The Influence of Push–Pull States on the Ultrafast Intersystem Crossing in Nitroaromatics

Abstract

The photochemistry of nitro-substituted polyaromatic compounds is generally determined by the rapid decay of its S1 state and the rapid population of its triplet manifold. Previous studies have shown that such an efficient channel is due to a strong coupling of the fluorescent state with specific upper receiver states in the triplet manifold. Here we examine variations in this mechanism through the comparison of the photophysics of 2-nitrofluorene with that of 2-diethylamino-7-nitrofluorene. The only difference between these two molecules is the presence of a diethylamino group in a push-pull configuration for the latter compound. The femtosecond-resolved experiments presented herein indicate that 2-nitrofluorene shows ultrafast intersystem crossing which depopulates the S1 emissive state within less than a picosecond. On the other hand, the amino substituted nitrofluorene shows a marked shift in its S1 energy redounding in the loss of coupling with the receiver triplet state, and therefore a much longer lifetime of 100 ps in cyclohexane. In polar solvents, the diethylamino substituted compound actually shows double peaked fluorescence due to the formation of charge transfer states. Evaluation of the Stokes shifts in different solvents indicates that both bands correspond to intramolecular charge transfer states in equilibrium which are formed in an ultrafast time scale from the original locally excited (LE) state. The present study addresses the interplay between electron-donating and nitro substituents, showing that the addition of the electron-donating amino group is able to change the coupling with the triplet states due to a stabilization of the first excited singlet state and the rapid formation of charge transfer states in polar solvents. We include calculations at the TD-DFT level of theory with the PBE0 and B3LYP functionals which nicely predict the observed difference between the two compounds, showing how the specific S(π-π*)-T(n-π*) coupling normally prevalent in nitroaromatics is lost in the push-pull compound.

Solvent-dependent dual fluorescence of the push–pull system 2-diethylamino-7-nitrofluorene

“…We note that these values are not from a Gaussian deconvolution but merely represent the maxima values -deconvolution in a series of solvents has been published previously. 30 The spectrum of dm-ANF in methanol has not been reported previously, although a peak absorption wavelength has been reported for this compound at 420 nm 38 and 432 nm. 34 Methanol was chosen as the solvent to illustrate the similarity of the steady-state spectra because the separation of the two fluorescence bands is clearest in this solvent.…”

“…We employed PBE0/6-311+G(d,p) for consistency with previous studies, and because this level of theory was found to describe the excited state properties of de-ANF reasonably well. 30 Minimum energy structures (zero imaginary frequencies) were first optimized on the ground state. Excited state minima were then identified by optimization from ground state initial guesses with the planes defined by CNC and ONO being perpendicular to the aromatic skeleton and in same plane, respectively.…”

A general method to describe intersystem crossing dynamics in trajectory surface hopping

“…Examples of molecules where ISC is important range from aldehydes [37] and small aromatic compounds like benzene, [38] naphthalene, anthracene, and their carbonylic derivatives [39][40][41][42][43][44][45][46][47][48][49][50][51] to nitrocompounds. [43,[52][53][54][55][56][57][58][59][60][61] Furthermore, ISC has been reported for thio-substituted, [62][63][64][65][66][67][68] aza-substituted, [69] bromo-substituted, [70] and canonical nucleobases. [71][72][73] From a theoretical point of view, ISC can be explained by the interaction of states of different multiplicity by spin-orbit coupling (SOC), which is a relativistic effect.…”

Mechanism of Ultrafast Intersystem Crossing in 2‐Nitronaphthalene

“…In contrast, ISC was expected to be considerably slower when the SOCs are very small, as is the case in organic molecules composed solely of light atoms of the first period. For certain classes of organic molecules, such as nitro polycyclic aromatic hydrocarbons (NPAHs)3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14 and the closely related nitrobenzene derivatives,15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25 ISC has been measured to occur in an ultrafast sub‐picosecond timescale, challenging this paradigm.…”