Excitons and a Charge-Separated Pair in Thin Crystals of Oxotitanium(IV) Phthalocyanine As Revealed by Femtosecond Time-Resolved Absorption and Time-Correlated Single Photon Counting

A photochemical study was performed by means of femtosecond time-resolved absorption spectroscopy on novel polymorphs of thin crystalline oxotitanium(IV) phthalocyanine (OTiPc)n (n = 2−4), β-OTiPc-L (phase I) and α-OTiPc-S (phase II), where the letters “L” and “S” mean “lying” molecular orientation and “standing” molecular orientation, respectively. A rapid change of the well time-resolved absorption spectrum in a range of 420−620 nm revealed that a part of the intrinsic exciton of β-OTiPc-L with the absorption maximum around 550 nm was in a lifetime of 0.5 ps converted to a charge-separated pair on the excitation of the 400 and 800 nm laser. The absorption bands at 430, 510, and ∼860 nm were left for a lifetime of 3 ps. The formation of the cationic π-radical inferred to be 2(OTiPc)n-1+ is responsible for the absorption bands at 510 nm and that of the anionic species of 2(OTiIIIPc)- is responsible for the band at 430 nm instead of the characteristic bands of the anionic π-radical of metallophthlocyanine around 600 nm. While the charge-separated pair disappeared in 10 ps, another transient absorption at 490 nm was survived on the excitation of the 400 nm laser, which is assigned to the triplet exciton formed via intersystem crossing from higher state(s) of the singlet exciton. The time-correlated single photon counting technique revealed that a novel fluorescence of α-OTiPc-S with a peak at 10 420 cm-1 decaying biexponentially with a short (48−64 ps) lifetime and a long (101−416 ps) lifetime is assigned to the trapped excitons. A 10-fold increase in the initial intensity of the fluorescence at 77 K compared with at 298 K indicates that the yield of long-lived and fluorescent exciton is smaller than 0.1 on the excitation of α-OTiPc-S at 298 K.