Experimental Evidence for a Selection Rule of Intersystem Crossing to the Excited Quartet States:  Metallophthalocyanines Coordinated by 4-Amino-TEMPO

Abstract

Metallophthalocyanines (MPcs; M = Zn, Mg) coordinated by 4-amino-TEMPO (ATEMPO) in the excited quartet (Q1) state, which are constituted by the excited triplet MPc and doublet ATEMPO, have been studied by a time-resolved electron paramagnetic resonance (TREPR) method. Two components were observed for both the Zn and Mg complexes, and are reasonably assigned to the Q1 and doublet states by calculating the zero field splitting and g value, respectively. The polarization pattern of the Q1 ZnPc-ATEMPO is A/E, and is opposite to that (E/A) of the Q1 MgPc-ATEMPO, where the A and E denote absorption and emission of the microwaves, respectively. The Q1 TREPR spectrum with the E/A polarization has been observed for the first time. The A/E and E/A polarization patterns are reproduced by selective intersystem crossing (ISC) to |Q1,±1/2‘〉 and to |Q1,±3/2‘〉, respectively. The selective ISC to |Q1,±1/2‘〉 is reasonably explained by the z component of SOC between the Q1 and higher excited doublet states, while the ISC to |Q1,±3/2‘〉 originates from the x and y components of SOC. These experiments have proved that the selectivity of ISC to the Q1 state is dependent on the components of SOC.

Excitation Energy Transfer and Exchange‐Mediated Quartet State Formation in Porphyrin‐Trityl Systems

“…The opposite multiplet polarisation pattern observed for ZnTPP‐trityl and MgTPP‐trityl ( a / e vs. e / a ) might suggest a strong correlation between the quartet state polarisation and the polarisation of the triplet precursor ( aaaeee vs. eeeaaa ), i.e. a direct influence of the relative initial triplet state populations and the sign of D T , in agreement with previous observations [55] . However, an exception is known in the literature, where the polarisation pattern of the quartet state formed in a free‐base porphyrin‐verdazyl system was found to differ from that of the corresponding porphyrin triplet state, [56] implying that the mechanism is likely to be more complex and that other factors, such as the signs of D Q , D TR , and J TR or the proximity of vibrational states for mixing, also need to be taken into consideration [19, 22] …”

“…ad irecti nfluence of the relative initial triplet state populations and the sign of D T ,i na greement with previouso bservations. [55] However,a ne xception is known in the literature, where the polarisation pattern of the quartet state formed in af ree-base porphyrin-verdazyl system was found to differ from that of the corresponding porphyrin triplet state, [56] implying that the mechanism is likelytobemore complex and that other factors, such as the signs of D Q , D TR ,a nd J TR or the proximity of vibrational states for mixing, also need to be taken into consideration. [19,22] The qualitative similarity of the transientc wE PR spectrar ecorded for ZnTPP-trityl at X-(9.75 GHz) and Q-band (34.0 GHz) frequencies (cf.…”

“…In particular, the complete absence of multiplet polarisation in the spectrum of H 2 TPP-trityl is intriguing and cannot readily be explained since spin-selective spin-orbit inducedi ntersystem crossing from the doublet to the quartet excited state should alwaysl ead to the observation of multiplet spin polarisation. [17,19,48,49,55] This apparent inconsistency can be resolved by assuming that doublet-quartet mixingi sf ast on the EPR time scale:B y the time of detection, as ignificant part (or all) of the multiplet polarisation might already have decayed. However,t oe xplain the experimental observations, these equilibrationd ynamics would need to be considerably faster in H 2 TPP-trityl as compared to ZnTPP-trityl or MgTPP-trityl, implying as maller D and/or J TR in H 2 TPP-trityl (cf.…”

Dual Chromophore‐Nitroxides: Novel Molecular Probes, Photochemical and Photophysical Models and Magnetic Materials

“…Recently, some chromophores bonded to nitroxides have been studied by time-resolved ESR (TRESR) spectroscopy to investigate directly the excited multiplet states consisting of a photoexcited triplet chromophore and doublet nitroxides (41)(42)(43)(44)(45)(46)(47)(48)(49)(50)(51)(52)(53)(54)(55)(56)(57). The D 1 and QA 1 states, which consist of a T 1 chromophore and doublet NR, have been observed in solution and in frozen solutions for several systems (Fig.…”

Reaching Optimal Light‐Induced Intramolecular Spin Alignment within Photomagnetic Molecular Device Prototypes

“…Regarding dual P/SC functional elements alternative to anthracene, there are the silole luminophore,74, 75 and pyrene,10c, f as well as metallo‐porphyrins76 or metallo‐phtalocyanines,77 and even corannulene78 or fullerene79 derivatives.…”