One- to Two-Exciton Transitions in Perylene Bisimide Dimer Revealed by Two-Dimensional Electronic Spectroscopy

The excited-state energy levels of molecular dimers and aggregates play a critical role in their photophysical behavior and an understanding of the photodynamics in such structures is important for developing applications such as photovoltaics and optoelectronic devices. Here, exciton transitions in two different covalently bound PBI dimers are studied by two-dimensional electronic spectroscopy (2DES), a powerful spectroscopic method, providing the most complete picture of vibronic transitions in molecular systems. The data are accurately reproduced using the equation of motion-phase matching approach. The unambiguous presence of one-exciton to two-exciton transitions are captured in our results and described in terms of a molecular exciton energy level scheme based on the Kasha model. Furthermore, the results are supported by comparative measurements with the PBI monomer and another dimer in which the interchromophore distance is increased

Time-Resolved Structural Dynamics of Extended π-Electron Porphyrin Nanoring

Molecular structure design inspired by naturally occurring light harvesting systems has been intensely pursued over the last couple of decades. Interesting new structures include the π- conjugated porphyrin nanorings, which show promising features such as ultrafast excited-state delocalization, leading to suppressed radiative rates, superradiance with increasing temperature and energy transfer times comparable to their natural counterparts. An mportant question to be addressed in such systems is the role and time scale of structural motions and how they affect excited-state delocalization. Here it is shown that porphyrin nanorings which are not rigidified by a template are structurally heterogeneous in the ground state and evolve dynamically on a tens of picoseconds timescale. In the excited state a structural relaxation of the porphyrin nanorings is observed, on a picosecond timescale. Furthermore ultrafast excitation delocalization is observed, by anisotropy measurements, being insensitive to structural motions of the nanorings

Exciton-Exciton Annihilation as a Probe of Exciton Diffusion in Large Porphyrin Nanorings

The photophysical behavior of natural and artificial cyclic supramolecular structures has been intensively investigated in the past decade. Among the artificial structures, large fully π-conjugated porphyrin nanorings are of particular interest because of their electronic, structural, and topological features, which make them suitable candidates for light-harvesting applications. A number of factors affect the efficiency with which such structures harvest and transmit energy. For instance, under intense irradiation conditions, the efficiency of light harvesting can be quenched by competing processes, such as exciton–exciton annihilation. Here, we investigate the pump fluence dependence of the transient absorption spectra of a series of porphyrin nanorings ranging in circumference between 13 and 52 nm (10–40 porphyrin units). This allowed the isolation of a fluence-dependent fast-decaying component in all but the smallest nanorings studied. This effect has been assigned to exciton–exciton annihilation and fit to a one-dimensional exciton diffusion model, which confirms that the exciton size and/or its mobility are inversely proportional to the nanoring size

Exciton Dynamics in Synthetic Multi-Chromophoric Model Systems

Investigating the excitonic properties of synthetic multichromophoric model systems can give insights into the behaviour of larger and more intricate structures, such as the photosynthetic complexes found in autotrophs or materials with applications in the area of organic photovoltaics (OPV). The unique properites of excitons depend critically on the electronic excited states of these systems, which present non-local character, and have short lifetimes. Hence, in order to characterise their dynamics it is helpful to employ laser spectroscopic techniques with ultrafast time resolution. Among these, the most widespread is broadband femtosecond transient absorption (fsTA), a two-pulse technique which has the drawback of being intrinsically ambiguous on the excitation frequency. A way to overcome this disadvantage is presented by two-dimensional electronic spectroscopy (2D-ES). In 2D-ES, the introduction of a third pulse allows the recovery of spectra in which excitation and detection frequencies are correlated on a two-dimensional surface. 2D-ES and fsTA have been used in a complementary fashion throughout this thesis in order to investigate photophysical processes in a range of different synthetic multichromophoric model systems. Experiments have been performed on a series of covalently-bound perylene bisimide (PBI) J-dimers. Here 2D-ES allowed us to identify a one- to two-exciton state transition in the strongly coupled dimer, which vanishes in the monomer or when the coupling is weakened. Such a transition is purely electronic in character, as confirmed by the calculated spectra, and its energy allowed us to estimate the excitonic coupling strength. We further report fsTA and 2D-ES studies of a subphthalocyanine-Zn porphyrin (SubPc-O-ZnTPP) heterodimer. fsTA allowed us to characterise the excitation energy transfer (EET) between the SubPc and ZnTPP moieties, which is well reproduced by the Forster model, while 2D-ES was used to observe sub-ps spectral diffusion, which is shown to be too fast to influence the incoherent EET. These studies were extended to larger systems. fsTA has been performed on a range of fully-conjugated porphyrin nanorings. fsTA and transient anisotropy on six-membered rings, with or without an inner template, revealed structural dynamics in the ground and in the excited states of the untemplated structure, which do not disrupt the exciton delocalisation. Finally, fsTA at increasing pump fluences allowed us to study exciton-exciton annihilation (EEA) dynamics in nanorings made up of 10, 20, 30 and 40 porphyrin units. Experiments confirmed that the exciton size is approximately 20 repeating units, and comparison with a one-dimensional diffusion model allowed estimation of the exciton diffusion coefficients, which decrease as the ring size increases, a result assigned to the increased static disorder experienced in larger structures

Diels-Alder Cycloaddition to the Bay region of perylene and its derivatives as an attractive strategyfor PAH core expansion: theoreticaland practical aspects

PAHs (polycyclic aromatics hydrocarbons), the compound group that contains perylene and its derivatives, including functionalized ones, have attracted a great deal of interest in many fields of science and modern technology. This review presents all of the research devoted to modifications of PAHs that are realized via the Diels–Alder (DA) cycloaddition of various dienophiles to the bay regions of PAHs, leading to the π-extension of the starting molecule. This type of annulative π-extension (APEX) strategy has emerged as a powerful and efficient synthetic method for the construction of polycyclic aromatic hydrocarbons and their functionalized derivatives, nanographenes, and π-extended fused heteroarenes. Then, [4 + 2] cycloadditions of ethylenic dienophiles, -N=N-, i.e., diazo-dienophiles and acetylenic dienophiles, are presented. This subject is discussed from the organic synthesis point of view but supported by theoretical calculations. The possible applications of DA cycloaddition to PAH bay regions in various science and technology areas, and the prospects for the development of this synthetic method, are also discussed