Photophysical Properties of Long Rodlike Meso−Meso-Linked Zinc(II) Porphyrins Investigated by Time-Resolved Laser Spectroscopic Methods

Abstract

The molecular design of directly meso-meso-linked porphyrin arrays as a new model of light-harvesting antenna as well as a molecular photonic wire was envisaged to bring the porphyrin units closer for rapid energy transfer. For this purpose, zinc(II) 5,15-bis(3,5-bis(octyloxy)phenyl)porphyrin (Z1) and its directly meso-meso-linked porphyrin arrays up to Z128 (Zn, n represents the number of porphyrins) were synthesized. The absorption spectra of these porphyrin arrays change in a systematic manner with an increase in the number of porphyrins; the high-energy Soret bands remain at nearly the same wavelength (413-414 nm), while the low-energy exciton split Soret bands are gradually red-shifted, resulting in a progressive increase in the exciton splitting energy. The exciton splitting is nicely correlated with the values of cos[pi/(N + 1)] according to Kasha's exciton coupling theory, providing a value of 4250 cm(-1) for the exciton coupling energy in the S(2) state. The increasing red-shifts for the Q-bands are rather modest. The fluorescence excitation anisotropy spectra of the porphyrin arrays show that the photoexcitation of the high-energy Soret bands exhibits a large angle difference between absorption and emission dipoles in contrast with the photoexcitation of the low-energy exciton split Soret and Q-bands. This result indicates that the high-energy Soret bands are characteristic of the summation of the individual monomeric transitions with its overall dipole moment deviated from the array chain direction, while the low-energy Soret bands result from the exciton splitting between the monomeric transition dipoles in line with the array chain direction. From the fluorescence quantum yields and fluorescence lifetime measurements, the radiative coherent length was estimated to be 6-8 porphyrin units in the porphyrin arrays. Ultrafast fluorescence decay measurements show that the S(2) --> S(1) internal conversion process occurs in less than 1 ps in the porphyrin arrays due to the existence of exciton split band as a ladder-type deactivation channel, while this process is relatively slow in Z1 (approximately 1.6 ps). The rate of this process seems to follow the energy gap law, which is mainly determined by the energy gap between the two Soret bands of the porphyrin arrays.

Multistep Energy Transfer in Single Molecular Photonic Wires

“…1 However, strongly coupling porphyrin arrays exhibit the disadvantage of forming so-called energy sinks due to different local interactions of the chromophores. 3 Furthermore, as molecular photonic wires have to operate at the single molecule level they have to be studied at this individual level as well. Therefore, the chromophores used have to exhibit special spectroscopic characteristics, such as high photostability and fluorescence quantum yield.…”

Femtosecond Coherent Spectroscopic Study of Zn(II)porphyrin Using Chirped Ultrashort Pulses

“…Emission spectrum with photoexcitation at 405 nm shows relatively strong S2 fluorescence similar to other mesosubstituted Zn II porphyrins. [16][17][18][19][20][21][22]24,25 From the absorption and emission spectra, the Stokes-shift was estimated to be about 630 cm −1 , which is quite similar to that of Zn Femtosecond pump-probe spectroscopy gives a direct insight into the dynamics of coherent wave packet motions and subsequent vibronic relaxation processes of molecules in the condensed phase. [1][2][3][4][5][6][7][8] We have studied impulsively photoinduced vibrational coherent motions in the electronic ground and excited states of Zn II DPP.…”

“…As like other metalloporphyrins, the S1 state of Zn II DPP is formed within ~1.6 ps following the Soret band excitation and decays to the ground state on a much slower time scale. 16 The slow 1.6 ps S2 → S1 internal conversion process for Zn II DPP is attributable to unfavorable Franck-Condon factors for nonradiative decay between the nearly parallel S2 and S1 PESs. 23 It is observed that the amplitude for each decay component depends on the pump/probe wavelengths as shown in Figure 3.…”

“…[13][14][15] Among these, porphyrins are one of the most attractive building block elements due to their desirable molecular characteristics such as rigid planar geometry, high stability, intense electronic absorption, and small HOMO-LUMO energy gap, as well as flexible tunability of their optical and redox properties by appropriate metalation. [16][17][18] Thus it is indispensable to have a deep understanding into the excited states of porphyrin molecules in various forms of molecular photonic wires, because the light signal transmission is based on the excitation energy transfer in the excited states of porphyrin molecules. [19][20][21] In this work, we present time-resolved spectroscopic data on Zn(II)-5,15-diphenylporphyrin (Zn II DPP) in toluene such as pump-probe transient absorption, coherent vibrational oscillations, and RR spectra.…”

Trimer Porphyrin Star

“…coincide with each other indicating the weak intramolecular interporphyrin communication but the intensity of absorption has considerably increased. The Soret band of 4 is sharp and doesn't display broadening typical for closely linked multiporphyrins arising from splitting of energy levels which is a consequence of point-dipole excitation coupling [28,29] and reported as characteristic for planar meso-meso linked multiporphyrins. [30] Relatively large interchromophore distance and nonplanar positions of porphyrin units, which can be seen at the molecular model of 4 diminish that electronic communication as well as perpendicular turned benzene rings break conjugation between neighbouring π-electronic systems (Figure 1 (DABCO) were added.…”