Relaxation in the Triplet Manifold of 1-Nitronaphthalene Observed by Transient Absorption Spectroscopy

Abstract

Previous phosphorescence and triplet quantum yield determinations indicate that the primary photophysical channel for 1-nitronaphthalene is the formation of its lowest energy triplet state. Also, previous direct measurements of the decay of the fluorescence from this compound indicated that the crossing between the singlet and triplet manifolds is ultrafast (sub-100 fs). In this contribution we present a sub-picosecond transient absorption study of the relaxation of photoexcited 1-nitronaphthalene in methanol and other solvents. Our measurements reveal the time scale in which the fully relaxed T(1) state is formed. We have observed that the spectral evolution associated with this process takes place in time scales from one to a few tens of picoseconds. Specifically, the appearance of the absorption spectrum of T(1) in the visible region is accompanied by the decay of transient signals at wavelengths below 400 nm. Since the fluorescence lifetime of this compound is sub-100 fs, we assigned the picoseconds decaying signals below 400 nm to an intermediate triplet state which acts as a receiver state in the intersystem crossing step and from which the T(1) population accumulates. From the details of the spectral evolution and the effects of different solvents, we also conclude that T(1) formation and vibrational cooling within this state occur in similar time scales of between 1 and 16 ps. Mainly, our results provide direct evidence in support of the participation of an upper triplet state in the mechanism for intersystem crossing in this molecule. This is considered to be common in the photophysics of several nitrated polycyclic aromatic compounds and the most determinant feature of their primary photochemistry.

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

“…If nonadiabatic couplings are not available, it is still possible to calculate the propagator matrix P using other schemes, see, for example, Refs. [55,181]. However, for the gradient transformation no alternative schemes without nonadiabatic couplings are available, and, as an approximation K MCH ij has to be neglected in eqs.…”

“…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

“…However, for other NPAH derivatives investigated in the same study5 the fast sub‐100 fs decay time was attributed to a conformational relaxation in the initially populated excited state involving the re‐orientation of the nitro group. Later studies on 1NN using different solvents6 and sub‐ps‐resolved transient absorption spectroscopy experiments7 reassured that the decay of S 1 occurs within 100 fs and it was established that relaxation within the triplet manifold (T n –T 1 ) proceeded on a time scale of 1–16 ps.…”

“…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.…”

Fotofísica e propriedades dinâmicas de sistemas moleculares

“…O 1-nitronaftaleno também foi estudado por Zugazagoitia et al [187] e Reichardt et al [188] , onde estudos experimentais de espectroscopia de absorção transitória mostram a existência de um estado tripleto que atua como intermediário entre o estado excitado singleto e os estados tripletos. Posteriormente o sistema evolui através de uma CI para o estado tripleto fosforescente.…”