Transient absorption anisotropy study of ultrafast energy transfer in porphyrin monomer, its direct meso–meso coupled dimer and trimer

Abstract

Transient absorption anisotropies of the Zn(II)porphyrin monomer, its direct meso–meso coupled dimer, and trimer are measured in the B (Soret) band region by sub-30 fs laser pulses. It is shown that detailed information on the electronic structures and energy transfer dynamics can be obtained from the anisotropy and the magic angle data. The anisotropies of all three molecules exhibit remarkable behaviors in the first 200 fs region. Experimental observations can be accounted for adequately by treating the transient absorption signal as an explicit sum of ground state bleach and excited state stimulated emission contributions. In the monomer, the anisotropy decay denotes an oscillatory feature followed by a 100 fs time constant exponential decay. It is argued that the Bx and By transitions of the porphyrin monomer are nondegenerate with an energy splitting of ∼170 cm−1. Furthermore, equilibration of the excitation energy within the B states is slightly underdamped. Excitation of the monomerlike B band of the dimer (trimer) leads to ultrafast ∼30 fs (60 fs) anisotropy decay and a subsequent rise with ∼60 fs (70 fs) time constant. It is concluded that the anisotropy decay is due to the ultrafast energy transfer to the low-energy exciton split B band, while the rise is due to the redshift of the excited state stimulated emission spectrum thereby decreasing the contribution of the excited state stimulated emission component in the overall transient absorption signal.

Femtosecond time‐resolved spectroscopy on biological photoreceptor chromophores

“…Excitation into the Soret band at 435 nm was chosen for technical reasons as the Q-band absorption was not accessible with the experimental setup used. Nonetheless, a number of studies focusing on the ultrafast light-induced processes in porphyrins and porphyrin-like systems have consistently shown ultrafast relaxation from the initially populated Soret into the Q-band manifold [22,84,85,[108][109][110][111][112], thus they will have relaxed into the S 1 state manifold within the best accessible time resolution of the streak camera. Figure 12A displays the fast time evolution of the fluorescence spectrum of PChla dissolved in methanol after excitation of PChla into the Soret band at 435 nm.…”

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.…”

“…28,29 This band is always strongly activated by the B-band excitation in the RR spectrum due to the π-π* transition of porphyrin macrocycle. 10,[16][17][18][19][20] Since these two normal modes involve relatively large vibrational motions of porphyrin ring, their intensities would be strong due to a large change in polarizability. In the previous result on Zn II TPP, however, the φ 10 mode is weakly enhanced presumably due to the contribution of pure phenyl translation motion without porphyrin ring vibration.…”

“…[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.…”

Coherent Electronic and Nuclear Dynamics for Charge Transfer in 1-Ethyl-4-(carbomethoxy)pyridinium Iodide

“…Similar anisotropy decays have been observed for systems of electronically coupled chromophores. 46,[89][90][91][92][93][94] However, ECMPI is not an excitonic system and the absorption spectrum of its radical does not indicate the presence of multiple transitions in the 400 nm region. 34 This leads us to conclude that nuclear relaxation is the origin of the sub-picosecond change in the anisotropy.…”