Characteristic Electronic Perturbation by Asymmetric Arrangements of

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Characteristic Electronic Perturbation by Asymmetric Arrangements of para-Aminophenyl Substituents in Free-Base Porphyrins Sung Cho, Yunhui Lee, Hyoung Soon Han, Hyo Kyoung Lee, and Seungwon Jeon J. Phys. Chem. A, Just Accepted Manuscript • Publication Date (Web): 13 Jun 2014 Downloaded from http://pubs.acs.org on June 20, 2014

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The Journal of Physical Chemistry

Characteristic Electronic Perturbation by Asymmetric Arrangements of para-Aminophenyl Substituents in Free-Base Porphyrins

Sung Cho,* Yunhui Lee, Hyoung Soon Han, Hyo Kyoung Lee, Seungwon Jeon*

Department of Chemistry, Chonnam National University, Gwangju 500-757, Korea

ACS Paragon Plus Environment

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ABSTRACT We have investigated the perturbed electronic properties of meso-substituted free-base porphyrins with symmetric and asymmetric arrangements of substituents using time-resolved spectroscopic measurements and theoretical calculations. The extent of electronic perturbation by substituents in meso-substituted porphyrins is mainly affected by the isoenergetic condition of frontier MOs of porphine and substituent units, non-orthogonal geometry, and geometrical arrangement of substituents. By using the asymmetric arrangements of para-aminophenyl and pentafluorophenyl substituents, we can induce the electron-rich condition on porphine unit and the intramolecular charge transfer character in the excited state. On the basis of this work, we can gain further insight into the energetic and geometric factors of substituents, the interaction between porphine and substituent units, and the perturbed photophysical and electronic properties by substituents, which provides a firm basis for further understanding of the catalytic activities or photophysical properties of porphyrins in porphyrin-based molecular catalysts and electronics.

Keywords: Heterocyclic macrocycles, Substituent effect, Electron donating/withdrawing groups, Charge transfer state, Charge density


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INTRODUCTION Porphyrins, a class of tetrapyrrolic macrocycles with four meso-carbons with an [18] πconjugation pathway, have emerged as a good candidate for a metal host for photo- or electrocatalytic reactions1-5 and as a molecular building block for molecular electronics.6-11

The

catalytic activities and electronic properties of porphyrins can be tuned by the substitution of a central metal and by various arrangements of substituents at meso- and β-carbons in the porphine unit. Generally, electron-donating or -withdrawing substituents are widely used to control the electron density on the porphine unit and to induce intramolecular charge transfer (CT) character in the excited states.12-13 For the best performance of porphyrin-based catalysts or molecular devices, we should consider several parameters to choose proper substituents such as electrondonating/-withdrawing ability, steric effect, and structural symmetry. According to Gouterman’s four-orbital theory, the weak absorption bands and small oscillator strength at the S1 state (Q-bands) of porphyrins can be understood by the configuration interactions between two HOMOs and two LUMOs.14

We are interested in how the

degeneracies of two HOMOs and two LUMOs can be perturbed by substituents at the mesopositions and their geometric arrangements. In this study, we are focused on the interactions between porphine and substituent units, the intrinsic characters of isolated substituents, and their symmetric and asymmetric arrangements. In this context, we have investigated a systematic series of meso-substituted free-base(FB) porphyrins with symmetric arrangements; FFFF (tetrakis(pentafluorophenyl)porphyrin),

AAAA

(tetrakis(para-aminophenyl)porphyrin),

MMMM (tetrakis(mesityl)porphyrin), and PPPP (tetrakis(phenyl)porphyrin), as well as their related porphyrins with asymmetric arrangements; FFFA (5-(para-aminophenyl)-10,15,20-


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tris(pentafluorophenyl)porphyrin),

FAFA

bis(pentafluorophenyl)porphyrin),

MMMN

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(5,15-bis(para-aminophenyl)-10,20(5-(2-methoxynaphthyl)-10,15,20-

tris(mesityl)porphyrin), MMMm (5-(1,5-dimethoxyphenyl)-10,15,20-tris(mesityl)porphyrin), MTMT (5,15-bis(mesityl)-10,20-bis(thiophenyl)porphyrin), and MaMa (5,15-bis(mesityl)10,20-bis(para-propenylamidephenyl)porphyrin (Scheme 1).

Scheme 1. Schematic Molecular Structures of FB Porphyrins with Symmetric and Asymmetric Arrangements of Substituents.


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On the basis of our works, we have revealed that the electronic perturbation by substituents in porphyrins is strongly affected by the isoenergetic condition of frontier MOs of porphine and substituent units, non-orthogonal geometry, and geometrical arrangement of substituents. Specifically, the asymmetric arrangements of para-aminophenyl and pentafluorophenyl substituents give rise to characteristic electronic perturbations of an electron-rich condition on the porphine unit as well as an intramolecular CT character in the excited states.

para-

Aminophenyl substituent acts as an electron donating group and participates the extension of πconjugation of the HOMO of porphine unit while other substituents cannot contribute to the formation of frontier molecular orbitals. Collectively, the characteristic properties of paraaminophenyl-substituted porphyrins are caused by the unique MO interactions between the porphine and para-aminophenyl units, which can be controlled by the alternating arrangements of heterogeneous substituents.


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EXPERIMENTAL METHODS Sample Preparation and Steady-state Spectroscopic Measurements. Symmetric and asymmetric FB Porphyrins were prepared according to the reported methods15-20 (detailed synthetic procedures in the Supporting Information). FFFF (H2TPFPP), PPPP (H2TPP), and MMMM (H2TMP) were purchased from Sigma-Aldrich. THF (ACS reagent grade, ≥ 99.0% purity) solvent was purchased from Sigma-Aldrich and used without further purification. UV-vis absorption spectra were recorded with a Cary-5000 UV-vis-NIR spectrometer, and steady-state fluorescence spectra were measured using a Hitachi F-2500 fluorometer.

Time-Resolved Fluorescence Measurements. A time-correlated single-photon counting (TCSPC) system was used for the spontaneous fluorescence measurements. The system consisted of a picosecond diode laser (PicoQuant). The excitation wavelength was fixed at 375 nm for all experiments. The excitation beam was focused onto a temperature-controlled cuvette containing the sample using a 5 cm focal length lens with s polarization. The fluorescence from the sample was collected with a magic angle (54.7°) in order to prevent polarization-dependent signals and was focused onto a monochromator (Dongwoo Optron) by a 2′′ plano-convex lens pair and detected using a APD detector (ID Quantique). The full width at half-maximum (fwhm) of the instrument response function obtained by using pump scattering was typically ∼180 ps in our TCSPC system. The number of fluorescence photons per unit time was always maintained at

Characteristic Electronic Perturbation by Asymmetric Arrangements of

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