Excited-state behavior of N-phenyl-substituted trans-3-aminostilbenes: Where the "m-amino effect" meets the "amino-conjugation effect"

The electronic spectroscopy and photochemistry of the trans isomers of 3-(N-phenylamino)stilbene (m1c), 3-(N-methyl-N-phenylamino)stilbene (m1d), 3-(N,N-diphenylamino)stilbene (m1e), and 3-(N-(2,6-dimeth-ylphenyl)amino) stilbene (m1f) and their double-bond constrained analogues m2a-m2c and m2e are reported. When compared with trans-3-aminostilbene (m1a), m1c-m1e display a red shift of the S₀ → S₁ absorption and fluorescence spectra, lower oscillator strength and fluorescence rate constants, and more efficient S₁ → T₁ intersystem crossing. Consequently, the N-phenyl derivatives m1c-m1e have lower fluorescence quantum yields and higher photoisomerization quantum yields. The corresponding N-phenyl substituent effect in m2a-m2e is similar in cyclohexane but smaller in acetonitrile. This is attributed to the weaker intramolecular charge transfer character for the S₁ state of m2 so that the rates for intersystem crossing are less sensitive to solvent polarity. It is also concluded that N-phenyl substitutions do not change the triplet mechanism of photoisomerization for m1 in both nonpolar and polar solvents. Therefore, the "m-amino conjugation effect" reinforces the "m-amino effect" on fluorescence by further reducing its rate constants and highlights the N-phenyl-enhanced intersystem crossing from the "amino-conjugation effect" by making S₁ → T₁ the predominant nonradiative decay pathway.