Photophysical Characterization of Chromophores and Arrays

We carry out fundamental studies of the relationships between molecular composition, electronic structure, and photophysical properties of tetrapyrrole chromophores and arrays.  The three basic tetrapyrrole families are porphyrins, chlorins and bacteriochlorins.  The three macrocycles have zero, one or two reduced pyrrole rings, respectively.  Native chromophores from each class are shown in Figure 1, the unsubstituted parents are shown in Figure 2, and examples of synthetic analogs bearing a variety of substituents and different central metal ions (or the free base, M = 2H) are given below.

Figure 1 Native tetrapyrroles from the three families.


Figure 2.  Parent unsubstituted tetrapyrroles from the three families.


 All three classes absorb strongly in the near-ultraviolet and weaky in the green-orange spectral regions, complemented in chlorins by stronger absorption in the red and in bacteriochlorins by stronger still absorption in the near infrared region (Figure 3). The long-wavelength (Qy) absorption band of synthetic chlorins and bacteriochlorins have been tuned in small increments from 600 to 900 nm (Figure 4)

Figure 3.  Absorption specta of the three native tetrapyrroles shown in Figure 1 under the solar umbrella.

 

Figure 4.  Absorption specta of representative synthetic chlorins and bacteriochlorins.


The synthetic porphyrins, chlorins and bacteriochlorins are synthesized in the laboratory of Prof. Jonathan Lindsey at North Carolina State University.  At Washington University, we us static and time-resolved (femtoseconds to seconds) absorption and fluoresence spectroscopy to elucidate the manner in which changes in macrocycle type and subsitutents can be used to tune properties such as the absorption and emission wavelengths and intensities, excited state lifetimes, and excited state decay pathways (fluorescence, internal conversion and intersystem crossing).

Collectively we analyze the optical, redox and other physicochemical properties of the chromophores.  For example, the effects of macrocycle type (porphyrin, chlorin, bacteriochlorin), peripherial substitutents, and central metal ion can be analyzed in terms of the effects on thr four frontier molecular orbitals using Gouterman's four-orbital model (Figure 5).  Some of the results from such an analysis for a series of syntehtic bacteriochlorins is given in Figure 6.

Figure 5. The optical spectra of tetrapyrroles can be analyzed using the four-orbital model.

Figure 6.  An example of the analysis of the wavelength (energy) of the near-infrared Qy band of a series of synthetic bacteriochlorins.


A new motif for tuning the electronic properties of tetrapyrroles is obtained by tightly coupling a perylene dye to the tetrapyrrole.  Figure 7 shows that the absorption spectrum of the perylene-porphyrin dyad is not simply the sum of the spectra of the constituents.  In particualr, substantial intensity has been shifted from the near-ultraviolet into the longer wavelength features.  This shift results in effectively panchromatic absorption from the violet to red regions for the porphyrin construct.  Absorption to longer wavelenths is provided by the chlorin and bacteriochlorin analogs. The dramatic changes in spectral properties here appears to be derived primarily from the effects of strong perylene-tetrapyrrole coupling on the configuration-interaction energy (involving electron-electron repulsion, electronic wavefunction overlap, etc.), as complemented by changes in molecular orbital energies.  Having access to these two motifs may provide tools for tuning the spectrtal properties of tetrapyrrole chromophores in unexpected ways.

Figure 7.  Structures of perylene and porphyrin benchmark monomers and peryelen-porphyrin dyad.


We are also interested in the mechanisms, rates, and efficiences of energy and electron transfer in multi-chromophore arrays.  An example is the cyclic hexamer depicted in Figure 8.

Figure 8. Structure and energy transfer pathwyas in a cyclic hexamer host bearing a central guest chromophore.