The photophysics of aminoperylene (APe) in various solvents, including a room-temperature ionic liquid, has been investigated by steady-state and femtosecond transient absorption spectroscopies. The ultrafast excited-state dynamics originates from the solvation of the polar S₁ state and not from a transition from a locally-excited to a charge-transfer state, as found with perylene-dimethylaniline. Addition of acid yields the protonated form APeH⁺, which exhibits similar photophysical properties than perylene, due to the suppression of the charge-transfer character of the S₀–S₁ transition. However, excited-state proton transfer, resulting to the formation of APe in the S₁ state, is observed in methanol.

We present results of a femtosecond spectroscopy study of the ring-opening dynamics of the photochromiccompound trimethyl-1′H-spiro[fluorene-9,1′-pyrrolo[1,2-b]pyridazines]-2′,3′,6′-tricarboxylate (also known asdihydroindolizine and abbreviated as DHI) in solvents of different polarities. We follow the ring-openingdynamics of photoexcited DHI by probing the transient response in the visible region between 450 and 700nm, as well as in the fingerprint region between 1100 and 1800 cm^-1. We conclude that photoexcited DHIconverts into the ring-opened betaine isomer while remaining in the electronic excited state. Subsequentelectronic excited-state decay on a time scale of 40-80 ps results in regeneration of ground-state DHI (0.75-0.9quantum yield) or betaine photoproduct, the exact value for DHI quantum yield recoveries and rates beingsolvent dependent. Figure Steady state of DHI in ACN-d₃, DCM-d₂, and TCE (A).Transient spectra of DHI at different pulse delays after 400 nm laserexcitation in ACN-d₃ (B), in DCM-d₂ (C), and in TCE (D).

Polarization-sensitive ultrafast infrared measurements on photoinduced electron transfer in donor-acceptor pairs in polar acetonitrile show distinct contributions from loose and tight ion pairs. Highly anisotropic signals from tight ion pairs reveal the importance of mutual orientation of the reactants (see picture) and thus the need to refine theoretical models based on spherical species that solely involve reaction distances.

The excited-state dynamics of the methylperylene/tetracyanoethylene (MPe/TCNE) donor−acceptor complex has been investigated in various solvents using femtosecond transient absorption spectroscopy. The transient spectra reveal the formation of two types of ion pairs: The first (IP1), constituting the major fraction of the total ion-pair population, is characterized by a broad and red-shifted absorption spectrum compared to that of the free MPe cation and by a subpicosecond lifetime, whereas the second (IP2) has a spectrum closer to that of MPe cation and a lifetime of a few picoseconds. A substantial polarization anisotropy was observed with IP1 but not with IP2, indicating a relatively well-defined structure for the former. The reaction scheme that best accounts for the observed dynamics and its solvent dependence involves the simultaneous excitation of complexes that differ by their electronic coupling. The more coupled complexes have a high absorption coefficient and thus yield IP1, which undergoes ultrafast charge recombination, whereas the less coupled complexes have a lower probability to be excited and lead to the longer-lived IP2.

The photophysics and excited-state dynamics of nitroperylene (NPe) in solvents of various polarities and viscosities, including a room-temperature ionic liquid, have been investigated by femtosecond-resolved transient absorption spectroscopy. The excited-state absorption spectrum was found to depend substantially on solvent polarity. In the most polar solvents, it is very similar to that of the NPe radical cation generated upon bimolecular quenching by an electron acceptor, denoting a substantial charge-transfer character of the S₁ state. Contrary to smaller nitroaromatic compounds, NPe in the S₁ state does not undergo ultrafast intersystem crossing (ISC) but decays mainly by internal conversion (IC). In nonprotic solvents, IC involves low-frequency modes with large amplitude motion associated with the nitro group and depends on both the solvent viscosity and polarity. It takes place on a 100 ps time scale in acetonitrile, while in cyclohexane, it is slow enough for ISC to become competitive. Moreover, both the fluorescence quantum yield and the excited-state dynamics were found to differ, depending on which side of the S₀−S₁ absorption band excitation was performed. This dependence is explained by the inhomogeneous nature of the absorption spectrum arising from a distribution of twist angles of the nitro group relative to the aromatic plane. On the other hand, such excitation wavelength effects were not observed in protic solvents, where the excited-state lifetime was found to be substantially shorter than that in nonprotic solvents. This behavior is rationalized in terms of a H-bonding interaction, which limits the torsional disorder of NPe and favors ultrafast nonradiative deactivation of the excited state. Transient absorption measurements performed for comparative purpose with nitropyrene in acetonitrile confirm the occurrence of ultrafast ISC in smaller nitroaromatic compounds.

Ultrafast infrared transient absorption spectroscopy is used to study the photoinduced bimolecular electron transfer reaction between perylene in the first singlet excited state and 1,4-dicyanobenzene in acetonitrile and dichloromethane. Following vibrational marker modes on both donor and acceptor sides in real time provides direct insight into the structural dynamics during the reaction. A band narrowing on a time scale of a few tens of picoseconds observed on the antisymmetric CN stretching vibration of the dicyanobenzene radical anion indicates that a substantial part of the excess energy is channeled into vibrational modes of the product, despite the fact that the reaction is weakly exergonic. An additional narrowing of the same band on a time scale of several hundreds of picoseconds observed in acetonitrile only is interpreted as a signature of the dissociation of the geminate ion pairs into free ions.