose 2PA into single-photon-absorbing states is enhanced as a result of the broken center-of-inversion symmetry and whose triplet states can be monitored by room-temperature phosphorescence. 5,15-Diaryl-syn-dibenzoporphyrins (DBPs) and syn-dinaphthoporphyrins (DNPs) were synthesized by [2 + 2] condensation of the corresponding dipyrromethanes and subsequent oxidative aromatization. Butoxycarbonyl groups on the meso-aryl rings render these porphyrins well-soluble in a range of organic solvents, while 5,15-meso-aryl substitution causes minimal nonplanar distortion of the macrocycle, ensuring high triplet emissivity. A syn-DBP bearing four alkoxycarbonyl groups in the benzo rings and possessing a large static dipole moment was also synthesized. Photophysical properties (2PA brightness and phosphorescence quantum yields and lifetimes) of the new porphyrins were measured, and their ground-state structures were determined by DFT calculations and/or X-ray analysis. The developed synthetic methods should facilitate the construction of π-extended porphyrins for applications requiring high two-photon triplet action cross sections.

Chart 1. Structures of the Target Porphyrins

Scheme 1. Synthetic Approach to syn-DBPs and syn-DNPs via Oxidative Aromatization

Chart 2. Dipyrromethanes with C1 Synthons at the 1- and 9-Positions

Scheme 2. Synthesis of β-Substituted Dipyrromethanes

Reagents and conditions: (a) p-TsOH, NBu₄Cl, CH₂Cl₂, r.t., 24 h (3), 48 h (4), 72 h (5). (b) p-TsOH, AcOH, 24 h (6), 48 h (7). (c) H₂/Pd(OH)₂/C, THF, r.t., 24–48 h. (d) (i) TFA/CH₂Cl₂ 1:1, 20 °C, 1 h; (ii) NaHCO₃.

Scheme 3. Synthesis of Target Porphyrins

Reagents and conditions: (a) (i) Zn(OAc)₂·2H₂O (10 equiv), MeOH, reflux, 2 h; (ii) DDQ (3 equiv), r.t., 2 h. (b) (i) Zn(OAc)₂·2H₂O (10 equiv), C₆H₆, Ar, reflux, 2 h; (ii) air, reflux, 12–18 h. (c) DDQ (3 equiv), THF, reflux, 30–40 min. (d) HCl conc./CH₂Cl₂. (e) Pt insertion: (method 1) Pt(acac)₂/benzoic acid, 130–135 °C, 2–6 h; (method 2) Pt(acac)₂/PhCN, microwave, 250 °C (∼200 kPa, 105–145 W), 40 min. Pd insertion: Pd(Ac)₂/PhCN, microwave, 250 °C (∼200 kPa, 105–145 W), 40 min. (f) DDQ (5 equiv), toluene, reflux, 10 h. (g) DDQ (3 equiv), THF, r.t., 10 min.

Figure 1

(A) Absorption and (B) phosphorescence spectra of porphyrins Pt–18 through Pt–22 in DMA. The absorption spectra were normalized by the corresponding Q-band maxima; the inset shows the zoomed Q-band region. The phosphorescence spectra were normalized by the respective phosphorescence quantum yields (Table 1).

Figure 2

(top) ORTEP view of the X-ray crystallographic structure of porphyrin Pt–19 (50% thermal ellipsoids) with two stacked nonidentical porphyrin molecules in the cell. All of the hydrogen atoms have been omitted for clarity. (bottom) View of the plain porphyrin skeleton of Pt–19, showing the small nonplanar distortion of the macrocycle and tilts of the two meso-aryl substituents. The aryl group between the two benzo rings is rotated by 83° relative to the porphyrin mean-square plane.

Figure 3

Relative 2PA cross sections (integrated intensities of 2P-excited phosphorescence normalized by the phosphorescence quantum yield) of PtTCHP (D_4h) and the synthesized Pt porphyrins. The data are scaled in such a way that the relative 2PA cross section of the most absorbing porphyrin, Pt–21, equals 1.0 at 770 nm.