Exceptional Near-Infrared Fluorescence Quantum Yields and Excited-State Absorptivity of Highly Conjugated Porphyrin Arrays

Abstract

We show that meso-to-meso ethyne-bridged (porphinato)zinc(II) oligomers (PZnn structures) define exceptional low band gap organic materials that possess both large magnitude NIR S1 --> S0 fluorescence quantum yields and substantial S1 --> Sn absorptive cross-sections, tunable over a wide 850-1400 nm spectral window. These PZnn species possess fluorescence quantum yields (phif values) comparable to the highest reported for NIR laser dyes in the 750-900 nm regime; importantly, these emitters do not suffer from commonly cited tricarbocyanine dye drawbacks of poor photostability and substantial phif sensitivity to solvent polarity. Furthermore, tauo (kr-1) values determined using the Strickler-Berg method highlight the close correlation of fluorescence quantum yields with S0 --> S1 integrated oscillator strength and demonstrate a rare if not unique example of broad NIR spectral domain fluorescence energy modulation, where phif magnitudes follow a simple Strickler-Berg relationship.

Excitation of Highly Conjugated (Porphinato)palladium(II) and (Porphinato)platinum(II) Oligomers Produces Long-Lived, Triplet States at Unit Quantum Yield That Absorb Strongly over Broad Spectral Domains of the NIR

“…53,54, The syntheses of PPd 2 , PPd 3 , PPt 2 , and PPt 3 were accomplished through metal-mediated cross-coupling of appropriately brominated and meso-ethyne-functionalized (porphinato)zinc(II) precursors. 57,63,80,81 Scheme 1 highlights the synthetic route to the PPd 2 and PPt 2 structures, while the analogous path to the PPd 3 and PPt 3 species is shown in Scheme S1 of the Supporting Information.…”

“…Representative transient absorption spectra recorded at various time delays are shown in Figure 2 (PPd 2 , PPt 2 ) and Figure 3 (PPd 3 , PPt 3 ), along with the analogous dynamical data obtained for the corresponding PZn n benchmarks. 54,66 The early timedelay transient absorption spectra (t delay < 500 fs, Figures 2 and 3, black lines) of each PZn n , PPd n , and PPt n chromophore share several common features: (a) bleaching signatures of the groundstate Q-and B-state derived bands, (b) transient absorption in the spectral region between these two bleaching signatures, and (c) intense excited-state absorption manifolds that extend over a broad NIR energy regime. At early time delays, the NIR transient absorption signatures (Figure 2) observed for PZn 2 , PPd 2 , and PPt 2 are dominated by respective absorption bands centered at 980 nm (full width at half-maximum (fwhm) ) 660 cm -1 ), 888 nm (fwhm ) 930 cm -1 ), and 848 nm (fwhm ) 1580 cm -1 ).…”

“…This effect has been verified in benchmark PZn n compounds as a function of conjugation length; for example, the measured ISC rate for PZn 5 is so small relative to the corresponding radiative and internal conversion rate constants that its excited triplet state remains virtually unpopulated following S 0 f S 1 excitation. 54 Because the heavy-atom-derived ISC rates in PPd n and PPt n species are much faster than the intrinsic 59,66,87,88 porphyrin nonradiative decay processes, ISC quantum yields approach unity, even for the PPd 3 and PPt 3 derivatives; T 1 f T n excited-state absorptions are intense in comparison to those of their PZn n analogs at 500 ps < t delay < 50 µs. As a result, the molar absorptivities of the PPd n and PPt n excited triplet states are universally large, as indicated by the T 1 f T n molar extinction coefficients compiled in Table 1.…”

“…Transient Absorption Spectroscopy. The femtosecond 54 and nanosecond 55 transient optical systems used for the experiments in this work have been described previously. All samples that were interrogated via transient pump-probe spectroscopic methods were deoxygenated via three successive freeze-pumpthaw cycles prior to measurement.…”

One- and two-photon absorption of highly conjugated multiporphyrin systems in the two-photon Soret transition region

“…[25][26][27] Furthermore, comprehensive studies of highly conjugated PZn oligomers highlight that broad, high oscillator strength NIR excited-state absorptive transition manifolds ͑S 1 → S n , T 1 → T n ͒ are a common characteristic for meso-to-meso macrocycle-macrocycle linkage topologies that include ethyne, butadiyne, and related proquinoidal bridging motifs. 14,[28][29][30][31] For TPA-based applications such as two-photon microscopy, the simultaneous presence of one-photon absorption or excited-state absorption processes can undermine these gains. Two-photon fluorescence microscopy, in particular, will suffer from undesirable increases in excitation volume and photobleaching 32,33 in the presence of such processes.…”