It is shown that intramolecular hydrogen bonding can give rise to very fast nonradiative desactivation of the S1 -state of indigo dyes. In those derivatives, which lack the possibility of internal hydrogen bonds, hydrogen bonding to suitable solvent molecules can provide the channel for fast radiationless transitions. As a consequence, drastic effects on the S1 -state lifetime are observed both as a function of solvent and/or temperature.

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We studied the effect of deuteration on the radiative decay of the triplet sublevels of naphthalene and some halogenated derivatives. We found that the influence of deuteration is much more pronounced in the heavy atom substituted than in the parent hydrocarbons. The strongest change upon deuteration is in the radiative decay of the out-of-plane polarized spin state T x . These findings are consistently related to a second order Herzberg-Teller (HT) spin-orbit coupling. Though we found only a small influence of deuteration on the total radiative rate in naphthalene, a significantly larger effect is observed in the rate of the 00-transition of the phosphorescence. This result is discussed in terms of a change of the overlap integral of the vibrational groundstates of Tx and S0 upon deuteration.