In this work we present the synthesis, time-resolved spectroscopic characterization and computati... more In this work we present the synthesis, time-resolved spectroscopic characterization and computational analysis of a bichromophore composed of two very well-known naturally occurring dyes: 7-hydroxycoumarin (umbelliferone) and 1,2-dihydroxyanthraquinone (alizarin). The umbelliferone donor (Dn) and alizarin acceptor (Ac) moieties are linked to a triazole ring viaσ bonds, providing a flexible structure. By measuring the fluorescence quantum yields and the ultrafast transient absorption spectra we demonstrate the high efficiency (∼85%) and the fast nature (∼1.5 ps) of the energy transfer in this compound. Quantum chemical calculations, within the density functional theory (DFT) approach, are used to characterize the electronic structure of the bichromophore (Bi) in the ground and excited states. We simulate the absorption and fluorescence spectra using the TD-DFT methods and the vertical gradient approach (VG), and include the solvent effects by adopting the conductor-like polarizable continuum model (CPCM). The calculated electronic structure suggests the occurrence of weak interactions between the electron densities of Dn and Ac in the excited state, indicating that the Förster-type transfer is the appropriate model for describing the energy transfer in this system. The average distance between Dn and Ac moieties calculated from the conformational analysis (12 Å) is in very good agreement with the value estimated from the Förster equation (∼11 Å). At the same time, the calculated rate constant for energy transfer, averaged over multiple conformations of the system (3.6 ps), is in reasonable agreement with the experimental value (1.6 ps) estimated by transient absorption spectroscopy. The agreement between experimental results and computational data leads us to conclude that the energy transfer in Bi is well described by the Förster mechanism.
The transient absorption spectra with sub-picosecond resolution from the S1 state of azulene in d... more The transient absorption spectra with sub-picosecond resolution from the S1 state of azulene in different solvents (acetonitrile, cyclohexane, methanol) have been measuredin the range 350-700 nm. The spectrum is characterised by the presence of a strong peak at 380 nm and a weaker one at 440 nm. Small differences can be observed due to solvent effects. The temporal behaviour of
A diagrammatic perturbative approach is adopted to derive the expression for the third-order pola... more A diagrammatic perturbative approach is adopted to derive the expression for the third-order polarization, p(8) which originates the anti-Stokes emission in a time-resolved CARS experiment. The various contributions to P(3) are calculated assuming that laser fields are off-resonance with respect to any electronic level of material system. The resonant part of the polarization consists of two terms, in which the roles of the pump and probe pulses at frequency oJ1 are exchanged. The global expression allows the direct calculation of the signal time profile in a transient CARS experiment once a model is assumed for the laser pulse shape.
ABSTRACT We report the molecular dynamics of several solvated ions present in aqueous ionic solut... more ABSTRACT We report the molecular dynamics of several solvated ions present in aqueous ionic solutions (HCl, HNO3, NaCl, and NaNO3) investigated by femtosecond time-resolved optical Kerr effect measurements. From the accurate fitting of decay curves, measured over 5-6 decades of intensity range, it has been possible to characterize (i) an instantaneous electronic response, (ii) a fast contribution of water, (iii) a rotational relaxation of the individual ions (tau(rot) similar to 2-3 ps), and (iv) a slow concentration-dependent decay connected to the direct ion-ion interactions. These results will be of interest to those seeking to measure electro-optic effects in aqueous solutions as these experiments give new information on the nature of the aqueous state.
Assigning the vibrational modes of molecules in the electronic excited state is often a difficult... more Assigning the vibrational modes of molecules in the electronic excited state is often a difficult task. Here we show that combining two nonlinear spectroscopic techniques, transient 2D exchange infrared spectroscopy (T2D-IR-EXSY) and femtosecond stimulated Raman spectroscopy (FSRS), the contribution of the C═C and C═O modes in the excited-state vibrational spectra of trans-β-apo-8'-carotenal can be unambiguously identified. The experimental results reported in this work confirm a previously proposed assignment based on quantum-chemical calculations and further strengthen the role of an excited state with charge-transfer character in the relaxation pathway of carbonyl carotenoids. On a more general ground, our results highlight the potentiality of nonlinear spectroscopic methods based on the combined use of visible and infrared pulses to correlate structural and electronic changes in photoexcited molecules.
Two-dimensional pump-probe infrared spectroscopy is utilized to study the structural proprieties ... more Two-dimensional pump-probe infrared spectroscopy is utilized to study the structural proprieties of Glycine-L-Alanine-Methylamide in D 2 O and DMSO solutions. Experimental results are compared to computational predictions. Preliminary calculations confirm the presence of a CO-CO coupling stronger in D 2 O than in DMSO as experimentally observed.
The spectrum of highly excited electronic states of pyrene in n-octane solution and the relaxatio... more The spectrum of highly excited electronic states of pyrene in n-octane solution and the relaxation processes in the SI singlet state are investigated by femtosecond transient absorption spectroscopy. Three different UV excitation wavelengths (360, 307, and 290 nm) are utilized to excite the molecule at the edge of the SI -SO transition and to higher S, -SO transitions. The buildup of SI population has been observed to be completed in the first 2 ps, with a time constant of 300-400 fs, depending on the excitation wavelength. The evolution of the absorption spectra on a larger time scale is attributed to the vibrational cooling in the SI state; its rate corresponds to an exponential relaxation with time constant of about 4 ps. The evidence of the formation of pyrene excimer at long time delays in highly concentrated solutions is reported.
The aim of this paper is to provide the main pieces of information concerning the application of ... more The aim of this paper is to provide the main pieces of information concerning the application of transient absorption (TA) spectroscopy with sub-picosecond laser pulses. A description of the experimental apparatus and of some detection schemes are included together with the most common mathematical formulas utilized to analyze the signals. The results, recently obtained in our laboratory and presented here, concern the investigation of the excited state dynamics of simple molecular systems. Examples of the measurements of the relaxation processes occurring in the lowest excited states of some aromatic molecules will be discussed in order to show the potentiality of the technique.
Highly excited electronic states of pyrene in ethanol solution and the relaxation processes in th... more Highly excited electronic states of pyrene in ethanol solution and the relaxation processes in the S1 singlet state are investigated by femtosecond transient absorption spectroscopy. UV excitation at 290 nm is utilized to excite the molecule in some vibronic level of S6 state. From this state internal conversion (IC) takes place and the build-up of S1 population has been observed
The exciton generation and recombination dynamics in semiconductor nanocrystals are very sensitiv... more The exciton generation and recombination dynamics in semiconductor nanocrystals are very sensitive to small variations in dimensions, shape and surface capping. In the present work CdTe quantum dots are synthesized in water using 3-mercaptopropionic acid and 1-thioglycerol as stabilizers. Nanocrystals with an average dimension of 4.0 ± 1.0 and 3.7 ± 0.9 nm were obtained, when 3-mercaptopropionic acid or 1-thioglycerol, respectively, was used as a capping agent. The steady-state characterization shows that the two types of colloids have different luminescence behavior. In order to investigate the electronic structure and the dynamics of the exciton state, a combined study in the time domain has been carried out by using fluorescence time-correlated single photon counting and femtosecond transient absorption techniques. The electron-hole radiative recombination follows the non-exponential decay law for both colloids, which results in different average decay time values (of the order of tens of nanoseconds) for the two samples. The data demonstrate that the process is slower for 1-thioglycerol-stabilized colloids.
The relaxation dynamics of unsubstituted porphyrin (H2P), diprotonated porphyrin (H4P2+), and tet... more 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).
The fluorescence spectra of unsubstituted porphyrin (H2P), diprotonated porphyrin (H4P2+), and is... more The fluorescence spectra of unsubstituted porphyrin (H2P), diprotonated porphyrin (H4P2+), and isoelectronic tetraoxaporphyrin dication (TOxP2+) have been measured in solution at room temperature. The S2-->S0 fluorescence has been observed, much more intense for TOxP2+ than for H4P2+ and H2P. In the TOxP2+ case, the S2-->S0 fluorescence spectrum is remarkably sharp and shows an excellent mirror symmetry with respect to S0-->S2 absorption. On the contrary, the spectra of H4P2+ and H2P are shifted and more extended with respect to the absorption counterparts. The differences have been attributed primarily to the change of the equilibrium geometry upon excitation, larger in H2P and H4P2+ than in TOxP2+ and in the case of H4P2+ to the nonplanar conformation of the macrocycle. Also the S1-->S0 spectra of H2P, H4P2+, and TOxP2+ have been measured and more qualitatively discussed. The S1 and S2 fluorescence decays have been observed for H4P2+ and TOxP2+ exciting with ultrashort pulses. The S2 lifetime of TOxP2+ is of the order of the temporal resolution of our experimental apparatus, whereas that of H4P2+ is shorter. The S2-->S0 quantum yield of TOxP2+ has been estimated to be 0.035, approximately 3 orders of magnitude higher than that of H4P2+. It is proposed on the basis of ab initio model calculations that excited states of the H4P2+(CF3COO-)2 complex with charge-transfer character are responsible of the increased extension of the S2-->S0 spectrum with respect to that of H2P.
The photochemical reactivity of colchicine and thiocolchicine is described. Although the irradiat... more The photochemical reactivity of colchicine and thiocolchicine is described. Although the irradiation of colchicine gave a well-known transposition reaction to -and γ-lumicolchicines, thiocolchicine did not react. Femtosecond transient spectroscopy of colchicine showed a strong band with maximum at 510 nm appearing at τ ) 0. It disappeared within few hundred femtoseconds, leaving a broad structureless band with a maximum around 470 nm. A second band is observed around 410 nm. The analysis in time showed that the 510-nm component appeared instantaneously and decayed following a biexponential low with time constants of 300 ( 100 fs and 40 ps. The kinetics at 420 nm has a measurable rise time of 300 ( 150 fs. Quantum mechanical calculations on colchicine showed that this absorption is due to a S 1 f S 11 transition. In thiocolchicine, the instantaneous formation of a structure with maxima out of the investigated spectral region was observed. A strong absorption around 650 nm indicated the presence of a band with a maximum at longer wavelengths (> 700 nm) and a peak around 380 nm, which partially coincides with the ground-state absorption and therefore strongly affected by its bleaching. The instantaneous formation of an absorption around 650 nm and its rapid (∼500 fs) decay was observed. At shorter wavelengths (400 nm), the decay was fitted with a biexponential curve with the first time constant of about 80 ps. The second part of the decay had a very long tail up to 500 ps. Transient spectroscopy and configuration interaction calculations are in agreement with a mechanism involving a disrotatory cyclization of colchicine in its first excited singlet state. The lack of reactivity observed in thiocolchicine was explained by considering the presence of efficient ISC to the triplet state.
Intramolecular electron transfer in the excited state of a [(bpy)Re I (CO) 3 (L)] + complex (bpy ... more Intramolecular electron transfer in the excited state of a [(bpy)Re I (CO) 3 (L)] + complex (bpy ) 2,2′-bipyridine), in which L contains a pendant azacrown ether that acts as an electron donor (L ) N- [4-(4,7,10,13-tetraoxa-1-azacyclopentadecyl)benzoyl]-4-aminopyridine), has been studied directly using picosecond and nanosecond time-resolved UV-visible absorption spectroscopy. Picosecond studies show that the metal-to-ligand chargetransfer (MLCT) state produced on excitation, [(bpy •-)Re II (CO) 3 (L)] + , undergoes forward electron transfer with a rate constant of k FET ) 2.0 × 10 9 s -1 to generate a ligand-to-ligand charge-transfer (LLCT) state, [(bpy •-)Re I (CO) 3 (L •+ )] + , in which the metal has been reduced back to Re(I) and charge separation has been effected between the bipyridine and azacrown ligands. Nanosecond studies show that the LLCT state returns to the ground state by back electron transfer from the bipyridine to azacrown ligand, with a rate constant of k BET ) 5.3 × 10 7 s -1 . Studies of complexes in which the azacrown complex is protonated, or is absent, demonstrate that intramolecular electron transfer to form the LLCT state does not occur in these cases. Forward electron transfer in the azacrown complex takes place on the picosecond time scale: it is weakly exoergonic and occurs in the Marcus normal region, with electronic coupling between the azacrown ligand and the rhenium metal center of ca. 100 cm -1 . Back electron transfer takes place on the nanosecond time scale: it is strongly exoergonic and occurs in the Marcus inverted region, with much weaker electronic coupling between the bipyridine and azacrown ligands. The rapid formation of a long-lived charge-separated state indicates that this molecule has a suitable design for a photochemical device.
The transient spectra of azulene in solution have been measured in the spectral region 600-350 nm... more The transient spectra of azulene in solution have been measured in the spectral region 600-350 nm at room temperature, pumping into the S 1 and S 2 states with femtosecond pulses and probing with a delayed femtosecond white light continumm. The spectra are a combination of ground-state bleaching, stimulated emission, and excited-state absorption. The latter component gives a direct information on excited states which may be not active in the ground-state absorption. S 1 f S n and S 2 f S n absorptions have been discussed with the help of ab initio calculations of the MCSCF/CAS type with the 6-31G* basis set and including perturbative corrections. The calculated S 0 f S n , S 1,eq f S n , and S 2,eq f S n vertical excitation energies and oscillator strengths are in satisfactory agreement with the experimental results. It has been found that electronic states, weakly active in the ground-state absorption, occur with high intensity in the femtosecond transient spectra, in particular in the energy range 36000-44000 cm -1 above the ground-state energy.
Proceedings of the National Academy of Sciences, 2008
High-pressure methods are increasingly used to produce new dense materials with unusual propertie... more High-pressure methods are increasingly used to produce new dense materials with unusual properties. Increasing efforts to understand the reaction mechanisms at the microscopic level, to set up and optimize synthetic approaches, are currently directed at carbon-based solids. A fundamental, but still unsolved, question concerns how the electronic excited states are involved in the high-pressure reactivity of molecular systems. Technical difficulties in such experiments include small sample dimensions and possible damage to the sample as a result of the absorption of intense laser fields. These experimental challenges make the direct characterization of the electronic properties as a function of pressure by linear and nonlinear optical spectroscopies up to several GPa a hard task. We report here the measurement of two-photon excitation spectra in a molecular crystal under pressure, up to 12 GPa in benzene, the archetypal aromatic system. Comparison between the pressure shift of the exciton line and the monomer fluorescence provides evidence for different compressibilities of the ground and first excited states. The formation of structural excimers occurs with increasing pressure involving molecules on equivalent crystal sites that are favorably arranged in a parallel configuration. These species represent the nucleation sites for the transformation of benzene into amorphous hydrogenated carbon. The present results provide a unified picture of the chemical reactivity of benzene at high pressure.
In this work we present the synthesis, time-resolved spectroscopic characterization and computati... more In this work we present the synthesis, time-resolved spectroscopic characterization and computational analysis of a bichromophore composed of two very well-known naturally occurring dyes: 7-hydroxycoumarin (umbelliferone) and 1,2-dihydroxyanthraquinone (alizarin). The umbelliferone donor (Dn) and alizarin acceptor (Ac) moieties are linked to a triazole ring viaσ bonds, providing a flexible structure. By measuring the fluorescence quantum yields and the ultrafast transient absorption spectra we demonstrate the high efficiency (∼85%) and the fast nature (∼1.5 ps) of the energy transfer in this compound. Quantum chemical calculations, within the density functional theory (DFT) approach, are used to characterize the electronic structure of the bichromophore (Bi) in the ground and excited states. We simulate the absorption and fluorescence spectra using the TD-DFT methods and the vertical gradient approach (VG), and include the solvent effects by adopting the conductor-like polarizable continuum model (CPCM). The calculated electronic structure suggests the occurrence of weak interactions between the electron densities of Dn and Ac in the excited state, indicating that the Förster-type transfer is the appropriate model for describing the energy transfer in this system. The average distance between Dn and Ac moieties calculated from the conformational analysis (12 Å) is in very good agreement with the value estimated from the Förster equation (∼11 Å). At the same time, the calculated rate constant for energy transfer, averaged over multiple conformations of the system (3.6 ps), is in reasonable agreement with the experimental value (1.6 ps) estimated by transient absorption spectroscopy. The agreement between experimental results and computational data leads us to conclude that the energy transfer in Bi is well described by the Förster mechanism.
The transient absorption spectra with sub-picosecond resolution from the S1 state of azulene in d... more The transient absorption spectra with sub-picosecond resolution from the S1 state of azulene in different solvents (acetonitrile, cyclohexane, methanol) have been measuredin the range 350-700 nm. The spectrum is characterised by the presence of a strong peak at 380 nm and a weaker one at 440 nm. Small differences can be observed due to solvent effects. The temporal behaviour of
A diagrammatic perturbative approach is adopted to derive the expression for the third-order pola... more A diagrammatic perturbative approach is adopted to derive the expression for the third-order polarization, p(8) which originates the anti-Stokes emission in a time-resolved CARS experiment. The various contributions to P(3) are calculated assuming that laser fields are off-resonance with respect to any electronic level of material system. The resonant part of the polarization consists of two terms, in which the roles of the pump and probe pulses at frequency oJ1 are exchanged. The global expression allows the direct calculation of the signal time profile in a transient CARS experiment once a model is assumed for the laser pulse shape.
ABSTRACT We report the molecular dynamics of several solvated ions present in aqueous ionic solut... more ABSTRACT We report the molecular dynamics of several solvated ions present in aqueous ionic solutions (HCl, HNO3, NaCl, and NaNO3) investigated by femtosecond time-resolved optical Kerr effect measurements. From the accurate fitting of decay curves, measured over 5-6 decades of intensity range, it has been possible to characterize (i) an instantaneous electronic response, (ii) a fast contribution of water, (iii) a rotational relaxation of the individual ions (tau(rot) similar to 2-3 ps), and (iv) a slow concentration-dependent decay connected to the direct ion-ion interactions. These results will be of interest to those seeking to measure electro-optic effects in aqueous solutions as these experiments give new information on the nature of the aqueous state.
Assigning the vibrational modes of molecules in the electronic excited state is often a difficult... more Assigning the vibrational modes of molecules in the electronic excited state is often a difficult task. Here we show that combining two nonlinear spectroscopic techniques, transient 2D exchange infrared spectroscopy (T2D-IR-EXSY) and femtosecond stimulated Raman spectroscopy (FSRS), the contribution of the C═C and C═O modes in the excited-state vibrational spectra of trans-β-apo-8'-carotenal can be unambiguously identified. The experimental results reported in this work confirm a previously proposed assignment based on quantum-chemical calculations and further strengthen the role of an excited state with charge-transfer character in the relaxation pathway of carbonyl carotenoids. On a more general ground, our results highlight the potentiality of nonlinear spectroscopic methods based on the combined use of visible and infrared pulses to correlate structural and electronic changes in photoexcited molecules.
Two-dimensional pump-probe infrared spectroscopy is utilized to study the structural proprieties ... more Two-dimensional pump-probe infrared spectroscopy is utilized to study the structural proprieties of Glycine-L-Alanine-Methylamide in D 2 O and DMSO solutions. Experimental results are compared to computational predictions. Preliminary calculations confirm the presence of a CO-CO coupling stronger in D 2 O than in DMSO as experimentally observed.
The spectrum of highly excited electronic states of pyrene in n-octane solution and the relaxatio... more The spectrum of highly excited electronic states of pyrene in n-octane solution and the relaxation processes in the SI singlet state are investigated by femtosecond transient absorption spectroscopy. Three different UV excitation wavelengths (360, 307, and 290 nm) are utilized to excite the molecule at the edge of the SI -SO transition and to higher S, -SO transitions. The buildup of SI population has been observed to be completed in the first 2 ps, with a time constant of 300-400 fs, depending on the excitation wavelength. The evolution of the absorption spectra on a larger time scale is attributed to the vibrational cooling in the SI state; its rate corresponds to an exponential relaxation with time constant of about 4 ps. The evidence of the formation of pyrene excimer at long time delays in highly concentrated solutions is reported.
The aim of this paper is to provide the main pieces of information concerning the application of ... more The aim of this paper is to provide the main pieces of information concerning the application of transient absorption (TA) spectroscopy with sub-picosecond laser pulses. A description of the experimental apparatus and of some detection schemes are included together with the most common mathematical formulas utilized to analyze the signals. The results, recently obtained in our laboratory and presented here, concern the investigation of the excited state dynamics of simple molecular systems. Examples of the measurements of the relaxation processes occurring in the lowest excited states of some aromatic molecules will be discussed in order to show the potentiality of the technique.
Highly excited electronic states of pyrene in ethanol solution and the relaxation processes in th... more Highly excited electronic states of pyrene in ethanol solution and the relaxation processes in the S1 singlet state are investigated by femtosecond transient absorption spectroscopy. UV excitation at 290 nm is utilized to excite the molecule in some vibronic level of S6 state. From this state internal conversion (IC) takes place and the build-up of S1 population has been observed
The exciton generation and recombination dynamics in semiconductor nanocrystals are very sensitiv... more The exciton generation and recombination dynamics in semiconductor nanocrystals are very sensitive to small variations in dimensions, shape and surface capping. In the present work CdTe quantum dots are synthesized in water using 3-mercaptopropionic acid and 1-thioglycerol as stabilizers. Nanocrystals with an average dimension of 4.0 ± 1.0 and 3.7 ± 0.9 nm were obtained, when 3-mercaptopropionic acid or 1-thioglycerol, respectively, was used as a capping agent. The steady-state characterization shows that the two types of colloids have different luminescence behavior. In order to investigate the electronic structure and the dynamics of the exciton state, a combined study in the time domain has been carried out by using fluorescence time-correlated single photon counting and femtosecond transient absorption techniques. The electron-hole radiative recombination follows the non-exponential decay law for both colloids, which results in different average decay time values (of the order of tens of nanoseconds) for the two samples. The data demonstrate that the process is slower for 1-thioglycerol-stabilized colloids.
The relaxation dynamics of unsubstituted porphyrin (H2P), diprotonated porphyrin (H4P2+), and tet... more 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).
The fluorescence spectra of unsubstituted porphyrin (H2P), diprotonated porphyrin (H4P2+), and is... more The fluorescence spectra of unsubstituted porphyrin (H2P), diprotonated porphyrin (H4P2+), and isoelectronic tetraoxaporphyrin dication (TOxP2+) have been measured in solution at room temperature. The S2-->S0 fluorescence has been observed, much more intense for TOxP2+ than for H4P2+ and H2P. In the TOxP2+ case, the S2-->S0 fluorescence spectrum is remarkably sharp and shows an excellent mirror symmetry with respect to S0-->S2 absorption. On the contrary, the spectra of H4P2+ and H2P are shifted and more extended with respect to the absorption counterparts. The differences have been attributed primarily to the change of the equilibrium geometry upon excitation, larger in H2P and H4P2+ than in TOxP2+ and in the case of H4P2+ to the nonplanar conformation of the macrocycle. Also the S1-->S0 spectra of H2P, H4P2+, and TOxP2+ have been measured and more qualitatively discussed. The S1 and S2 fluorescence decays have been observed for H4P2+ and TOxP2+ exciting with ultrashort pulses. The S2 lifetime of TOxP2+ is of the order of the temporal resolution of our experimental apparatus, whereas that of H4P2+ is shorter. The S2-->S0 quantum yield of TOxP2+ has been estimated to be 0.035, approximately 3 orders of magnitude higher than that of H4P2+. It is proposed on the basis of ab initio model calculations that excited states of the H4P2+(CF3COO-)2 complex with charge-transfer character are responsible of the increased extension of the S2-->S0 spectrum with respect to that of H2P.
The photochemical reactivity of colchicine and thiocolchicine is described. Although the irradiat... more The photochemical reactivity of colchicine and thiocolchicine is described. Although the irradiation of colchicine gave a well-known transposition reaction to -and γ-lumicolchicines, thiocolchicine did not react. Femtosecond transient spectroscopy of colchicine showed a strong band with maximum at 510 nm appearing at τ ) 0. It disappeared within few hundred femtoseconds, leaving a broad structureless band with a maximum around 470 nm. A second band is observed around 410 nm. The analysis in time showed that the 510-nm component appeared instantaneously and decayed following a biexponential low with time constants of 300 ( 100 fs and 40 ps. The kinetics at 420 nm has a measurable rise time of 300 ( 150 fs. Quantum mechanical calculations on colchicine showed that this absorption is due to a S 1 f S 11 transition. In thiocolchicine, the instantaneous formation of a structure with maxima out of the investigated spectral region was observed. A strong absorption around 650 nm indicated the presence of a band with a maximum at longer wavelengths (> 700 nm) and a peak around 380 nm, which partially coincides with the ground-state absorption and therefore strongly affected by its bleaching. The instantaneous formation of an absorption around 650 nm and its rapid (∼500 fs) decay was observed. At shorter wavelengths (400 nm), the decay was fitted with a biexponential curve with the first time constant of about 80 ps. The second part of the decay had a very long tail up to 500 ps. Transient spectroscopy and configuration interaction calculations are in agreement with a mechanism involving a disrotatory cyclization of colchicine in its first excited singlet state. The lack of reactivity observed in thiocolchicine was explained by considering the presence of efficient ISC to the triplet state.
Intramolecular electron transfer in the excited state of a [(bpy)Re I (CO) 3 (L)] + complex (bpy ... more Intramolecular electron transfer in the excited state of a [(bpy)Re I (CO) 3 (L)] + complex (bpy ) 2,2′-bipyridine), in which L contains a pendant azacrown ether that acts as an electron donor (L ) N- [4-(4,7,10,13-tetraoxa-1-azacyclopentadecyl)benzoyl]-4-aminopyridine), has been studied directly using picosecond and nanosecond time-resolved UV-visible absorption spectroscopy. Picosecond studies show that the metal-to-ligand chargetransfer (MLCT) state produced on excitation, [(bpy •-)Re II (CO) 3 (L)] + , undergoes forward electron transfer with a rate constant of k FET ) 2.0 × 10 9 s -1 to generate a ligand-to-ligand charge-transfer (LLCT) state, [(bpy •-)Re I (CO) 3 (L •+ )] + , in which the metal has been reduced back to Re(I) and charge separation has been effected between the bipyridine and azacrown ligands. Nanosecond studies show that the LLCT state returns to the ground state by back electron transfer from the bipyridine to azacrown ligand, with a rate constant of k BET ) 5.3 × 10 7 s -1 . Studies of complexes in which the azacrown complex is protonated, or is absent, demonstrate that intramolecular electron transfer to form the LLCT state does not occur in these cases. Forward electron transfer in the azacrown complex takes place on the picosecond time scale: it is weakly exoergonic and occurs in the Marcus normal region, with electronic coupling between the azacrown ligand and the rhenium metal center of ca. 100 cm -1 . Back electron transfer takes place on the nanosecond time scale: it is strongly exoergonic and occurs in the Marcus inverted region, with much weaker electronic coupling between the bipyridine and azacrown ligands. The rapid formation of a long-lived charge-separated state indicates that this molecule has a suitable design for a photochemical device.
The transient spectra of azulene in solution have been measured in the spectral region 600-350 nm... more The transient spectra of azulene in solution have been measured in the spectral region 600-350 nm at room temperature, pumping into the S 1 and S 2 states with femtosecond pulses and probing with a delayed femtosecond white light continumm. The spectra are a combination of ground-state bleaching, stimulated emission, and excited-state absorption. The latter component gives a direct information on excited states which may be not active in the ground-state absorption. S 1 f S n and S 2 f S n absorptions have been discussed with the help of ab initio calculations of the MCSCF/CAS type with the 6-31G* basis set and including perturbative corrections. The calculated S 0 f S n , S 1,eq f S n , and S 2,eq f S n vertical excitation energies and oscillator strengths are in satisfactory agreement with the experimental results. It has been found that electronic states, weakly active in the ground-state absorption, occur with high intensity in the femtosecond transient spectra, in particular in the energy range 36000-44000 cm -1 above the ground-state energy.
Proceedings of the National Academy of Sciences, 2008
High-pressure methods are increasingly used to produce new dense materials with unusual propertie... more High-pressure methods are increasingly used to produce new dense materials with unusual properties. Increasing efforts to understand the reaction mechanisms at the microscopic level, to set up and optimize synthetic approaches, are currently directed at carbon-based solids. A fundamental, but still unsolved, question concerns how the electronic excited states are involved in the high-pressure reactivity of molecular systems. Technical difficulties in such experiments include small sample dimensions and possible damage to the sample as a result of the absorption of intense laser fields. These experimental challenges make the direct characterization of the electronic properties as a function of pressure by linear and nonlinear optical spectroscopies up to several GPa a hard task. We report here the measurement of two-photon excitation spectra in a molecular crystal under pressure, up to 12 GPa in benzene, the archetypal aromatic system. Comparison between the pressure shift of the exciton line and the monomer fluorescence provides evidence for different compressibilities of the ground and first excited states. The formation of structural excimers occurs with increasing pressure involving molecules on equivalent crystal sites that are favorably arranged in a parallel configuration. These species represent the nucleation sites for the transformation of benzene into amorphous hydrogenated carbon. The present results provide a unified picture of the chemical reactivity of benzene at high pressure.
Uploads
Papers by Paolo Foggi