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A theoretical method for calculating rate constants for internal conversion (IC), intersystem crossing (ISC) and radiative (R) electronic transitions is presented. The employed method uses temperature-dependent quantum Green's functions, which give the opportunity to consider almost any nth-order polynomial perturbation operator and the influence of external electromagnetic fields on the rate constants. The rate constants of the IC, ISC and R processes are calculated for two important indocyanine molecules namely indocyanine green (ICG) and heptamethine cyanine (IR808) at the Franck-Condon level using the temperature-dependent quantum Green's function approach. Calculations at the time-dependent density functional theory level with the MN15 functional show that ICG and IR808 have only one triplet state below the S 1 state. The main deactivation channel of the S 1 state is the IC process with a large ( k (S IC → S 1 → S 0 )) rate constant of ∼10 9 s 11 s -1 . The estimated quantum yield of fluorescence ( φ fl ) is ∼0.001-0.24 for the two studied molecules, which agrees rather well with experimental values. Thus, the present approach enables calculations of the three kinds of rate constants and the quantum yield of fluorescence using the same computational methodology.

We have developed a new methodology for calculating contributions to the rate constants ( k IC ) of internal conversion that are induced by external electric ( k IC-E ) or magnetic ( k IC-M ) fields. The influence of the external electric and magnetic fields on the k IC was estimated for seven representative molecules. We show that the k IC-E contribution calculated at a field strength of 10 11 V m -1 is generally as large as the k IC rate constant in the absence of the external field. For indocyanine green, azaoxa[8]circulene, and pyromitene 567, the k IC-E contribution is as large as k IC already at a field strength of 10 9 V m -1 . Such electric-field strengths occur for example in plasmonic studies and in strong laser-field experiments. The induced effect on the k IC rate constant should be accounted for in calculations of photophysical properties of molecules involved in such experiments. The induced effect of an external magnetic field on k IC can be neglected in experiments on Earth because the magnetic contribution becomes significant only at very strong magnetic fields of 10 4 -10 5 T that cannot be achieved on Earth. However, the magnetic effect on the rate constant of internal conversion can be important in astrophysical studies, where extremely strong magnetic fields occur near neutron stars and white dwarfs.

A novel method for calculating rate constants for internal conversion ( k IC ) that simultaneously accounts for Duschinsky, anharmonic and Herzberg-Teller effects has been developed and implemented. This method has been applied to robust planar molecules like tetraoxa[8]circulene (4B), free-base porphyrin (H2P) and pyrometene (PM567) with small Duschinsky rotation ( i.e. with almost identical normal coordinates in the ground and excited states) and to poly[ n ]fluorenes (P[n]F) ( n such as the X-H (X = C, N and O) stretching modes that mix in the S -1 → S 1 → S 0 electronic transition. However, even in this case, the increase in k IC due to the Duschinsky effect does not exceed one order of magnitude. The calculations show that anharmonic contributions to k IC are larger than Herzberg-Teller contributions which in turn are larger than contributions from the Duschinsky effect ANH > HT > Du. We also show that an approximation, where only X-H bonds are considered in the k IC calculation, is accurate even for P[n]F ( n = 2-14).