Highly Efficient, Near‐Infrared Electrophosphorescence from a Pt–Metalloporphyrin Complex

Abstract

Organic light-emitting diodes (OLEDs) have been the subject of a significant research effort for the past two decades with a focus on devices that emit almost exclusively in the visible part of the electromagnetic spectrum.[1] Recently, there has been a growing interest in OLEDs that emit in the nearinfrared (NIR) region (700-2500 nm). [2][3][4] Applications for these NIR OLEDs are particularly interesting for nightvision-readable displays [5] and sensors.[6] The efficiency of OLEDs are markedly improved when fluorescent emissive dopants are replaced with phosphorescent heavy-metal complexes that can effectively harvest both the singlet and triplet excitons formed in electroluminescence, with wavelengths (l) ranging from the near-ultraviolet into the red (with peak emission at l max = 380-650 nm).[7-10] Herein, we report on an efficient NIR OLED that utilizes a phosphorescent Ptmetalloporphyrin dopant, with an external quantum efficiency (EQE) greater than 6 % at l max = 765 nm and a full width at half maximum of 31 nm (500 cm À1 ). Previously, two classes of phosphorescent complexes have been employed as dopants in NIR-emitting OLEDs. The first utilizes trivalent lanthanide cations (Ln 3+ ) as the emitting centers, for example, Er 3+ or Nd 3+ , chelated with chromophoric ligands to sensitize excitation-energy transfer to the lanthanide ion.[11] Schanze et al. have reported an NIR OLED utilizing Ln 3+ in conjunction with a porphyrin/polystyrene matrix, with EQE ranging from 8.0 10 À4 to 2.0 10 À4 % at approximately 1 mA cm À2 . [5] Similarly, a Nd(phenalenone) 3 -based OLED had an EQE of 0.007 % at l max = 1065 nm.[4] The second class of NIR OLEDs is transition-metal complexes, similar to those used in the visible region. A recent report of an electrophosphorescent device that used a cyclometalated [(pyrenyl-quinolyl) 2 Ir(acac)] complex as the phosphor gave l max = 720 nm and an EQE of 0.1 %. [6] A family of complexes that have shown intense absorption and emission in the red-to-NIR region of the spectrum are the metalloporphyrins. [12,13] There are a number of reports of OLEDs fabricated with [Pt(oep)], [Pt(tpp)] (oep = 2,3,7,8,12,13,17,18-ocatethylporphryin, tpp = 5,10,15,20-tetraphenylporphyrin), or analogues of these compounds as phosphorescent emitters, with emission maxima between 630 and 650 nm, [7,[14][15][16][17][18][19][20][21][22] however, there has been no apparent effort to shift the Pt-porphyrin-based OLED emission into the NIR region. Porphyrin chromophores with fused aromatic moieties at the b-pyrrole positions, for example, tetrabenzoporphyrin (bp), exhibit a bathochromic shift (relative to unsubstituted porphyrin) of the absorption and emission energy, owing to the expansion of the p-electronic system of the porphyrin core.[23] The addition of bulky groups to the meso positions of the porphyrin macrocycles with b-substituted pyrroles leads to the formation of nonplanar porphyrins, and further red-shifts the absorption spectra.[24] Coordination of a heavy-metal atom increases the rate of the int...

Organic Photovoltaics Using Tetraphenylbenzoporphyrin Complexes as Donor Layers

“…As shown previously, [5,6] the solution absorption spectra present an intense and sharp Soret band at wavelengths of l ¼ 430 nm and l ¼ 444 nm for PtTPBP and PdTPBP, respectively. The heavy central atoms and the ring structure affect the Qband, resulting in a narrow and intense peak at l ¼ 613 nm (full width at half-maximum (FWHM) ¼ 18 nm) for PtTPBP and l ¼ 629 nm for PdTPBP (FWHM ¼ 22 nm).…”

“…immediately prior to loading into a high vacuum (1-3 Â 10 À6 Torr) chamber. Both Pt and Pd tetraphenylbenzoporphyrin were synthesized according to literature [5], and purified by vacuum thermal gradient sublimation. The organic materials, copper phthalocyanine (CuPc) 99% (Aldrich), fullerene (C 60 ) 99.5% (MTR Limited), and 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP) 96% (Aldrich) were also purified by sublimation with one (C 60 ) or two (CuPc and BCP) cycles prior to use.…”

“…[5] COMMUNICATION www.advmat.de Table 1). The difference is primarily due to the higher V oc of the Pt compound.…”

“…1) and the doped thin-film spectra of PtTPBP (solution and doped thin film PL spectra of PtTPBP are nearly identical). [5] Indeed, the thin-film absorption spectrum of PtTPBP shows only a minor bathochromic shift and broadening relative to the solution spectrum. These spectra suggest that they are due to excimers, or to a low concentration of dimer/aggregate states.…”

“…[5] More relevant to OPVs is the extended electron conjugation of this molecule, relative to other Pt porphyrins, which results in higher optical extinction coefficients and a marked red-shift of the absorption in the visible region. PtTPBP is a nonplanar molecule with a saddle shape that may affect film morphology, and consequently, its electronic transport properties.…”