Molecular symmetry determines the mechanism of a very efficient ultrafast excitation-to-heat conversion in Ni-substituted chlorophylls

Abstract

In the Ni-substituted chlorophylls, an ultrafast (<60 fs) deactivation channel is created, which is not present in Ni-porphyrins. This observation prompted us to investigate in detail the mechanism of excitation-to-heat conversion in Ni-substituted chlorophylls, experimentally, using time-resolved laser-induced optoacoustic spectroscopy, and theoretically, using group theory approach. The Ni-substituted chlorophylls show exceptional photostability and the optoacoustic measurements confirm the prompt and very efficient (100%) excitation-into-heat conversion in these complexes. Considering their excellent spectral properties and the loss-free excitation-into-heat conversion they are likely to become a new class of versatile photocalorimetric references. The curious features of the Ni-substituted chlorophylls originate from the symmetry of a ligand field created in the central cavity. The central N-Ni(2+) bonds, formed via the donation of two electrons from each of the sp(2) orbitals of two central nitrogens to an empty [Formula: see text] hybrid centered on Ni(2+), have a considerable covalent character. The extreme rate of excited state relaxation is then not due to a ladder of the metal centered d-states, often invoked in metalloporphyrins, but seems to result from a peculiar topology of the potential energy surface (a saddle-shaped crossing) due to the covalent character of the N-Ni(2+) bonds. This is confirmed by a strong 0→0 character of electronic transitions in these complexes indicating a similarity of their equilibrium geometries in the ground (S(0)) and the excited states (both Q(X) and Q(Y)). The excitation energy is very efficiently converted into molecular vibrations and dissipated as heat, involving the central Ni(2+). These Ni-substituted pigments pose a fine exemplification of symmetry control over properties of excited states of transition metal complexes.

Intrinsic Photoprotective Mechanisms in Chlorophylls

“…Apparently,t he binding of Mg 2+ maximizes the S 1 -T 1 energetic separation and thus lowers the probability of ISC,which is inversely proportional to the S-T gap. [16,17] TheT 1 states of the pigments and their interactions with O 2 in solvents of varying coordination strength were characterized using the LIOAS technique with Ni-Phea and ferrocene as photocalorimetric references. Thenarrowing of the S-T gap in the Pd and Pt complexes indicates adecrease in spatial overlap between these two MOs,which is consistent with as ubstantial shift in electron density towards the core nitrogen atoms in LUMO of Pd-Phea and Pt-Phea (see the MO contours in Figure 2).…”

“…Thefluorescence emission maxima of Phea, Chla, and Zn-Phea, located near 665 nm, show only aw eak solvent effect and relatively small Stokes shifts (! [16,17] TheTD-DFT approach was used to calculate the relevant characteristics of the T 1 state and the electron density distribution on the frontier molecular orbitals (MOs) of the pigments.T he results obtained for Chla compare well to the previously published data, [8,18,19] which confirms the validity of the present approach. [7,14] Also the quantum yield of fluorescence emission (F F )d epends on CMI (Table 1) and the highest values were found for Chla, as in the series of metallosubstituted BChla.…”

Factors controlling the reactivity of divalent metal ions towards pheophytin a

“…Many such metallocomplexes, especially those with transition metal ions, are important catalysts in industry and the laboratory [1], and can be used as photosensitizers in various kinds of phototherapy and diagnostics. Often their functioning relies on those photophysical and photochemical features which result from the interactions of the π-electron system with the central metal ion [2–8]. In addition, the formation of metallosubstituted chlorophylls (Chls) and bacteriochlorophylls (BChls) is relevant from the biological point of view [9] and such modified pigments are useful as model compounds [6, 8, 10].…”

“…Often their functioning relies on those photophysical and photochemical features which result from the interactions of the π-electron system with the central metal ion [2–8]. In addition, the formation of metallosubstituted chlorophylls (Chls) and bacteriochlorophylls (BChls) is relevant from the biological point of view [9] and such modified pigments are useful as model compounds [6, 8, 10]. The chemistry of metalloporphyrins has been extensively explored for over a century and the literature on synthetic approaches to their preparation is indeed vast [11–13].…”

“…Recently, we have shown that the reactivity of porphyrinoids in metalation reactions is determined by both the extended aromatic π-electronic system and the peripheral groups, whose interplay creates the energetic barrier to be overcome [29]. Moreover, the type of bonding between the central metal ion and porphyrinoids depends on the structural details of the chelating molecules [6, 8]. Therefore, the effects observed for simple unsubstituted porphyrins cannot be generalized and directly applied to Chls.…”