Ultrafast Solvation Dynamics and Vibrational Coherences of Halogenated Boron-Dipyrromethene Derivatives Revealed through Two-Dimensional Electronic Spectroscopy

Abstract

Boron-dipyrromethene (BODIPY) chromophores have a wide range of applications, spanning areas from biological imaging to solar energy conversion. Understanding the ultrafast dynamics of electronically excited BODIPY chromophores could lead to further advances in these areas. In this work, we characterize and compare the ultrafast dynamics of halogenated BODIPY chromophores through applying two-dimensional electronic spectroscopy (2DES). Through our studies, we demonstrate a new data analysis procedure for extracting the dynamic Stokes shift from 2DES spectra revealing an ultrafast solvent relaxation. In addition, we extract the frequency of the vibrational modes that are strongly coupled to the electronic excitation, and compare the results of structurally different BODIPY chromophores. We interpret our results with the aid of DFT calculations, finding that structural modifications lead to changes in the frequency, identity, and magnitude of Franck-Condon active vibrational modes. We attribute these changes to differences in the electron density of the electronic states of the structurally different BODIPY chromophores.

Figure 1.

The chemical structures of BODIPY-2H (1) and BODIPY-2I (2) are displayed along with the normalized linear absorption (solid line) and fluorescence (dashed line) spectra of BODIPY-2H (blue) and BODIPY-2I (green).

Figure 2.

Absorptive 2DES spectra at magic angle polarization of BODIPY-2H (a, top) and BODIPY-2I (b, bottom) at three different waiting times: t₂ = 13 fs, 100 fs and 2.5 ps. The panels on the right display the linear absorption (blue) and emission (green) spectra as solid lines, the spectrum of the incoming probe pulse (shaded area), and the normalized projection of the 2DES spectra onto the λ₃ axis for three different waiting times: t₂ = 13 fs (dashed red), t₂ = 100 fs (dashed orange), and t₂ = 2.5 ps (dashed blue).

Figure 3.

(a) Plots of ν₊/_o for BODIPY-2H (blue) and BODIPY-2I (green) as a function of waiting time are shown. Values of ν _+/o are obtained from the normalized projection of the 2DES spectra onto the λ₃ axis. (b) Representative projections (blue dots) are shown with ν₊ indicated with (−) and ν_o with circles (o) for BODIPY-2H (top) and BODIPY 2I (bottom). The orange shaded area is the linear absorption spectrum and the light green shaded area is the emission spectrum.

Figure 4.

(top) Dynamic Stokes shift response functions (S(t)) generated by Eq. 1 from projections of the 2DES spectra onto the λ₃ axis and corresponding fit resulting from the sum of Gaussian and exponential decay functions (blue line) for (a) BODIPY-2H and (b) BODIPY-2I. Inset plots for (a) and (b) show the Stokes shift response functions for up to 2.5 ps and the main plots zoom in on the first 500 fs. (bottom) The dynamic Stokes shift response functions (S(t)) along with corresponding fit for three representative slices along the λ₁ axis for λ₁ = 505 (purple), 502 (blue), and 500 (cyan) nm for BODIPY-2H (c), and λ₁ = 529 (purple), 526 (blue), and 523 (cyan) nm for BODIPY-2I (d) are shown. S(t) generated from projecting the green boxed area of the 2DES spectrum onto λ₃ axis along with resulting fit are shown green (c,d).

Figure 5.

Representative residuals are shown in green for traces taken at the λ₁, λ₃ coordinate indicated with a green dot in the 2D spectra. The mean of the power spectra for each l₁, λ₃ coordinate is plotted in blue for (a) BODIPY-2H (top) and (b) BODIPY-2I (bottom).

Figure 6.

Nuclear motion associated with the assigned vibrational modes is indicated with arrows for (a) BODIPY-2H and for (b) BODIPY-2I

Figure 7.

The HOMO and LUMO molecular orbitals for BODIPY-2H (top) and BODIPY-2I (bottom).