Symmetry- and Solvent-Dependent Photophysics of Fluorenes Containing Donor and Acceptor Groups

Abstract

Three two-photon absorption (2PA) dyes (donor-π-donor (DPA2F), donor-π-acceptor (AF240), and acceptor-π-acceptor (BT2F); specifically, D is Ph2N-, A is 2-benzothiazoyl, and the π-linker is 9,9-diethylfluorene) are examined in a variety of aprotic solvents. Because the 2PA cross section is sensitive to the polarity of the local environment, this report examines the solvent-dependent linear photophysics of the dyes, which are important to understand before probing more complex solid-state systems. The symmetrical dyes show little solvent dependence; however, AF240 has significant solvatochromism observed in the fluorescence spectra and lifetimes and also the transient absorption spectra. A 114 nm bathochromic shift is observed in the fluorescence maximum when going from n-hexane to acetonitrile, whereas the lifetimes increase from 1.25 to 3.12 ns. The excited-state dipole moment for AF240 is found to be 20.1 D using the Lippert equation, with smaller values observed for the symmetrical dyes. Additionally, the femtosecond transient absorption (TA) spectra at time zero show little solvent dependence for DPA2F or BT2F, but AF240 shows a 52 nm hypsochromic shift from n-hexane to acetonitrile. Coupled with the solvatochromism in the fluorescence and large excited-state dipole moment, this is attributed to formation of an intramolecular charge-transfer (ICT) state in polar solvents. By 10 ps in AF240, the maximum TA in acetonitrile has shifted 30 nm, providing direct evidence of a solvent-stabilized ICT state, whose formation occurs in 0.85-2.71 ps, depending on solvent. However, AF240 in nonpolar solvents and the symmetrical dyes in all solvents show essentially no shifts due to a predominantly locally excited (LE) state. Preliminary temperature-dependent fluorescence using frozen glass media supports significant solvent reorganization around the AF240 excited state in polar solvents, and may also support a twisted intramolecular charge-transfer (TICT)-state contribution to the stabilization. Finally, time-dependent density functional theory calculations support ICT in AF240 in polar media and also allow prediction of the 2PA cross sections in the 0-0 band, which are much larger for AF240 than the symmetrical dyes.

Deep-blue emitting pyrene–benzimidazole conjugates for solution processed organic light-emitting diodes

“…This supports the assumption of relatively large degree of charge separation for the amine-containing molecules (3d and 3e) in the excited state. 32,33 The Stokes shis for the dyes 3a-3e in different solvents were calculated to know the structural reorganization occurring during electronic excitation. Stokes shi values in a particular solvent for the dyes follow the order 3a < 3d < 3c < 3e < 3b.…”

Relay proton transfer triggered twisted intramolecular charge transfer

“…[1][2][3] Though several mechanisms were propsed to demonstate formation of the ICT state, the twisted ICT (TICT) model proposed by Grabowski et al is widely accepted. [4][5][6][7][8][9][10][11][12] A close look reveals that other than polarity and viscosity, the hydrogen bonding also strongly influence the TICT process of numerous systems. 4,5 In other words, that the donor is perpendicular to other part of the molecule in the ICT state.…”

“…It is well documented that the TICT emission depends on the polarity as well as viscosity. [4][5][6][7][8][9][10][11][12] A close look reveals that other than polarity and viscosity, hydrogen bonding also strongly influences the TICT process of numerous systems. [13][14][15][16][17][18][19][20] Solvents can form hydrogen bonds with either the charge donor or the charge acceptor of the molecule.…”

T‐Shaped Benzimidazole Derivatives as Blue‐Emitting Materials: The Role of C2 Substituents on Photophysical Properties

“…Compound 4 c exhibited the longest wavelength emission in the series. This probably indicates significant structural reorganization during the photoexcitation of the dyes, that is, excimer formation or strong interchromophoric electronic coupling in the excited state. Similar observations were reported previously for pyrenoimidazole‐based blue‐emitting dyes containing pyrene at the C2 position of the imidazole unit .…”