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6.2: Excited states of carotenoids: electronic and vibrational relaxation

One of the most commonly encountered pigments in living organisms are carotenoids. These molecules determine yellow and orange color of autumn leaves, provide colors to fruits and vegetables (tomatoes, oranges), fish and even some birds, such a canary. Besides the obvious ability to aesthetically please one’s eyes, carotenoids are also important for their biological functions. Perhaps the most important one is the absorption of light used for photosynthesis in plants, bacteria and algae. The main photosynthetic pigments, chlorophylls, do not absorb green and yellow light (this is why they look green). In the meantime, our Sun is a yellow star, and radiates most of its energy in the green-yellow range of the visible spectrum. To make use of all this energy, photosynthetic pigment-protein complexes have evolved to contain carotenoids in addition to chlorophylls. Typical carotenoids absorb in blue-green specral range (420-550 nm) and help chlorophylls in capturing solar energy. In some marine organisms that live in deeper waters, where only blue and green light penetrates, carotenoids are actually the main light absorbing pigments.

 

Fig. (1) A: Structural formula of b-carotene, one of the most widespread carotenoids in nature. B: Energy level scheme of a typical carotenoid. C: Absorption spectrum of b-carotene dissolved in hexane. The absorption correspons to the transition from S0 to S1 state (blue arrow). Transitions corresponding to S2àS0 stimulated emission (green arrow), S1àS2 excited state absorption (orange arrow) and ), S2àSN excited state absorption (purple arrow) contribute to pump-probe signals. Wiggly lines show radiationless relaxation processes.

 

All carotenoids have a characteristic polyene structure – a long chain of conjugated double bonds (Fig. 31A). Carotenoids are untypical pigments – they exhibit virtually no fluorescence. This is caused by a peculiar energy level structure: the strong absorption of carotenoids in blue-green spectral range is determined by the transition to the second excited state S2. The extinction coefficient due to this absorption can be up to 150000 mol∙l-1∙cm-1. In the meantime, the transition from the ground state to the first excited state S1 is optically forbidden (the symmetry of the S1 molecular orbital is identical to the one of the ground state S0, which implies that the matrix element describing the probability of optical transition between S0 and S1 is zero).

The structure, energy level scheme and absorption spectrum of a widespread carotenoid, b-carotene (this is the pigment that gives carrots their orange color) is shown in Fig. 31A. When the pigment in the ground state absorbs a photon, it is transferred to the second excited state S2, which is very short lived and quickly relaxes to the optically forbidden state S1, which, in turn relaxes to the ground state. All relaxation steps (wiggly lines in Fig. 31B) are radiationless. These processes can be observed and the energy level scheme can be tested by performing PP experiment on b-carotene [27].

The dynamics of transient absorption in b-carotene (solvent: n-hexane) is shown in Fig. 32. The data is plotted as kinetic traces at selected representative probe wavelengths (A) and transient spectra at selected delays. A quick glance at the data reveals that the kinetic traces at different wavelengths are very different, as are the transient spectra. This indicates that an excited b-carotene does not just decay to to the ground state via fluorescence, as a ‘usual’ dye molecule (see Fig. 12 and Eq. ). If this were the case, we would observe a monoexponential relaxation with identical kinetic behavior at all probe wavelengths. Instead, the observed pump-probe dynamics is determined by a number of different processes, which are discussed below.

The difference absorption kinetics probed at 420 nm (Fig. 32A, black line) is mainly determined by the ground state bleach (GSB). It is obvious from looking at the transient spectra recorded at different delay times: the region from 400 to 490 nm is all negative and resembles the inverted absorption spectrum shown in Fig. 31C. Both spectra feature a characteristic ‘three-finger’ vibrational structure, and, as expected, the bleaching signal is negative. The kinetic trace at 420 nm shows that the absorption of the ground state decreases immediately after the excitation and the signal goes back to zero within roughly 20 ps. This is the time it takes b-carotene to return to the ground state after the excitation.

 

Fig. (2) Difference absorption dynamics in b-carotene dissolved in n-hexane. Panel A shows difference absorption kinetics after exciting the molecules with 400 nm light and probing at 420 nm (black), 495 nm (red), 550 nm (green) and 590 nm (blue). Panel B shows transient absorption spectra at delay times equal to 100 fs (red), 300 fs (green) and 1 ps (blue).

 

At the wavelengths immediately to the red from the red-most absorption maximum, one could expect to observe a negative contribution the difference absorption signal due to stimulated emission (SE). Indeed, the kinetic trace at 495 nm (Fig. 32A, red curve) shows a negative signal immediately after the excitation; however, this signal lasts only 200 fs or less. Later, it is replaced by a small induced absorption (IA). Such fast dynamics at SE wavelength shows that the state into which b-carotene was excited only lives for 200 fs. At the same time, only SE disappears, but the overall signal does not go to zero within 200 fs, neither at this wavelength, nor at GSB. This means that the original ground state population is not yet recovered, but the emitting state population is already lost. Transient absorption spectra at delays later than 200 fs exhibit an intense IA band that rises roughly within 200 fs (compare spectra at 100 fs and 300 fs in Fig. 32B). The maximum of this band is at approximately 550 nm. All these experimental facts put together suggest that the S2 state, into which b-carotene is excited, decays on 200 fs time scale. During this time, the molecule relaxes to S1 electronic state. The transitions from S1 to higher electronic states (orange arrow in Fig. 31B) are responsible for the intense excited state absorption (ESA) band centered at 550 nm.

The comparison of transient spectra at 300 fs and 1 ps reveals that the S1 absorption band becomes narrower and more intense during the time between these two spectra. This is indicative of the vibrational energy redistribution: when the molecule relaxes from the S2 to S1, it does it via the ladder of S1 vibrational sublevels. The population from the higher levels quickly relaxes to the lower ones (within 1 ps). The ensemble of vibrationally excited molecules is distributed over a broader range of energies and therefore has a wider ESA spectrum. Vibrational relaxation is reflected in the kinetic traces at 550 nm and 590 nm (Fig. 32A, green and blue curves). The redder wavelengths mostly represent the ESA of ‘hot’ S1 state: signal decays on the time scale of 1 ps, whereas the bluer wavelengths exhibit rise on the same time scale. Within 1 ps, an equilibirium distribution over the vibrational sublevels is reached and IS spectrum remains virtually constant, exhibiting simple decay with the lifetime of S1 state (~20 ps).

From the analysis above, it is clear that pump-probe experiments allow monitoring a complex excited state dynamics of b-carotene, including, electronic and vibrational relaxation and related spectral changes. Such complex dynamics is observed in all carotenoids, and in some is complicated further by the involvement of not just S2, ‘hot’ S1 and ‘cold’ S1 states, but also charge transfer states. [27].