Laura Moroni

Excited-state absorption and ultrafast relaxation dynamics of porphyrin, diprotonated porphyrin, and tetraoxaporphyrin dication.

The relaxation dynamics of unsubstituted porphyrin (H2P), diprotonated porphyrin (H4P2+), and tetraoxaporphyrin dication (TOxP2+) has been investigated in the femtosecond-nanosecond time domain upon photoexcitation in the Soret band with pulses of femtosecond duration. By probing with spectrally broad femtosecond pulses, we have observed transient absorption spectra at delay times up to 1.5 ns. The kinetic profiles corresponding with the band maxima due to excited-state absorption have been determined for the three species. Four components of the relaxation process are distinguished for H2P: the unresolvably short B --> Qy internal conversion is followed by the Qy --> Qx process, vibrational relaxation, and thermalization in the Qx state with time constant approximately 150 fs, 1.8 ps, and 24.9 ps, respectively. Going from H2P to TOxP2+, two processes are resolved, i.e., B --> Q internal conversion and thermal equilibration in the Q state. The B --> Q time constant has been determined to be 25 ps. The large difference with respect to the B --> Qy time constant of H2P has been related to the increased energy gap between the coupled states, 9370 cm-1 in TOxP2+ vs 6100 cm-1 in H2P. The relaxation dynamics of H4P2+ has a first ultrafast component of approximately 300 fs assigned as internal conversion between the B (or Soret) state and charge-transfer (CT) states of the H4P2+ complex with two trifluoroacetate counterions. This process is followed by internal CT --> Q conversion (time constant 9 ps) and thermalization in the Q state (time constant 22 ps).

Excited States of Porphyrin Macrocycles

S1 --> S(n) spectra of porphyrin, diprotonated porphyrin, and tetraoxaporphyrin dication have been measured in the energy range 2-3 eV above S1 at room temperature in solution by means of transient absorption spectroscopy exciting with femtosecond pulses. Highly excited pi pi* states not active in the conventional S0 --> S(n) spectrum have been observed. The experimental data are discussed on the basis of the time dependent density functional theory taking advantage of large scale calculations of configuration interaction between singly excited configurations (DF/SCI). The DF/SCI calculation on porphyrin has allowed to assign g states active in the S1 --> S(n) spectrum. Applying the same calculation method to tetraoxaporphyrin dication the S0 --> S(n) spectrum is reproduced relatively to the Q and B (Soret) bands as well as to the weaker E(u) bands at higher energy. According to our calculation the S1 --> S(n) transient spectrum is related to states of g symmetry mainly arising from excitations between doubly degenerate pi and pi* orbitals such as 2e(g) --> 4e(g). In the case of diprotonated porphyrin it is shown that the complex of the macrocycle with two trifluoroacetate anions plays a significant role for absorption. Charge transfer excitations from the anions to the macrocycle contribute to absorption above the Soret band, justifying the intensity enhancement of the S0 --> S(n) spectrum with respect to the other two macrocyclic systems.

Optical characterization of quasi-monomolecular layers of the nonlinear polymer polyDCHD-HSPresented at the LANMAT 2001 Conference on the Interaction of Laser Radiation with Matter at Nanoscopic Scales: From Single Molecule Spectroscopy to Materials Processing, Venice, 3–6 October, 2001.

The nonlinear optical properties of quasi-monomolecular layers of polyDCHD-HS spun on silver-coated plates were measured using surface plasmon spectroscopy at 1064 nm and with ps pulses. A surprisingly large value of χ(3) was obtained (|χ(3)| > 10−17 m2 V−2). This value was related to the presence of the silver film, whose nanostructured surface provides an electromagnetic mechanism for the enhancement of the nonlinearity through local field effects. This point was confirmed by SERS experiments performed on-resonance (515 nm) and off-resonance (647 nm) on the same samples. It is suggested that the experimental data can be accounted for by the existence of an interaction between the π-conjugated system of the polymer chain and the silver surface.

Excitation of crystalline all–trans retinal under pressure

Polycrystalline all–trans retinal (ATR) has been excited at 647.1 nm under pressure up to 9 GPa at room temperature and the resulting Raman spectrum has been measured. The large majority of Raman bands in the 700–1700 cm−1 range have positive pressure coefficients, i.e. bonds stiffen upon compression. The Raman spectrum is recovered after the compression–decompression cycle indicating that the molecular unit does not undergo an irreversible transformation under our experimental conditions. Model density functional (DF) calculations with the B3-LYP exchange-correlation functional and the 6-31G* basis set have been performed on 1-cis,3-trans,5-trans,7-trans,9-trans,1,5,9-trimethyldecapentaenal (TMDPL) mimicking the polyenic moiety of ATR. By shortening the single and double CC bonds of TMDPL up to 0.0015 A it was possible to fit the frequency dependence on pressure of the most intense polyenic bands with single and double bond compressibilities ≈0.00292 and 0.00117 A/GPa, respectively. The trans → cis isomerization in the crystal state under pressure and in the presence of the 647.1 nm excitation light is suggested by the Raman spectrum in the fingerprint region. A simplified model is proposed for the occurrence of this process in the crystal phase.

The transient absorption of 1,3,5-tri-tert-butyl-pentalene

The transient absorption of 1,3,5-tri-tert-butyl-pentalene in cyclohexane solution has been measured at room temperature exciting at 340 nm with femtosecond pulses. Two relaxation processes have been determined, the first, very rapid, with time constant 650 fs and the second, occurring on a picosecond timescale, with estimated decay time 13 ps. The former is due to the vibronic relaxation from upper S3 levels to the S3 minimum, the latter to the decay from S3 to S0. This interpretation is in good agreement with results of ab initio calculations on pentalene and its alkyl derivatives, 1,3,5-tri-ethyl, 1,3,5-tri-sec-propyl and 1,3,5-tri-tert-butyl. MCSCF/CAS calculations, perturbatively corrected within the scheme of quasi-degenerate perturbation theory (QDPT), of vertical S0 → Sn excitation energies allow us to study the dependence of excitation energies on substitution. Calculated S3 → Sn and S4 → Sn absorption spectra of pentalene at the S0, S3 and S4 equilibrium geometries indicate that vibronic levels of S3 rather than of S4 are excited by the strong pump pulse with subsequent decay within this state.

Photochemical Reactivity of 1,6-Methano[10]annulene

1,6-Methano[10]annulene solutions in cyclohexane have been subjected to continuous and pulsed UV irradiation. Photolysis occurs in both cases, giving naphthalene as a minor and major product, respectively. The wavelength dependence of the reaction in solution indicates that the photochemical process occurs, exciting 1,6-methano[10]annulene in the second and third singlet electronic excited states. The reaction kinetics has been determined under pulsed irradiation. From the time dependence of concentrations, along with support of density functional theory calculations and early published data, two mechanisms are proposed for naphthalene production. Reaction steps such as direct migration of the bridging methylene of 1,6-methano[10]annulene to cyclohexane and 1,6-methano[10]annulene isomerization to benzotropilidene have been identified. The calculated energy diagrams relative to the ground and lowest excited states allow one to relate these steps to processes such as electrocyclic closure and sigmatropic shift. The norcaradienic form of 1,6-methano[10]annulene results in the critical species for methylene migration and the sigmatropic [1,5] shift. The present results and those arising from photolysis in the gas phase are good examples of the photochemical reactivity of 1,6-methano[10]annulene.

Nonlinear properties of nm-thick layers of polyDCHD-HS on silver by using Surface Plasmon Resonance and Surface Enhanced Raman Scattering

Surprisingly large intensity-dependent variations of the refractive index of nm-thick films of polyDCHD-HS on silver were observed with Surface Plasmon Spectroscopy. SERS experiments suggested that the effect must be related to interactions with the nano-structured surface of the metal.