Transient absorption studies of the photochromic behavior of 3H-naphtho[2,1-b]pyran linked to a p-nitroaniline group

The photophysical and photochemical behavior of a 3H-naphtho[2,1-b]pyran compound substituted at position 8 by a p-nitroaniline group has been investigated by transient absorption spectroscopy in the femto/picosecond and nano/microsecond time domains. Measurements were undertaken at two different pump excitation wavelengths, adjusted in resonance with the lowest energy transition S₀-S₁ (abs λ_max 392 nm) and with a higher energy transition S₀-S_n (abs λ_max 265 nm), respectively. In both cases, the results show the contribution of three transient species to the photoinduced processes: the excited S₁ and T₁ states of the initial ring-closed molecule and a long-lived colored species ascribed to a ring-opened photoproduct, attesting the presence of some photochromic activity. The S ₁ and T₁ states are mainly localized on the p-nitroaniline substituent and do not contribute highly to the photochromic efficiency. The ring-opening photochromic process occurs essentially in ≤0.4 ps following excitation of the S_n state, in competition with the relaxation to the S₁ state via internal conversion.