Dramatic enhancement of intrinsic two-photon absorption in a conjugated porphyrin dimery

Promising applications of two-photon absorption (TPA) in high-density optical data storage, two-photon fluorescence microscopy, microfabrication, photodynamic therapy, and other areas of photonics have recently stimulated the search for novel materials with high efficiency of simultaneous (intrinsic) two-photon absorption (TPA). Porphyrins, notably porphyrin oligomers and polymers, deserve a particular attention since tetrapyrrolic chromophores are inherently suitable for medical and biological applications, and also because their photophysical behaviour can easily be optimized by changing the central metal, as well as by peripheral substitution. On the other hand, previous studies have revealed that the majority of regular porphyrins show rather small TPA cross sections, s2 , which, typically, do not exceed 1–10 GM (1 GM 1⁄4 10 50 cm s photon ) in the biologically important range of near-IR wavelengths. This fact alone has so far largely precluded tetrapyrrolic compounds from being used in real TPA-based applications. In this paper, we report on a p-conjugated porphyrin dimer molecule, which shows about 400 times greater two-photon absorptivity than the corresponding monomer. The intrinsic TPA cross section, measured with femtosecond pulses, is, s2 1⁄4 6 10 GM at 780 nm illumination wavelength. This value is comparable to or even exceeds the largest simultaneous TPA cross sections measured for organic chromophores, and is certainly the largest value measured for a tetrapyrrolic molecule.z It is known that in linear oligomers and multibranched conjugated systems the lowest-energy TPA-allowed transition is well separated from other higher TPA-energy levels, leading to characteristic peaks in near-IR TPA spectra. We have recently used this circumstance to study cooperative enhancement of s2 in a series of conjugated multibranched systems. In porphyrins, however, the relevant TPA transitions are known to be devoid of distinct spectral features, and rather form a broad continuum due to several overlapping electronic transitions and their vibronic satellites. Another peculiar feature of porphyrins consists in a possibility of strong resonance enhancement, which occurs if the TPA excitation frequency is close to one-photon-allowed Q-transition(s). Depending on chemical structure, the lowest Q-band is located at 620–740 nm, i.e. close to the important near-IR region of wavelengths. In the experiments with two different porphyrins described below, we found that their TPA spectra correspond to very broad features in the excitation region of 780–900 nm. Therefore, we chose l 1⁄4 780 nm for excitation, which is close to the Q-band, and measure s2 for both the parent porphyrin monomer and its dimer at this particular wavelength. We then invoke the knowledge of linear absorption properties from the groundand excited state to explain the very strong enhancement of s2 in dimer as compared to monomer. The chemical structures of [5,15-bis(3,5-bi-tert-butylphenyl)10,20-bis(trihexylsilylethynyl)porphyrinato]zinc(II) (monomer) and 5,50-(1,3-butadiyne-1,4-diyl)bis[[10,20-bis(3,5-bi-tert-butylphenyl)-15-(trihexylsilylethynyl)porphyrinato]zinc(II) (dimer) are shown in the insets in Fig. 1. Linear absorption and fluorescence spectra of monomer and dimer in polyvinylbutyral (PVB) film are presented in Fig. 1. The extended conjugation in the dimer manifests itself in a pronounced red shift of the absorption and fluorescence maxima relative to monomer. In order to maximize the resonance enhancement, we chose the excitation wavelength l 1⁄4 780 nm on the red side of dimer ’s longest wavelength absorption band. The excitation is performed by 150 fs time duration laser pulses with the repetition rate of 1 kHz (see electronic supplementary information (ESI)y for experimental details). Even though the linear extinction at 780 nm is rather small (the corresponding one-photon absorption (OPA) cross section is s1 3 10 17 cm), it can still surpass the TPA, especially at room temperature. In our recent paper we showed that this residual OPA in porphyrins originates from thermal population of higher-lying vibrational levels of the ground electronic state, and that one can reduce this side effect by lowering the temperature of the sample. Upon lowering temperature, the population of these states depletes, thus decreasing the efficiency of OPA. Fig. 2 shows y Electronic supplementary information (ESI) available: Experimental procedures. See http://www.rsc.org/suppdata/cp/b3/b313399k/ z Degenerate four wave mixing (DFWM) studies on a polymer analogue of this dimer indicate that it may have a two-photon cross section as high as 50 000 GM per macrocycle at 1064 nm, but DFWM is a much more indirect method of determining two-photon cross sections than the femtosecond two-photon excited fluorescence measurements reported here. P C C P w w w .c.o rg /p cp C OMMUN I C A T I O N