Abstract
The rates of ultrafast intersystem crossing in acceptor-bridge-donor molecules centered on Pt(II) acetylides are investigated. Specifically, a Pt(II) trans-acetylide triad NAP--Pt--Ph-CH2-PTZ [1], with acceptor 4-ethynyl-N-octyl-1,8-naphthalimide (NAP) and donor phenothiazine (PTZ), is examined in detail. We have previously shown that optical excitation in [1] leads to a manifold of singlet charge-transfer states, S*, which evolve via a triplet charge-transfer manifold into a triplet state 3NAP centered on the acceptor ligand and partly to a charge-separated state 3CSS (NAP--Pt-PTZ+). A complex cascade of electron transfer processes was observed, but intersystem crossing (ISC) rates were not explicitly resolved due to lack of spin selectivity of most ultrafast spectroscopies. Here we revisit the question of ISC with a combination and complementary analysis of (i) transient absorption, (ii) ultrafast broadband fluorescence upconversion, FLUP, which is only sensitive to emissive states, and (iii) femtosecond stimulated Raman spectroscopy, FSR. Raman resonance conditions allow us to observe S* and 3NAP exclusively by FSR, through vibrations which are pertinent only to these two states. This combination of methods enabled us to extract the intersystem crossing rates that were not previously accessible. Multiple timescales (1.6 ps to ∼20 ps) are associated with the rise of triplet species, which can now be assigned conclusively to multiple ISC pathways from a manifold of hot charge-transfer singlet states. The analysis is consistent with previous transient infrared spectroscopy data. A similar rate of ISC, up to 20 ps, is observed in the trans-acetylide NAP--Pt--Ph [2] which maintains two acetylide groups across the platinum center but lacks a donor unit, whilst removal of one acetylide group in mono-acetylide NAP--Pt-Cl [3] leads to >10-fold deceleration of the intersystem crossing process. Our work provides insight on the intersystem crossing dynamics of the organo-metallic complexes, and identifies a general method based on complementary ultrafast spectroscopies to disentangle complex spin, electronic and vibrational processes following photoexcitation.
Highlights
Ultrafast creation of the primary excited electronic state leads to a manifold of vibrationally hot states where cooling, state- and spin-dynamics are convolved
Comparing fluorescence upconversion (FLUP) and femtosecond-stimulated Raman (FSR) results for [1]–[3], alongside with transient absorption and transient infrared data, we show that the strength of radiative coupling to the electronic ground state remains constant during the decay process, at a value similar to that of absorption
Some of us previously reported a series of Pt(II) donor-acceptor complexes, Scheme 142 in which high-frequency acetylide stretching vibrations were used as a tool to manipulate excited state dynamics
Summary
Ultrafast (femtosecond) creation of the primary excited electronic state leads to a manifold of vibrationally hot states where cooling, state- and spin-dynamics are convolved. The disentangling of the dynamics of the initially populated Franck–Condon-excited state presents a difficult task. Such states are typically extremely short-lived with multiple ultrafast decay processes occurring in parallel, contrary to the frequently-used cascade model where vibrational relaxation, internal conversion and intersystem crossing are assumed to occur sequentially on different time-scales. Numerous observations of ISC in metal complexes have been reported in the past two decades, containing, for example Cr, Fe,11–17 Cu, Zn,19 Ni, Ru,21–26 Re, Os,28 Pt, Pd,20 or Mo.. ISC has been reported to occur in metal complexes over a large range of timescales, from o30 fs to hundreds of picoseconds, but it is not clear yet what 21652 | Phys.
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