Origin of Panchromaticity in Multichromophore–Tetrapyrrole Arrays

Abstract

Panchromatic absorbers that have robust photophysical properties enable new designs for molecular-based light-harvesting systems. Herein, we report experimental and theoretical studies of the spectral, redox, and excited-state properties of a series of perylene-monoimide-ethyne-porphyrin arrays wherein the number of perylene-monoimide units is stepped from one to four. In the arrays, a profound shift of absorption intensity from the strong violet-blue (B and B) bands of typical porphyrins into the green, red, and near-infrared (Q and Q) regions stems from mixing of chromophore and tetrapyrrole molecular orbitals (MOs), which gives multiplets of MOs having electron density spread over the entire array. This reduces the extensive mixing between porphyrin excited-state configurations and the transition-dipole addition and subtraction that normally leads to intense B and weak Q bands. Reduced configurational mixing derives from moderate effects of the ethyne and perylene on the MO energies and a more substantial effect of electron-density delocalization to reduce the configuration-interaction energy. Quantitative oscillator-strength analysis shows that porphyrin intensity is also shifted into the perylene-like green-region absorption and that the ethyne linkers lend absorption intensity. The reduced porphyrin configurational mixing also endows the S state with bacteriochlorin-like properties, including a 1-5 ns lifetime.

Electronic Structure and Excited-State Dynamics of Rylene–Tetrapyrrole Panchromatic Absorbers

“…The average peak molar absorptivity is ∼50,000 M −1 cm −1 on the basis of the measured values for PMI-P-PMI and comparisons with other arrays. 14,15,18 The span between the short-and long-wavelength maxima is comparable, 310 nm (9430 cm −1 ) for PMI-P-TMI and 297 nm (9470 cm −1 ) for PMI-P-PMI. The longestwavelength absorption feature is moderately bathochromically shifted for PMI-P-TMI (749 nm) versus PMI-P-PMI (728 nm).…”

“…As noted previously, the S 1 excited state of porphyrin−perylene arrays has bacteriochlorin-like characteristics in terms of the fluorescence yield and S 1 lifetime compared to those of a simple porphyrin. 18 Rate Constants for Excited-State Decay Pathways. The radiative (fluorescence) rate constant can be obtained from the measured Φ f and τ S and via the expression k f = Φ f /τ S , where the τ S is taken to be the lifetime τ 2 listed in Table 2.…”

“…We have previously discussed the roles played by MO mixing and CI in such dyads, which possess strong tetrapyrrole−rylene interactions mediated by the direct ethyne linker. 15,18 The manifestations of orbital mixing in defining (1) the MO characteristics and (2) the electronic properties of the excited states of the new dyads and triads, and for some analogues studied previously in comparison, were described above (see e.g., .…”

“…Prior studies on perylene-containing analogues, depending on the array and solvent, revealed small yet measurable (up to 20%) intersystem-crossing yields, although again nonradiative decay occurs primarily by internal conversion. 15,16,18 The findings presented in the Results section show that the increase in S 1 → S 0 radiative rate constant k f (Table 2) and associated increase in S 0 → S 1 absorption intensity (Figure 3) for P-PMI versus P-Ph in toluene results in a 2.7-fold increase in Φ f (0.38 vs 0.14) and 2.3-fold decrease in τ S (4.7 ns vs 11 ns). Thus, the enhanced optical properties of P-PMI are accompanied by a substantial increase in fluorescence and retention of a long singlet excited-state lifetime.…”

The chemical space of B, N-substituted polycyclic aromatic hydrocarbons: Combinatorial enumeration and high-throughput first-principles modeling

“…Historically, isomer counting of cyclic compounds has been based on the enumeration theorem named after George Pólya [33][34][35] . Exhaustive applications of this technique have been carried out by Lindsey et al to enumerate macrocyclic compound libraries 36,37 suitable for lightharvesting 38 . Baraldi et al 39,40 have applied Pólya's theorem and enumerated the stereoisomers of highly symmetric icosahedral topologies.…”