Strongly Absorbing π–π* States in Heteroleptic Dipyrrin/2,2′‐Bipyridine Ruthenium Complexes: Excited‐State Dynamics from Resonance Raman Spectroscopy

Abstract

A recently reported new class of ruthenium complexes containing 2,2'-bipyridine and a dipyrrin ligand in the coordination sphere exhibit both strong metal-to-ligand charge-transfer (MLCT) and pi-pi* transitions. Quantitative analysis of the resonance Raman scattering intensities and absorption spectra reveals only weak electronic interactions between these states despite direct coordination of the bipyridyl and dipyrrin ligands to the central ruthenium atom. On the basis of DFT calculations and time-dependent DFT (TD-DFT), we propose that the electronic excited states closely resemble "pure" MLCT and pi-pi* states. Resonance Raman intensity analysis demonstrates that a large amplitude transannular torsional motion provides a mechanism for relaxation on the pi-pi* excited-state surface. We assert that this result is generally applicable to a range of dipyrrin complexes such as boron-dipyrrin and metallodipyrrin systems. Despite the large torsional distortion between the phenyl ring and the dipyrromethene plane, pi-pi* excitation extends out onto the phenyl ring which may have important consequences in solar-energy-conversion applications of ruthenium-dipyrrin complexes.

Raman spectroscopy of dipyrrins: nonresonant, resonant and surface‐enhanced cross‐sections and enhancement factors

“…There are several reports of resonant cross‐sections for strongly enhanced Raman scatterers with visible excitation such as betaine‐30 (maximum cross‐section 1.6 × 10 −25 cm 2 sr −1 ), the nonlinear chromophore julolidinyl‐ n‐N , N ′‐diethylthiobarbituric acid (JTB) with a maximum cross‐section of 7 × 10 −24 cm 2 sr −1 , the azo dye Disperse Red (a maximum total cross‐section of 6.0 × 10 −25 cm 2 ), and [Ru(bpy) 3 ] 2+ (1.0 × 10 −24 cm 2 sr −1 ), which are similar in magnitude to the values reported here for the free dipyrrin systems. The cross‐sections reported here are also in good agreement with the cross‐sections for the ruthenium complex of a meso ‐aryl‐substituted‐dipyrrin (1.6 × 10 −24 cm 2 sr −1 for the 409 cm −1 band) . Resonant Raman cross‐sections for fluorescent dyes, R6G (1510 cm −1 band, 1.5 × 10 −24 cm 2 sr −1 with 532 nm excitation), and Crystal Violet (1650 cm −1 band, 6.7 × 10 −25 cm 2 sr −1 with 568 nm excitation) commonly used in SM‐SERS have been estimated by using the SERS effect to quench spontaneous emission .…”

“…Despite the nonplanarity of the phenyl ring there is also significant movement of charge‐density onto the phenyl ring during the π−π* transition. Similar changes in charge‐density over the phenyl ring have been proposed to explain the enhanced transannular torsional mode intensity in the resonant Raman spectra of the ruthenium complexes of dipyrrin ligands …”

“…The large intensity of mode 83 implies substantial structural reorganization of the phenyl ring in the π−π* excited state. As shown in dipyrrin complexes the structural reorganization of the phenyl ring is mostly associated with rotation of the ring about the transannular bond. It is appropriate to make some comparison between the electronic structures and resonant Raman features of phenyl‐substituted dipyrrins and tetraphenylporphyrin species.…”

“…Surprisingly, there are no publications involving the Raman spectroscopy of free dipyrrins. The SERS spectra of dipyrrin–BF 2 complexes have been reported and the only reported Raman cross‐sections for dipyrrin compounds are for the recently reported ruthenium complex . Boron complexes of dipyrrins are widely known for their excellent emission properties and the success of BODIPY™ complexes in fluorescence applications in particular, may have obscured the promising Raman properties of the free dipyrrin unit.…”

Bridging-arylene effects on spectroscopic and photophysical properties of arylborane–dipyrrinato zinc(ii) complexes

“…Dipyrrinato metal complexes exhibiting the CT-type excited states are characteristic at this stage since most of the excited states of dipyrrinato metal complexes are ascribed to pp* transition in the dipyrrinato ligand. [43][44][45][46][47][48][49][50][51][52] Especially, two stereoisomers of bis(5-{4-[bis (2,4,6-trimethylphenyl)boryl]phenyl}dipyrrinato)platinum(II), PtBph, possessing the square-planar and distorted tetrahedral geometries show different absorption/emission spectra not only in the MLCT/ pp*/p(aryl)-p(B) CT band but also in the ligand-centered (LC) band. Both crystallographic data and theoretical calculations suggest that a dihedral angle between the dipyrrinate and bridging phenylene moieties in the square-planar isomer is larger than that in the distorted tetrahedral one.…”