Thermally Activated Internal Conversion with 4-(Dimethylamino)benzonitrile, 4-(Methylamino)benzonitrile, and 4-Aminobenzonitrile in Alkane Solvents. No Correlation with Intramolecular Charge Transfer

Abstract

With 4-(dimethylamino)benzonitrile (DMABN), 4-(methylamino)benzonitrile (MABN), and 4-aminobenzonitrile (ABN) in an alkane solvent such as n-hexadecane, the fluorescence decay time τ and quantum yield Φf strongly decrease with increasing temperature. For DMABN in n-hexadecane, τ decreases from 3.43 ns at 25 °C to 0.163 ns at 284 °C, with a simultaneous drop in Φf from 0.14 to 0.006. Similar results are obtained for MABN and ABN. By measuring τ, Φf, and the intersystem crossing (ISC) yield ΦISC of the three aminobenzonitriles as a function of temperature in 2-methylpentane and n-hexadecane, covering a range from −151 to 284 °C, the rate constants for internal conversion (IC), ISC, and fluorescence are determined, together with their activation energies and preexponential factors. It is so established that DMABN, MABN, and ABN undergo efficient thermally activated IC. Upon increasing the temperature for DMABN in n-hexadecane from 18 to 287 °C, the IC yield ΦIC increases from 0.04 to 0.95. This goes at the expense of ISC and fluorescence, with a decrease from 0.81 to 0.04 for the yield ΦISC and from 0.15 to 0.005 for the fluorescence quantum yield Φf between these two temperatures. With MABN and ABN, likewise with ISC as the main decay channel at room temperature, IC becomes the dominating deactivation pathway of the first excited singlet state S1 at temperatures above 125 °C. The IC activation energies EIC have similar values for the three aminobenzonitriles in the alkanes: 31.3 kJ/mol (DMABN), 34.3 kJ/mol (MABN), and 34.8 kJ/mol (ABN), with preexponential factors of around 5 × 1012 s-1. The ISC activation energies EISC are considerably smaller, 3.9 kJ/mol (DMABN) and 5.6 kJ/mol (MABN and ABN), with relatively small preexponential factors of around 3 × 108 s-1, values in accord with the spin forbidden character of ISC. The different height of the barriers EIC and EISC makes their separate determination at the high and low parts of the available temperature range possible. Contrary to what has previously been postulated for 1-aminonaphthalenes, the similarity of the barrier heights EIC of DMABN, MABN, and ABN shows that the IC reaction of these molecules in alkane solvents is not governed by the energy gap ΔE(S1,S2) between the two lowest excited singlet states, which gap substantially increases in the series DMABN, MABN, ABN. Because intramolecular charge transfer (ICT) does not take place with any of these three aminobenzonitriles in alkane solvents, the thermally activated IC process reported here is mechanistically not related to ICT. The IC decay channel obviously should be taken into account in discussions of excited-state processes of DMABN and its derivatives at higher excitation energies.