Charge-Transfer-Type Fluorescence of 4-(1H-Pyrrol-1-yl)benzonitrile (PBN) and N-Phenylpyrrole (PP) in Cryogenic Matrixes:  Evidence for Direct Excitation of the CT Band

Abstract

The fluorescence spectrum of PBN in a neat argon matrix is excitation-wavelength-dependent: at short excitation wavelengths, it consists of dual emission assigned to a charge-transfer (CT) state and a much weaker band assigned to the locally excited (LE) state. The CT emission is broad and almost completely devoid of vibrational structure, whereas the LE band is characterized by vibrationally resolved emission. At long excitation wavelengths, only CT emission is observed, indicating that the CT state is populated directly by light absorption and not via the LE state. Comparison with jet-cooled spectra of the bare molecule allows the unambiguous assignment of the LE spectrum and the location of the 0,0 band. The matrix LE emission spectrum is blue-shifted with respect to that of the gas phase, showing that the dipole moment of the LE state is smaller than that of the ground state. The fluorescence spectrum of PBN in an argon matrix does not change appreciably when acetonitrile (AN) is added to the matrix, in contrast to the case of N-phenylpyrrol (PP) (Schweke, D.; Haas, Y. J. Phys. Chem. A 2003, 107, 9554), for which addition of AN results in the appearance of two well separated emission bands. The different photophysical behaviors of PP and PBN in an argon matrix (and in supersonic jets) are analyzed by a simple model that considers the restriction of large-amplitude motions in the matrix. The implications of these low-temperature studies for understanding the properties of these systems in liquid solution are discussed.

An anti-quinoid structure in dual fluorescence of fluozazene molecule and solvent effect of intramolecular charge transfer

“…2, the twist angle, \C2-N1-C6-C11, is negligibly small from both methods. The small twist angle implies that the first excited state of PP has almost planar geometry, which is consistent to the reported results by other authors [46,48,49]. The calculated structural parameters show that PP has also a Q-like geometry (listed in Fig.…”

Solvent Polarity Tunes the Barrier Height for Twisted Intramolecular Charge Transfer in N-Pyrrolobenzonitrile (PBN)

“…Dual fluorescence was also found in many other small D/A systems, e.g., phenyl pyrroles, − aryldisilanes , 4-(dimethylamino)­benzoic acid methyl ester (DMABME) and in the here investigated compound 4-(1H-pyrrol-1-yl)­benzonitrile (PBN) − ­(Figure , right) which is the pyrrol analogue of DMABN. PBN, in comparison to DMABN, exhibits a higher electron density at the donor moiety due to the pyrrol ring possessing six π-electrons.…”

“…On the contrary, symmetry equivalent bonds C 2′ –C 1′ and C 2 –C 3 and the central N–C 1 bond are shortened. Hence, 2A Min can be described as the quinoid structure, which was previously suggested. , …”

“…Hence, 2A Min can be described as the quinoid structure, which was previously suggested. 48,51 Unconstrained geometry optimization of PBN in the 2A (S 2 ) state at RICC2/cc-pVTZ level does not usually converge to a stationary point. In the course of the optimization the symmetry equivalent C 1′ −N bonds are elongated by about 0.02 Å, the central N−C 1 bond is shortened by about 0.02 Å (numbering scheme in Figure 1, right) and the pyrrol ring is tilted driving the system toward a surface crossing between the 2A (S 2 ) and 1B (S 1 ) states.…”

“…This should lead to an even more efficient ICT. Even though numerous experimental and theoretical studies of PBN in gas and liquid phase have been carried out to obtain deeper insights into the mechanism of dual fluorescence and its photochemistry, ,− there remain open questions concerning the involved photochemical processes and the nature of the ICT ,, as will be outlined below.…”