Photophysics of a cis axially disubstituted macrocycle: rapid intersystem crossing in a tin(IV) phthalocyanine with a half-domed geometry.

We have studied the photophysical properties of a tin(IV) phthalocyanine which coordinates two myristate groups through their carboxylate functionalities in a cis disposition at the tin center. Such a coordination mode, anisobidentate through the same side of the macrocycle, makes this phthalocyanine acquire a capped or half-domed shape. This bis myristate tin(IV) molecule shows an intersystem crossing channel which populates the triplet manifold with high efficiency and with a time constant of 300 ps, about an order of magnitude faster than planar phthalocyanines, including some previously reported tin(IV) phthalocyanines. For comparison purposes, we also include the description of a planar silicon(IV) phthalocyanine that keeps the more common stereochemistry, of trans type, with the same axial myristate groups. The characterization of these systems included steady state and time-resolved spectroscopy through femtosecond fluorescence up-conversion and transient absorption. We also studied the initial S(n) → S(1) internal conversion dynamics when these compounds are excited to upper states with 387.5 nm light. In addition, we include measurements of the rate for singlet oxygen production through the formation of an ESR-active adduct in aerated solutions. Such measurements indicate that, associated to its photophysics, the tin(IV) phthalocyanine produces (1)O(2) with an efficiency significantly larger than the silicon(IV) counterpart, making it an interesting option for sensitization applications. Finally, we performed excited state calculations at the TD-DFT level which describe the effects of the reduced symmetry together with the state ordering and indicate the presence of near dark intermediate states between the Q and B transitions for both of these macrocycles.

On the accessibility to conical intersections in purines: hypoxanthine and its singly protonated and deprotonated forms.

The dynamics following electronic excitation of hypoxanthine and its nucleoside inosine were studied by femtosecond fluorescence up-conversion. Our objective was to explore variants of the purinic DNA bases in order to determine the molecular parameters that increase or reduce the accessibility to ground state conical intersections. From experiments in water and methanol solution we conclude that both dominant neutral tautomers of hypoxanthine exhibit ultrashort excited state lifetimes (τ < 0.2 ps), which are significantly shorter than in the related nucleobase guanine. This points to a more accessible conical intersection for the fluorescent state upon removal of the amino group, present in guanine but absent in hypoxanthine. The excited state dynamics of singly protonated hypoxanthine were also studied, showing biexponential decays with a 1.1 ps component (5%) besides a sub-0.2 ps ultrafast component. On the other hand, the S(1) lifetimes of the singly deprotonated forms of hypoxanthine and inosine show drastic differences, where the latter remains ultrafast but the singly deprotonated hypoxanthine shows a much longer lifetime of 19 ps. This significant variation is related to the different deprotonation sites in hypoxanthine versus inosine, which gives rise to significantly different resonance structures. In our study we also include multireference perturbation theory (MRMP2) excited state calculations in order to determine the nature of the initial electronic excitation in our experiments and clarify the ordering of the states in the singlet manifold at the ground state geometry. In addition, we performed multireference configuration interaction calculations (MR-CIS) that identify the presence of low-lying conical intersections for both prominent neutral tautomers of hypoxanthine. In both cases, the surface crossings occur at geometries reached by out of plane opposite motions of C2 and N3. The study of this simpler purine gives several insights into how small structural modifications, including amino substitution and protonation site and state, determine the accessibility to conical intersections in this kind of heterocycles.

Ultrafast photosensitization of phthalocyanines through their axial ligands.

We have studied the energy transfer properties of a novel silicon phthalocyanine that coordinates two anthracene-9-carboxylate groups in the form of trans axial ligands. Our objectives were to generate a system with auxiliary chromophores that enhance the light absorption properties of the macrocycle in a specific region in the UV and to evaluate the efficiency and time scales for energy transfer. The ligand coordination through a carboxylate group directly attached to the anthracenic system allows for close proximity of the donor and acceptor chromophores. The energy transfer process was observed to be nearly 100% efficient and to occur on a time scale of 370 fs. From the energy relations of the donor and acceptor states and the observed dynamics, the initial energy transfer step is likely to involve upper electronic states of the phthalocyanine rather than the states of the lowest-energy vibroelectronic Q bands.