Reverse Saturable Absorption in the Near‐Infrared by Fused Porphyrin Dimers

Abstract

Macrocyclic dyes such as porphyrins and phthalocyanines are well known to exhibit strong reverse saturable absorption (RSA) in the visible-wavelength range.[1±5] Here we present results on a new class of macrocyclic dyes, specifically triply linked porphyrin dimers, with the goal of extending the region of RSA to longer wavelengths. Reverse saturable absorption is a phenomenon in which the absorption increases with light intensity because the excited state has a larger absorption cross-section, r ex , than the ground state, r gr , i.e., r ex /r gr > 1. In porphyrins and phthalocyanines the useful RSA region lies between the Q and Soret absorption bands, where the ground state absorption is low. In porphyrins the RSA region typically extends over the range 440±540 nm, [4,5] whereas in phthalocyanines the RSA band can extend out to 650 nm [4,6] and to 700 nm [7] for naphthalocyanines. A reasonable strategy for extending the RSA band to longer wavelengths is to increase the p-conjugation and red-shift the Q-band. Some attempts have been made to extend the RSA band by synthesizing anthralocyanines. [8] This route does shift the Q-band to longer wavelengths (~950 nm), however dye stability and solubility become serious limitations. Recent work on Osuka's triply linked, fused porphyrin dimers [9±13] and higher tape-like oligomers has shown that these materials have extremely red-shifted absorption spectra coupled with photo-and chemical stability. The dimers typically have Q-band maxima around 1000±1200 nm. A further requirement for reverse saturable absorption to be useful is that the excited-state lifetime must be comparable to, or longer than, the optical pulse of interest. This allows the excited state population to accumulate throughout the pulse.Generally this means that the dye must have an appreciable triplet yield, since the triplet excited state has a longer lifetime (microseconds) than the singlet state (nanoseconds). Preliminary photophysical measurements with fused porphyrin dimers suggested that they would not be useful for RSA applications since the excited-state lifetime is of the order of a few picoseconds. [12,13] However in this article we show that, by use of meso-bromo and meso-iodo substituents, we can achieve appreciable triplet yields, which allows us to measure tripletstate lifetimes and excited-state absorption spectra using nanosecond excitation. The spectra and structures of some of the materials discussed in this article are shown in Figure 1. Dimers 1 and 2a have been described previously [11] and dimer 3 was synthesized to increase the conjugation and red-shift the absorption features; it also gives a useful comparison with 2a and 2b to help understand the importance of the bromine and iodine for increasing the triplet yield. The quinoidal tetracyano dimer 1 has a Q-band peak at 958 nm, which is comparable with the anthralocyanines, however in dimers 2a and 2b the Q-band is shifted out to 1139 nm and 1152 nm respectively. The absorption spectra of 2a and 2b are extremely similar, so onl...

Triply Linked Porphyrinoids

“…[29c] Aq uinoidal porphyrin tape was synthesized from a meso-meso-linked quinoidal porphyrin dimer through oxidative fusion reaction ( Figure 6). [30] Reflectingi ts cross-conjugated structure, 17 displays absorption features different from those of the parentp orphyrin tapes probably due to the disruption of aromatic conjugated circuit as revealed by large bond-length alternation. 7,8-Dehydropurpurin is known as au nique p-extended porphyrin featuringadual-electronic state derived from the 18p aromatic and 20p antiaromatic conjugation circuits.…”

Enhanced Optical Limiting Effects in Porphyrin‐Covalently Functionalized Single‐Walled Carbon Nanotubes

“…Porphyrins are often used as visible chromophores to decorate the surfaces of semiconductor and metal nanoparticles, and they are also promising candidates for applications in optical limiters owing to their large RSA in the visible even near-infrared wavelength range. [13][14][15] Furthermore, an effective energy or electron transfer may exist in the functionalized SWNTs with porphyrins. [16] The photoinduced electron transfer can result in a large optical limiting effect, which has been observed in the PVK-modified SWNTs system.…”

Fused Pyrene–Diporphyrins: Shifting Near‐Infrared Absorption to 1.5 μm and Beyond

“…8b,c The conjugation of porphyrin dimers can be extended through several modes of substitution involving the meso, (β, β), (β, meso) and (β, meso, β) positions. For diporphyrins substituted with two alkyne groups at the terminal meso positions, the Q band is red‐shifted by 130 nm ( λ =1181 nm) relative to the parent dimer 9. In contrast, extending the conjugation in porphyrin dimers by benzannulating β,β‐pyrrolic positions red‐shifts the Q band by only 18 nm, and the resulting compounds have poor solubility 8b.…”