The sensitivity and specificity of the optical modality can be enhanced by the use of highly absorbing dyes as contrast agents. Novel cyanine dyes that absorb and emit light in the near infrared region of electromagnetic spectrum are disclosed. These dyes are useful for imaging, diagnosis and therapy of various diseased states. Particularly, the molecules of the invention are useful for optical diagnostic imaging and therapy, in endoscopic applications for the detection of tumors and other abnormalities, for localized therapy, for photoacoustic tumor imaging, detection and therapy, and for sonofluorescence tumor imaging, detection and therapy.